Valves Seen On Graham Slee's Desk!
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Topic: Valves Seen On Graham Slee's Desk!
Posted By: Graham Slee
Subject: Valves Seen On Graham Slee's Desk!
Date Posted: 29 Jan 2016 at 5:52pm
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Yes, the V (or T) word has been uttered at 'Slee towers'... And four valves complete with bases arrived today!  Does this mean I've gone crazy? Does this mean I've swallowed the marketing man's pill? I hope not, but I thought I'd try and hear what all the fuss is about and have a go at building a preamp line level stage to see just how good a modern performance can be gotten out of tubes. These are 12AX7 (ECC83) and 12AU7 (ECC82) tubes suitable for preamp use, and because they are very high output impedance (100k roughly) they'll need the help of some solid-state electronics to make them usable. Often these days they're used in what's called "plate voltage starvation circuits" -- that's what all these Audiophool things that run off 12V wall-warts are about. (see: http://audio-forum.gspaudio.co.uk/low-voltage-valve-tube-buffers-hybrids-amps_topic2785.html) But I'm not into making hi-fi fuzz boxes. I want to see how low I can make their distortion and how natural they can be made to sound. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Replies:
Posted By: ServerBaboon
Date Posted: 29 Jan 2016 at 6:03pm
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Just be careful you don't start tube rolling, not the same as joint rolling. ------------- Steve ------------- Various bits of GSP Kit ..well two so far, unless you count the cables that is. |
Posted By: DaveG
Date Posted: 29 Jan 2016 at 6:08pm
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Crazy? I couldn't possibly comment. Swallowed the pill? hmm.. unlikely I guess And speaking of tube rolling I have some NOS early 70's Philips matched 6189 SQ's that I am more than happy to to swap for an Accession!  ------------- Dave Michell Gorbe + HR PSU -> Cadenza Bronze -> SME V -> Elevator -> Accession -> Proprius -> B&W CM6 s2 | Cusat 50 & Spatia cables -> 
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Posted By: Graham Slee
Date Posted: 29 Jan 2016 at 6:18pm
Some of the prices of 'new old stock' are frightening. The ones I bought are cheapo Chinese from RS, which will get changed (rolled) for Electro-Harmonix if it works. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
ServerBaboon wrote:
Posted By: discrete badger
Date Posted: 29 Jan 2016 at 6:38pm
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We all know what happened the last time Graham decided to "dabble" in an area of audio electronics for which he wasn't previously known! ... what happened was the Bitzie and the Majestic. |
Posted By: ICL1P
Date Posted: 29 Jan 2016 at 7:16pm
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But I want a simple USB ADC with gain control for ripping by vinyl. ------------- Ifor ===== Reflex M & ACCESSION M, CuSat50, Majestic DAC, a Proprius pair. |
Posted By: CageyH
Date Posted: 29 Jan 2016 at 7:21pm
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I love the sound of my hybrid pre amp and hybrid power amp. It sounds superb, so I am really interested in what Graham can come up with. ------------- Kevin European loan coordinator, based near Toulouse, France. |
Posted By: Aussie Mick
Date Posted: 30 Jan 2016 at 4:38am
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Hmm. This raises questions! Increasingly I'm reading that valved-up amps are going more and more neutral in presentation and that solid state and valve pre and power amps are converging into some kind of common ground when it comes to sonic presentation. Graham, you've just stated you're interested in knowing how low you can get the distortion and that you're not into fuzz-box type constructions. If you're going to build a typical Slee component with openness, scale, clarity and neutrality, what is actually to be gained if you succeed? If such a device ended up sounding essentially like, for example, the Majestic, well, why build it? The valve version would be more expensive and less environmentally friendly, right? I'm more than happy to be corrected! So my question is, what are you hoping to gain by using valves? Mick. ------------- Rega RP8 - Apheta 2 - Accession MC Enigma PS -Solo ULDE (Focal Utopia) - PS Audio M700 - Fical Kanta No2 |
Posted By: JamesD
Date Posted: 30 Jan 2016 at 9:19am
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I'm not for or against valves but I'll find it interesting to see how good (or not) this 100+ year old invention will sound, through Graham's ears at least. I like it when what is considered "out-of-date" old technology still sounds great when used properly. Vinyl being an obvious example. ------------- Aren't ears brilliant |
Posted By: Fatmangolf
Date Posted: 30 Jan 2016 at 9:41am
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I've learned from Graham's previous experiments and bought most of the resulting products. I've got 4 valve guitar amps running and one of my mic preamps has HT valves in it so I'm curious. James - you've said what I was about to on vinyl and valves. ------------- Jon Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC. |
Posted By: morris_minor
Date Posted: 30 Jan 2016 at 9:42am
I think it's great that Graham is exploring the possibilities of glowing bottles, and if the results generate sales then that has to be good . . ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: discrete badger
Date Posted: 30 Jan 2016 at 11:12am
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I agree - I think it's potentially quite a commercially-astute move. There is a group of audiophiles out there who are quite convinced that valves sound better, and they just won't buy a solid-state amp no matter how good it sounds. They have expectation bias, but they also have money. If a product were to exist that has the characteristically excellent Slee sonics but ticks their "glowing tube" box too, it might prove a tempting proposition for them....
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Posted By: morris_minor
Date Posted: 30 Jan 2016 at 11:40am
Making the assumption that the sound quality of a GS valve (pre)amp is up to the standard we all expect it to be, the visual design (exposed valves etc) needs to be carefully considered since it seems that folk like to see the glow rather than just know it's there.  However, this is jumping the gun somewhat . . . (but can I sign up as an beta tester, please Graham?  )------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: DaveG
Date Posted: 30 Jan 2016 at 12:09pm
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I think there is still a place for valves. I've heard a few high end valve amps - AR, Jadis etc and the results were stunning but the price was equally stunning. I rather like the old glowing bottles for some reason. I still own an old Unison Research S4 which is not without merit, but ultimately lacks some low end control & my current amp outperforms it by some margin without heating the flat. Many years ago - just after CD started to become mass market, a mate of mine showed me his, to my eyes, completely bizarre system. TT was some budget Dual or Garrard. The amp was a Leak Stereo 20 played through what looked like a pair of bronze radiators (yup ESL 57's). It was a really great sound and the memory has stayed with me. Best of all he found the whole lot in a skip!!! Let's not forget that some of Grahams phono stages have been praised for their 'valve like' presentation but with solid state convenience. I've never actually heard a valve phono stage. An interesting experiment anyway, even if it's not intended to be brought to market ------------- Dave Michell Gorbe + HR PSU -> Cadenza Bronze -> SME V -> Elevator -> Accession -> Proprius -> B&W CM6 s2 | Cusat 50 & Spatia cables ->
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Posted By: CageyH
Date Posted: 30 Jan 2016 at 4:39pm
Both my pre and power amps have valves in them, but neither are exposed. I would not buy an exposed valve audio product with a 7 month old daughter in the house. ------------- Kevin European loan coordinator, based near Toulouse, France. |
Posted By: Ash
Date Posted: 30 Jan 2016 at 4:59pm
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I would not buy any audio product that gets really hot, due to safety/fire concerns. Not the sort of thing I'd be happy to leave switched on all the time then leave unattended in the house when I go to work. Majestic/Proprius oscillating warmth in a cool place is as hot as I'm happy to go. Anything hotter, in my personal opinion, is poorly designed electronics.
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Posted By: CageyH
Date Posted: 30 Jan 2016 at 5:08pm
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You don't like "pure" class A then Ash? ------------- Kevin European loan coordinator, based near Toulouse, France. |
Posted By: BackinBlack
Date Posted: 30 Jan 2016 at 5:25pm
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This will be an interesting exercise. There is no question that good (sounding) valve pre and power amplifiers exist; but I would be surprised if they do not all use the valves within their original design parameters, particularly the HT voltages required by the designer. The success of such amplifiers relies heavily on well designed power supplies and for power amplifiers also well designed and built transformers. As all of us here know, this almost inevitably means increased cost for components and certification (assuming they want to remain legal) and weighty transformers for power amps. There is also inevitably concern over the safety of devices using potentially lethal voltages. It seems that in an attempt to dispense with these difficult and expensive aspects some "clever" designers have chosen to go with standard cheap low voltage "HT" power supplies, with varying degrees of success. If I understand correctly what Graham wants to investigate is the practicality of the design and performance of valves in unorthodox circuits using low "HT" voltages, where the valves are outside their design operating envelopes. If successful then I can fully understand that this could open up a new market sector for GSP. I'm keeping and open mind. ------------- Just listen, if it sounds good to you, enjoy it. |
Posted By: Ash
Date Posted: 30 Jan 2016 at 5:34pm
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I only like what sounds good, isn't a hazard and isn't physically supermassive. I don't understand why anyone would think that electronics should sound best when operating hot and generating lots of heat. What's the rationale?? I'll never understand the world of hi-fi. I have always been very careful with whose judgment I decide to trust. Spend long enough researching a subject and reasoning with a logical mind and eventually it becomes clear who is a genuine expert in their field of research and who is simply pretending to be an expert, who only has the right answers because you're asking the wrong questions.
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Posted By: Drewan77
Date Posted: 30 Jan 2016 at 5:35pm
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Apart from cannibalising my Dads' old valve radiogram to make a rudimentary separates setup when I was a student, I have not had any popping, banging or glowing in my system since. I was quite fond of the smell though. I recall being told that valves performance 'improves' and then gradually declines over time whereas solid state tends to have a much longer period of stability and 'lasts longer'. Is that so? There's also the issue of tube rolling, another area of audiophile nervosa for us all to fret over whilst searching for the holy grail. In spite of all that, if Graham eventually puts his name to something, I'm sure it will be quite exceptional. ------------- Older than I once was, younger than I'll be ............................. Andrew |
Posted By: morris_minor
Date Posted: 30 Jan 2016 at 5:47pm
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I remember the smell too, Andrew! And the issue of tube rolling is akin to cartridge swapping. But I'd be tempted to stick with whatever valves a GSP unit came with. If they're good enough for Graham . . . ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: JamesD
Date Posted: 30 Jan 2016 at 6:21pm
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Perhaps I am over thinking things but we humans are used to seeing, hearing, smelling and feeling the things around us. I think there will always be people (myself being one) that likes to see and perhaps hear what makes something work. Steam engines, car engines, spinning records, reel to reel tape recorders, etc. Being able to see something working perhaps enables suitably inclined people to feel more involved or attached. The same with valves I guess. In a few years when we all drive electric cars the petrol engine will be seen as a redundant, noisy, polluting, inefficient way of propelling a car but there will always be people who enjoy the sound of a nicely tuned engine and a polished lump of metal under the bonnet. By comparison audio electronics is pretty much a black, invisible art which can only be heard working rather than be seen working. Just my opinion! ------------- Aren't ears brilliant |
Posted By: DaveG
Date Posted: 30 Jan 2016 at 6:45pm
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I agree completely. We don't just hear with our ears, the whole thing is processed through our brains and is informed by our opinions, expectations, physical state, emotions & memories. This is why everyones take is slightly different. People tend to have an attachment to all those things you listed. It may may be just sentiment but they are somehow evocative of, I dunno, a simpler, more understandable world. I read a thread (linked in the mains cable topic) where someone suggested listening is not subjective at all, but intuitive. I tend to agree. It perhaps take an experienced/ trained professional ear, rather than a so called golden one to listen subjectively. ------------- Dave Michell Gorbe + HR PSU -> Cadenza Bronze -> SME V -> Elevator -> Accession -> Proprius -> B&W CM6 s2 | Cusat 50 & Spatia cables ->
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Posted By: Ash
Date Posted: 30 Jan 2016 at 7:11pm
| For me, in some ways, the less equipment I have to plug in, the better. I don't get attached to audiophile equipment unless it serves a purpose; I can still enjoy music without any of it and see much of the hobby as a materialistic distraction/obsession. However, my current equipment selections do allow me to further enjoy the music in a way that I could not otherwise do. I was enjoying music long before owning any hi-fi devices but my current system brings the music back to what it should have sounded like before mediocre computer electronics got in the way. |
Posted By: discrete badger
Date Posted: 30 Jan 2016 at 7:55pm
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Pure Class A (the circuit classification, as opposed to "marketing department Class A") intrinsically produces a lot of heat. This doesn't mean it's poorly designed, it's just a characteristic of the high bias in the circuit. It certainly solves - completely - the problem of crossover distortion. At the cost - as is so often the case - of introducing other problems. For some people, the balance of benefits versus costs will make it their choice. Personally, it's not for me. I prefer my Class A amps to be very small and low power and act as correction to bigger amplifiers in the same case. Then you get the benefit but save most of the costs. That's a good point JamesD. I think it's a question of admiration for the object itself versus enjoying the utility it creates. Steam trains are a good example - I think they're wonderful machines, far more inspiring and majestic than (say) a modern ElectroStar. Seeing, smelling, and experiencing them at a heritage railway on the weekend is a great thing. But I certainly know which one I'd want to be hauling me to work Monday to Friday. For me, personally, the primary value in hi-fi is the sound quality and the connection it brings to the music; that's to say I only care about the utility. The admiration for the machines themselves is secondary. I want the ElectroStar not the A4. I accept that for many people it's different - about the admiration of the kit as well as what it can do. Now it just turns out that many of the boxes I've found myself enjoying the music with have some very clever thinking inside them, and I can also very much admire some of the ingenious thinking that has gone into them. For example, the point source concentric electrodes in the ESLs, the impedance correction in the Solo UL etc.
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Posted By: Graham Slee
Date Posted: 30 Jan 2016 at 8:33pm
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Well, 4 pages in less than 30 hours must be close to the record on here! Or it may have even beaten it. Such then is the popularity of valves! It shows most have a soft spot for the old beasties even if it's a misplaced one... Having used lots of valved gear early in my career I am glad solid-state done properly lifted the veil off all that musically interesting stuff I'd have never heard otherwise! But I also like the old world engineering which paved the way for (the few) better things today. I will not be running valves outside their designed operating parameters. I will (if my experiments work...) be running the valves shown, at their lowest designed plate voltages, and in doing so there will be no lethal voltages. To do that will require solid-state techniques - for example: the plates (anodes) will not be aware that they're not driving a resistive load connected to HT, because they are unable to think! However, let's be simplistic about this and say if they were able to think, they'd think they were being utilised in a normal (for valves) way. Valves produce their warmth because of even order harmonics. This was best described by Marcus Graham Scroggie under his pen name Cathode Ray in Wireless World some years ago, and I will attempt to repoduce the entire article in this thread at some point. I will be amazed if the valves used in this experiment do the same! Why? Because if my experiment succeeds they will not produce the nice tubey flattened bottom (or top depending on phase) which causes those even-order harmonics. I am looking for neutrality. So why muck about with valves when it can be done with solid-state (as I may have shown...)? I suppose it's because so many people are attracted to that warming glow of bottles stuck on the top of equipment. I call them sooners: they'd sooner see valves than hear how real music can sound. And here I may have a problem. My research so far shows the nicest sounding valves I can obtain (not NOS - they need to be in production) have their filaments completely enclosed making it nigh on impossible to see their glow. But, here's the trick many employ: a LED nicely fits in the valve base, and I have an excess stock of orange LEDs that should outlast me.... So yes, it is mainly a commercial experiment.  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: shashirao
Date Posted: 31 Jan 2016 at 12:18am
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The LED idea sounds like those television channels that show the yule log burning...warming the hearts and homes of many during the holidays. Alas I live in Minnesota...we get real winter...and if it gets cold...or if we get buried under 20 foot of snow...I am dusting off and firing up my vaccum power amp and bask in its warmth and glow :) But seriously I think we have a great thing going with the solid state world and hope we don't get overtly distracted with tubing :)
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Posted By: Graham Slee
Date Posted: 31 Jan 2016 at 2:11am
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The promised (38 year old) article by M G Scroggie aka Cathode Ray can be downloaded here: (each page is over 1 MB!) https://www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-1.pdf - www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-1.pdf https://www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-2.pdf - www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-2.pdf https://www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-3.pdf - www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-3.pdf https://www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-4.pdf - www.hifisystemcomponents.com/downloads/articles/negative-feedback-and-non-linearity-4.pdf The pages were taken on my camera as my Adobe program and my Brother printer/copier decided not to talk (all computers are *astards). The article is from Wireless World October 1978, but is still as valid today. Most will find the article heavy going because it is rather technical, and to fully understand it will require foundation level maths, but by skim-reading to the graphs which show asymmetrical sine waves, the reader will receive at least some education into what even-order harmonics is about - what valves without solid-state assistance do in abundance! What I do with valves is in my spare time, and although it could have a commercial outcome, at present it is to satisfy my own curiosity. Like many of you who might think valves offer something "special", although my distant memories of valves suggest the opposite, I would like to find just how much can be achieved. The current state of solid-state offerings (mine excepted) might be such that valves do in-fact sound better. And as the younger electronics design engineer tends to be digital-first -- analogue last, it might be because of that. Some of us however, are older, but the customer might not be able to differentiate solid state designed by the experienced against solid-state designed by the inexperienced. Therefore the experienced (me) might have to offer tubes/valves to be accepted by the public at large. I don't mind designing all the things people suggest to me if there is some payback, but to date I am only considered good for phono stages and headphone amps. After spending most of my adult life doing far more than that (for example: understanding the application of the above mentioned article in all kinds of audio circuitry) I am getting rather *issed off by being passed by on the other things we make. And if putting valves in things gets us more noticed, I will therefore do so. On the other hand I could be talking sh*t ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: shashirao
Date Posted: 31 Jan 2016 at 4:43am
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One of the terms I consistently come across when people (listeners) describe the sound of tube-based equipment - 'warm' is still the most commonly used term. Warm analog sound seems to the aspect that attracts many to tube based equipment. Of course I am making a broad assumption here. So my own example. Before the Revelation C or any Graham Slee products, I owned (and still do) all tube based amplifiers. Pro-Project Tubebox as the pre-amp. And Yaqin power amp with EL-34 tubes for amplification. Life was all fine and warm. However with the Revelation, Reflex loaner and now the Accession, I am not missing the "warm analog sound". In fact this is as analog as it gets. Since most of my listening material is from pre 90s, it is predominantly acoustic instruments with some "recent innovations in electronic sounds". And with these products I am getting only closer to hearing the real acoustic instrument being played live in my room. And thus...if the future product involves tube...I am quite confident it will be based on sound sound decisions/design. And after I acquire the Proprius and the Majestic...I will eagerly queue up for anything new :)
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Posted By: morris_minor
Date Posted: 31 Jan 2016 at 8:43am
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Maybe the appeal of a "warm" sound is as an antidote to the "cold", sterile digital sound put out by a lot of devices? Also, I like the idea of a little orange LED providing the valve glow! Maybe, Graham, you should save yourself a lot of work, and put fake tubes on top of a modified Proprius case with suitable LED illumination? Double the price, and watch the orders roll in . . .
 ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Graham Slee
Date Posted: 31 Jan 2016 at 10:28am
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The "working design" of the Proprius originally had a dome in the top which glowed and changed colour but I thought it too corny. I think somebody mentioned an ADC? The trouble with that is we're a one-man designer outfit, and me, the one-man designer, doesn't do everything - I don't do PIC programming and wouldn't have a clue where to start, plus it's something I don't intend to spend my latter years busting my brain with. Companies do that sort of thing by bringing in or sub-contracting design, or just buying what Shenzhen has for sale. I therefore make what I can mentally absorb, and for USB (which is all people want these days) it is a Burr Brown USB CODEC, and they only do actual CODECs up to 48kHz. I have already found that the Bitzie and Majestic don't sell, and that is because the USB input only does 48kHz, and so I refuse to waste my precious years designing another loss maker. The phono preamps and headphone amps were the result of what I did for fun. Things I designed to obtain outright musical performance for myself, and so is the Proprius. I'm pleased as punch that so many have enjoyed listening with them and have served a good purpose. The Lord sayeth whatever you make, make it for a fit and proper purpose, which means don't fanny about! I think placing valves on equipment, and some don't actually do anything, is fannying about at the customers expense if they don't serve a proper purpose. If they don't bring anything better to the table then somebody is taking the piss and you're paying to have it taken out of you. I also think that lighting up valves with LEDs to pretend they're glowing is also taking the piss. But, it seems that is what people want, and to cater for people's wants I therefore must also take the piss out of the customer?? Maybe I also need to start advertising amplifier power in PMPO? It might attract the younger generation? For those who don't understand PMPO, it is a completely false assumption of power that is either made up by unscrupulous marketing departments, or still as falsley calculated as being the short circuit current of the power supply multiplied by its output voltage: the short circuit current can be tens if not hundreds of amps depending on the imagination of the company marketing department, and multiplied by even the most modest of voltages (3.3V or 5V on computational type devices) results in hundreds of watts! As Paul Daniels would say "That's magic!" but the younger generation fall for it all the time (except Ash). (by the way, the short circuit current multiplied by the output voltage gives zero watts! Why? because there is no voltage if it is short circuited!!! Unfortunately most people don't use the "free program" that came with their brain called common sense) I can also remember how to make a light show. I had to be able to do so because music alone was too boring for the younger generation of my youth also. Instead of the gently changing colours some "hi-fi" emits I could do a sound to light?? Instead I have made the decision to stick with analogue! I know at least two people out there who would love me to do an analogue audio preamp of the "old school", and I have discovered that some in my generation and above still want to spend on such sensible things. As for the valves, I will find if they offer anything new. If I find they are just a diversion down a blind alley - which is what I suspect - then they'll be going in the bin... Or maybe I just use them as decoration to grab more customers?? And qualify their use by lying? ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Ash
Date Posted: 31 Jan 2016 at 11:17am
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You don't need a particularly powerful (in terms of Joules per second) amplifier to fill even a moderately large room with space-filling music, as long as the speakers are up to the job. I played around with a pair of 2W speakers a few years ago and they went up really quite loud in the domestic environment... O_O As Graham has said already, dB isn't a linear scale either; it's logarithmic. It's quite easy to reach uncomfortable listening volume. I personally don't like very high volumes, especially with speakers, as it just increases adverse room interactions IMO, messing up coherence and frequency balance, as well as disturbing everyone else with the noise. So yeah, I could get myself a dedicated listening room with full acoustic treatment (somehow) but it's easier to listen to a well designed/implemented open baffle speaker pair in the nearfield at moderate volume, where almost all the audible room interaction is gone, the dB SPLs for the listening distance is adequate enough to hear bass all the way down low and where I get a more spatial full-body experience than just having K1000 drivers sat in front of my ears. 2000 Watt speakers or amplifiers?? So how much of that is actually usable or even remotely significant?? Wow, they must think I'm an idiot. I'm many things but not an idiot. Trust nothing except your own experimentation, a sensible electronics engineer like Graham and a little self-reflection.
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Posted By: JamesD
Date Posted: 31 Jan 2016 at 6:47pm
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When I saw Graham at his roadshow I believed him to be a human rather than a freakishly intelligent wild animal with the ability to speak English and walk convincingly on his back legs... As a result he is cursed with the need for the necessary evil called money to keep him in the manner to which he has become accustomed and to allow him to pay his loyal employees. Provided he isn't making false claims about anything I can't see the problem with him making products that will sell even if he doesn't particularly think that they are the best route for audio perfection. If the rewards from selling these items allow him to further develop his top of the line products and perhaps gets his name out there then all the better in my humble opinion. Despite recent events I'm still hoping that one day I'll be able to afford his DAC and amps. They'll probably last as long as I do. Which is good in one respect, but perhaps not so good for GSP sales figures... ------------- Aren't ears brilliant |
Posted By: Graham Slee
Date Posted: 31 Jan 2016 at 8:47pm
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One of the things I've realised about valves is the complete lack of support in my simulation software (Number One Systems Easy-SPICE). However, after much thought I realised the "amplifier sub-circuit component" could be married-up with the available sub-circuit SPICE models, and whilst they look nothing like a valve -- and can't be edited to look like them -- it seems the simulations work. This is important to see the actual operating characteristics, otherwise it's just dabbling for dabbling's sake. It is also hard to trust in such simulations until you actually build something and measure its performance to correlate simulation findings. Without being able to simulate it will be an absolute waste of effort. Interestingly though the 12AX7 designed for a minimum HT of 90V shows useful gain operated on 48V. Another point of interest is that without grid stopper resistor the phase margin is vastly reduced confirming what people find reporting a very bright sound. So the model might be actually doing its job. We'll see. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 01 Feb 2016 at 9:44am
Love that bit  Being born into privilege does have its advantages. And I know if Grandad hadn't moved from his Stavely coal board house to take up residence in a semi in Thurnscoe where he became a pit under-manager, and had my Mother not worked in service for some market stall traders in Sheffield during WWII etc etc, I would not have had the chance by the age of 60 to work my way up to owning a detached overlooking the golf course car park, back of the driving range, and some chicken sheds. Anyway, back to these valve thingy's: I am now going to see if I can utilse the double triode 12AX7 in a differential amplifier of some sort or another. The problem with valves in today's world is their output impedance. Power valves required a matching transformer to be able to drive low impedance speakers because their plate impedances were so high. Preamp valves have even-higher output impedances as can be seen on this data sheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AX7.pdf The table on page 2 shows Rp values of 100k and higher. Obviously that isn't going to drive any of today's solid-state inputs without considerable insertion loss. And this is why so many valve products are in fact hybrids which use solid-state output devices of one kind or another to be able to have the low output impedance required. It should make us all laugh out loud at those who belittle solid-state whilst listening to their solid-state assisted valved equipment, completely unaware that it is partially solid-state. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 02 Feb 2016 at 7:13am
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CONCLUSION Yes, even before putting them into a circuit I have come to my conclusion for what it's worth. Having checked the simulation model against the data and the simulation returning the same information I think it is obvious that the SPICE simulation is telling it as it is. For the safe supply voltage of 48 volts the plate voltage will be obviously lower, and to obtain linearity the plate voltage would have to be approximately half supply voltage to obtain a symmetrical signal swing. 1. The calculated cathode voltage required to bias the valve properly on this supply is quite low at 0.5VDC which tallies with the valve data. As such the ac voltage which can be accepted at the grid will be the inverse square root of 2 by half that voltage which is 175mV. This is the maximum allowable input voltage before clipping. Absolutely of no use for most signal levels. 2. The plate resistance required to obtain the signal swing is approximately 100 kilohms and that is the output impedance. It is absolutely of no use at all in driving any other equipment. 3. Without negative feedback the distortion is approximated at greater than 1% (possibly nearer 5%), so not even worth switching on the analyser. 4. The open loop gain is approximately 28dB which for a gain goal of 8dB (for a preamp) allows just 20dB of negative feedback, which is 20dB short of Bell Laboratories Harold Black's 40dB (1927) which still holds true today. 5. Therefore applying negative feedback becomes empirical which is another word for messing about. 6. The resulting reduction in distortion would be quite little and not reach my 0.1% target. 7. The minimum HT voltage set by the valve manufacturers (90V) is obviously there for a reason, and that reason being that it allows for an "acceptable" distortion figure, but definitely not high fidelity. 8. For the valve to be able to perform to near high fidelity standards the HT voltage would need to be near its maximum of 300 volts, which increases costs significantly. 9. The necessary solid-state support circuitry required to make the valve "work", which means lowering output impedance to be compatible with todays amplifiers, and to reduce distortion to acceptable levels, is just as complex as the sold-state circuitry which can do the same job without a valve being in sight! Therefore why bother? This experiment, cut short for the common sense resons set out above, conclusively shows (IMO) that all low voltage circuits utilising the common preamp valves of which there are many very similar types, will result in unacceptable levels of harmonic distortion. Also that any of these valve circuits running at voltages below 48 volts, of which there are many, often running on much lower voltages such as 12 volt wall-warts (although many specify just the wall-wart mains supply voltage possibly to mislead the customer), will provide a considerable amount of distortion. That those devices are not high fidelity by nature of their distortion. That those who find satisfaction using such devices must therefore prefer to hear the music with the addition of the effect known as fuzz. That any device utilising such valves operating on low voltage supplies and claiming high fidelity distortion levels cannot be actually using the valves, and therefore they are there for decoration, and deceit. Given the results of this research (albeit short lived) the customer reading this thread has at least been informed. NEXT! ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BackinBlack
Date Posted: 02 Feb 2016 at 8:50am
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I have no direct experience, not having examined or dismantled a modern boutique valve preamp, but, I suspect that some of the valve products may use a 12v AC wall wart, then a step up transformer to provide the necessary HT and heater voltages. At the power levels required these transformers could be quite small. I presume this circumvents some of the regulations relating to AC mains powered devices and the testing/certification requirements. ------------- Just listen, if it sounds good to you, enjoy it. |
Posted By: morris_minor
Date Posted: 02 Feb 2016 at 10:13am
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For ease of reading, Graham's posts in this thread have been edited together as http://www.blog-gspaudio.co.uk/valves-on-a-graham-slee-product/ - this Blog post . ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: BAK
Date Posted: 02 Feb 2016 at 5:03pm
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A proper "valve job" has been demonstrated by Graham in this post. He has given us many of the reasons why the electronics industry has evolved from valves, to transistors, to hybrid multi-transistor packages, and to integrated circuit operational amplifiers. (I have worked on them all.) What the electronics industry is working on for the future is to bypass electrical energy all-together... to processing signals in the light energy realm only. Light energy transmitters and receivers are already here with fiber optics; and we have light energy data storage on CDs and DVDs; presently all of these need electrical transformations. The light energy amplifiers are close at hand, some are in the prototype stages. Now, how do we make light energy transform into sound without electrical energy? That is a very big hurdle. ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: Graham Slee
Date Posted: 02 Feb 2016 at 11:45pm
Among the implementing measures applicable to CE marking (without which the free movement of goods within the EC is prohibited) is the low-voltage directive (LVD). This sets an upper voltage limit within equipment of 75V (ac or DC) after which the product must meet one of the safety standards for mains powered products. The same now applies in many US states, Canada, most of Asia and New Zealand, and some countries insist on certification. But in this thread I've been mainly referring to low voltage supplied valve circuits, and how they can be implemented to give acceptable measured results. The problem is that for linearity and voltage gain, valves require DC voltages beyond those considered safe without the need of certification. My interest has been in producing a preamp gain stage to give a relatively gentle boost in the region of 8dB (x2.5) using the type of valves made for preamplification, of which the 12AX7, ECC83 and equivalent types were/are made to do so. These are often used in add on buffer stages and images of the insides of such products are often posted online by owners. It can be seen from capacitor voltage ratings that the supply voltages are often in the region of 12VDC, and such circuits are referred to as plate starved. Because of the quite high output impedances of such valves their outputs are of no use unless buffered by solid-state circuitry, which reduces output impedance suitable to drive other solid-state equipment. People often buy these items because the advertising leads them to believe there will be an improvement because of the valves(s), but regularly find the distortion they produce to be unacceptable. Since my conclusion I have had a go at further simulation using bipolar transistor output buffering of the said valve types, and through bootstrapping have managed to increase voltage gain marginally. It may be that with around the 30dB of negative feedback, and improved linearity that the transistor output stage makes possible, that it reduces distortion sufficiently to make this intermediate (48 volts DC) voltage idea viable. However, as I may have said, making up the circuit without sufficient evidence that it will stand a chance of having reasonably low distortion would be to waste one's efforts and materials. But the point is that without the support of solid-state electronics the said valves do not stand a chance of bringing anything to the table worth having. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 02 Feb 2016 at 11:48pm
Reminds me of an episode of The Avengers (1960's/70's British TV drama). ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 03 Feb 2016 at 12:45am
OK, dare me to build it...  But where's the valve? My simulator won't do valves but the amplifier subcircuit (the "triangle") will use the 12AX7 SPICE model I managed to find on Github. Other models stated minimum voltages. The Q1 BJT (solid-state) buffer has just below unity gain and bootstraps the 12AX7 anode load (R8,9) via C9. This improves open-loop gain by around 12dB. For it to drive this plus the output (indicated by R3) and the NFB via C10, it is given a constant current emitter load (Q2,3, R10.11) to ensure output linearity. A summing amplifier configuration is employed to rid it of common mode distortion (hopefully), and gain is approx. R12/R6. The valve terminals are: grid - pin 2; cathode - pin 3; anode (plate) - pin 1. R5 is grid stopper resistor which brings the GBW product down at a "gentle pace" and in doing so provides a generous 108 degree phase margin along with 30dB gain margin (see below).  Flat from 10Hz to 100kHz.  Plot showing open versus closed loop gain. Nearly 30dB of loop gain/negative feedback. Might be sufficient for low distortion?? Heater circuit and decoupling omitted as it wasn't necessary to be able to model it. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BAK
Date Posted: 03 Feb 2016 at 4:09am
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I see in the 2nd graph... the violet trace is the closed loop gain curve giving 8 dB gain above the zero line. The 30dB gain margin between the violet and green curves, for others to understand, would be the "headroom" built into the design. The 108 degree phase margin is needed to make the circuit very stable (not become an oscillator); the phase margin helps reduce distortion. How low will be the IMD and THD? the spice model can only approximate the result. Graham, if you want to try this, go for it. A three or four transistor may suffice to give better specs... your choice. I know you have many design resources to draw from. PS: I still have working valve amplifiers that I built 45 years ago, but these were not hifi, just 1 for an intercom and another was my 1st 50c5 power amp (1.5 watt) I built at age 16. (Of course, these amps have only been turned on about 2 times in the past 20 years.) ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: CageyH
Date Posted: 03 Feb 2016 at 12:42pm
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It's interesting to note that my "Class A" tube based preamp has manufacturers claims for THD of 0.06%. This uses ECC88s though, which I changed to E88CCs as they are of better quality. ------------- Kevin European loan coordinator, based near Toulouse, France. |
Posted By: Graham Slee
Date Posted: 04 Feb 2016 at 12:56pm
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Prototype: It isn't even wired yet! Just thought I'd show you as it is now, because it might not look this good again (because it may have gone in the scrap bin!)  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BAK
Date Posted: 04 Feb 2016 at 1:41pm
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Simply beautiful ! ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: BackinBlack
Date Posted: 04 Feb 2016 at 7:33pm
Now all you need is one of these: and a few of these:  Nice quiet power supplies. No certification problems, although ROHS might take a different view considering the heavy metals etc, then there's the sulphuric acid and the hydrogen released when recharging. Ah well back to the wall warts. Sorry just couldn't resist, I'm truly interested in how this investigation develops. I'm not a particular fan of valve amplification, but keep an open mind. ------------- Just listen, if it sounds good to you, enjoy it. |
Posted By: Graham Slee
Date Posted: 04 Feb 2016 at 10:13pm
How they used to hard wire valve sets in production beats me. It's taken half a day (with interruptions) to part wire it. Will try and complete the component wiring tommorow. It doesn't help having to wire solid state devices to thin air.  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 05 Feb 2016 at 6:57pm
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Didn't manage to build the entire thing but had a play with one valve section only. Will need to use a trimmer in the cathode to get the bias spot on. With a ridiculously low plate voltage of around 12 volts I measured 0.5% THD, 16dB gain and 65dB S/N. More to come (maybe). ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 07 Feb 2016 at 3:54am
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A great resource for valve design can be found here: http://www.valvewizard.co.uk/Common_Gain_Stage.pdf It has taught me through design exercises that using low B voltages (48-50V in my case) that there is little to be gained from the use of valves unless used at the voltages they are designed for - and here we are talking about greater than 200 volts HT. I am sure I can utilise the 12AX7 high-mu valve on 48-50V as a voltage amplifier, but getting the low distortion high fidelity circuits should have, is another story. Although HT supplies are quite high the signal swings obtainable with good linearity are quite small. This is very old technology after all. And after all my years of solid-state design, my adventure into valves thus far says to me that valve technology is about using a sledgehammer to crack a nut! Linearity therefore suffers as supply voltage is decreased, and although my badly biased 12AX7 voltage amplifier achieved 0.5% THD (at 1kHz), it might be able to be improved upon through biasing the plate voltage to 2/3rds the supply voltage (32 volts at the anode). The stage alone should be able to swing 11 volts rms, but this is without a load and as such academic. With an IEC 1938 (DIN standard) load of 22k the maximum rms voltage is reduced to 4 volts. Here we are using a 50 volt supply, a £10 valve, and a considerable amount of hardware "real estate" when compared to a 18 volt supply, a 50p op-amp and an offcut of vero/strip board which severely out-performs the valve, for a similar component count. However, the public's impression of valves is the complete opposite of the truth, and valves are a money magnet as a number of business concerns have discovered. There are hundreds of thousands of prospective buyers of hifi gear each year. There are hundreds of businesses there to satisfy that want. Google Adwords keyword checker makes it plain that most of those people are searching for valve/tube hi-fi! Valves aren't magic but the public has been led to believe they are, and would rather spend their money on any valve product over and above the solid-state equivalent. Search results are important for a business to survive, and no amount of search engine optimisation is able to compete with the "magic of the tube". As I have explained (albeit without any mathematical or other proof) that low voltage valve circuits are exrtremely poor at high fidelity, the sales of such equipment are incredible. Search engine results put valve equipment manufacturers way higher than their solid-state counterparts -- simply because more people search for tube gear than solid-state. The popularity of tubes is sky high. It is impossible for any small manufacturer of solid-state high fidelity products to attract anywhere near such a following no matter how good the products are. Emerson (the philosopher) was wrong in saying if a man invents a better mousetrap, the whole world will beat a path to his door. Now, should the mousetrap have incorporated a valve, then he would have been right. The 12 volt powered valve circuits do distort badly and can't swing much of a signal. You might not agree but I've proved it to myself and for me that's all that matters. But they sell! And people blame the products being used with such highly distorted valve products for the distortion. I should know. My products often get the blame. A recent customer was going to return a Solo UL and keep the valve line "conditioner" because he could not imagine the 12V valve product as being at fault. It took quite a lot of persuasion to get him to listen without the valve conditioner, and even then his brainwashing took a full week to be undone, before his own ear-brain interface and common sense took over. Such is the power of the valve over the human mind. Therefore, whether the valve is any good at all, I do have to use it. I have to try harder to make it deliver some resemblance of good sound. It is the only thing stood between our company and the success it deserves. We have to offer a valved product, but for what reason? Is it to sell valved products? Does it mean turning my back on all the great solid state designs I've done? No! It is simply to allow us to reappear on the "radar" -- to be seen on search engine results. When people come to our websites people buy. But if they never visit...?? So "adversary" = valves, and I know the meaning of adversary, and the devil is in the detail. The customer has been led into temptation and there is nothing for it than to assist in that temptation. Hopefully next post will be more about the results I'm getting, and hopefully it will lead to a good sounding product. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: morris_minor
Date Posted: 07 Feb 2016 at 4:03pm
I've had a look at the specs of a few valve amps, and distortion figures are often vague or missing completely. Those that are published properly are generally worse than solid state amps. It seems that folk buy solid state amps based on specs, but valve amps based on, er, nothing much at all other than the glow. (Or am I missing something here? Maybe people just like the sound of distortion?)
 ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Drewan77
Date Posted: 07 Feb 2016 at 4:19pm
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I have attended a few Northern 'bake-offs' with hi-fi buffs over the years and there seems to be a lot of snobbery around this topic - people drooling & dribbling over valve equipment & talking a language the rest of us seem to be excluded from. Having said that, one of the guys lives 4 miles from me and his huge horn speakers and 'valve-everything' does produce some of the most enjoyable music I have heard. Most others are not to my taste though - overly warm sounding, lacking in dynamics & unengaging. I don't get it. ------------- Older than I once was, younger than I'll be ............................. Andrew |
Posted By: Graham Slee
Date Posted: 07 Feb 2016 at 4:43pm
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I don't either... My ISO-TECH IDM72 multimeter from RS Components decided not to autorange and shorted the plate to ground, but I didn't notice it was doing that until it shorted the second plate to ground with a loud arcing crack! I tested the meter on the 48 volt rail too, and it wanted to short out the power supply! Thank you RS Components!!! They've been going down the pan for years IMO. No matter what variation in cathode current neither valve will play ball anymore. The moral of the story? Don't buy British Chinese -- buy Chinese Chinese!!! Chinese-Chinese works because that's the way the Chinese want it, and to snatch the sales they obviously need to make British-Chinese gear fail. Anyway, RS wins no matter how the coin lands, as they are the only quick way of obtaining replacement valves... which their meter just welded up!  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Ash
Date Posted: 07 Feb 2016 at 5:55pm
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Valves ultimately wear out in the short-term and need to be replaced, they heat up, their use introduces distortion, they take up a lot more space, are more fragile than solid-state components, need associated SS components anyway and require dangerous voltages to work best. Looks like common sense isn't very common. There are too many delusional pseudo-educated people looking towards more delusional pseudo-educated people. There's no consistency either so no point in being polite about it.
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Posted By: Richardl60
Date Posted: 07 Feb 2016 at 5:55pm
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It is interesting to see Graham's take with valves. I have never owned any valve equipment and out of concerns over longevity and consistency probably never would. Other than a valve amp being fed by Era gold V & Elevator when I bought my first GSP products I have had no experience with any valve gear for over 25 years. Demos and shows back in the mid to late 80s heard some great sounds from Audio Research pre & power amps which did appear exceptionally good, at a price. I used to believe that valve gear tended to produce higher than transistor measured distortions but in use this was not an audible feature and from my understanding if we'll designed and executed would use Terms such as sweet sounding, natural and open as descriptive words. Whilst not up to date with current valve gear on sale and from comments here appears that perhaps there is a 'valve' is everything mentality leading to cheap, poorly designed and executed models which like most things give the genre a bad name to what may be a costly and very high end market? If will be very interesting to see how Graham's project develops but dont think I would be in the market for one even if one did ever progress to production. Richard |
Posted By: ServerBaboon
Date Posted: 07 Feb 2016 at 7:21pm
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Over the years I have heard both good and bad valve amplifiers ( and Trannnies ) and have found most of the time it is often the implementation that is the important bit. Will add that the better sounding valve amps were always the life endangering high voltages ones not 48V ones. ------------- Steve ------------- Various bits of GSP Kit ..well two so far, unless you count the cables that is. |
Posted By: ServerBaboon
Date Posted: 07 Feb 2016 at 7:26pm
..oh and very expensive. ------------- Steve ------------- Various bits of GSP Kit ..well two so far, unless you count the cables that is. |
Posted By: Graham Slee
Date Posted: 07 Feb 2016 at 8:51pm
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As M G Scroggie AKA Cathode Ray pointed out in his article about negative feedback and non-linearity, it is the benign even-order second harmonic distortion which valves produce in abundance which is "inaudible". I say inaudible in quotation marks because although it is audible, it is only like that to a trained ear. It doesn't sound like distortion - it sounds kind of nice. However, it is not high fidelity. It is an addition to the music being reproduced, so what you hear isn't the truth, and fidelity in this context means exactness. You can't have the valve sound without valve distortion. If it sounds very valvey it has that distortion. So how can we arrive at 0.06%? Quite easy if you remove the second harmonic! ------------------------ Funny how these buggered 12AX7s now seem to be working... So I decided I may as well listen to the darned heap. All I can say is there must be a lot of sh*t solid-state if this sounds better. Anyway, try it for yourself if you want...  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: ServerBaboon
Date Posted: 07 Feb 2016 at 10:10pm
Having been to several HiFi Shows in my life I can confirm there is a lot, and when you find a a brand you like you then perhaps you should explore it though they may not always be on demo at HiFi shows.------------- Steve ------------- Various bits of GSP Kit ..well two so far, unless you count the cables that is. |
Posted By: Graham Slee
Date Posted: 08 Feb 2016 at 10:36pm
However, once the grid current was increased and a solid-state current source was added to each plate, the following miracle happened... I managed to get roughly the gain I set out to obtain. This being close to 6dB (x2) and probably suitable for a preamp. All measurements shown are into a 200 kilohm load on the AP.  The frequency response is a little curtailed being -3dB at roughly 27kHz. Possibly the grid stopper resistor is a bit too large.  Now, the distortion doesn't look all that pretty but this is a 12AX7 valve after all (from 1947!). And virtually all of this is second harmonic, which the ear finds acceptable and even nice. It is at rated output: 0dBu = 775mV. It will go to +20dBu (7.75 volts rms) only increasing to 4%. I shall have to photograph the FFT trace which shows a spectrum analyser representation of the harmonics in a subsequent post.  However, a lot of music and its nuances will be at a much lower level, and here at -26dBu we beat 0.06% by a small margin: we can claim 0.05% THD at 1kHz.  But what about high frequency distortion? Here we see 0.12% THD at 10kHz - only double the 0.06% THD target. Remember, this is at 10kHz where distortion often rises rapidly.  The question often asked is "what is its intermodulation distortion". Intermodulation causes "new" or alien sounds to appear and might not be too big an issue on loud passages, but we wouldn't want the delicate sounds of instruments to be changed. Here we see it at 0.3% for a -20dBu (77.5mV) output.  By far the biggest surprise was the signal to noise ratio: at nearly -82dB (A wtd.) it is as good as many of the broadcast audio small-signal circuits I've designed in the past using op-amps.  And here's the "rat's nest" responsible. Crosstalk was amazingly decent at better than -74dB. It uses two switched mode power supplies: 48V for the "HT", and 24V for the two series wired 12V heaters (don't worry - this wouldn't be the case on a production unit). The orange LEDs aren't for show - although they'll get used to add effect in the valve bases should this ever get made into a product. No, they're part of the plate (anode) current sources and comprise: the LEDs themselves; a small signal bipolar transistor (per channel); and two resistors which set the plate (anode) current. These replace the plate resistor allowing an increase in gain and greater linearity from the 12AX7 valves. Solid-state came to the rescue and if it can reap a performance like the above from a single 12AX7 triode (per channel) running on (starved) 48V "HT" then solid-state can't be all that bad?? The only way to use this circuit and get the "benefit" of its characteristics would be to buffer it with an op-amp stage, which I did in a roundabout way - I connected its output to the Majestic line input for a listen. The input was vinyl via a Gram Amp 2 Communicator (all I could scrounge). The valve character could be made out as the addition of harmonic content (second harmonic - even-order) on louder passages, whilst it retained all the delicate detail at lower levels, and tempo was just as good as it always is. It showed no sign of straining, and delivered an unobtrusive but tuneful bass line - demonstrating bass power where needed. The valve sound isn't overbearing and I'd say, the just sub-1% THD is about the right amount to give a hint of valve sound without going silly. This therefore might be something worth taking to the next stage. Maybe it could be used for the preamp I've been looking into for quite a few years? We'll see. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: morris_minor
Date Posted: 09 Feb 2016 at 8:07am
Fascinating stuff, Graham!
------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Graham Slee
Date Posted: 09 Feb 2016 at 8:56am
Hasn't it always been like that? Somebody posted a video recently about an AP presentation on the subject of distortion - I can't remember exactly where?? And if it were possible to reduce the distortion to solid-state levels it wouldn't sound like valves... And if so, wouldn't it make more sense to buy solid-state?? But this next revelation might be of interest: There are defined ways of measuring power amps, but for preamps it is often the case of setting the signal level to get the lowest possible distortion - which I did to get 0.05%. And it is truthful.... ...but it doesn't tell you what it's doing at rated output (0dBu in my case). Or at maximum overload (which I've shown). All food for thought. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 10 Feb 2016 at 7:45am
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OK Bob, I get the picture - you expect better of me... So here it is:  Better than 0.2% THD at rated output!  And at -23dBu -- the sort of level it will be doing at 12 'o clock volume control position... Disappearingly low sub 0.02% THD!!!  But that's not all! The gain is increased too! More than the 8dB I needed.  And a much wider frequency response... nearly 55kHz. That's within 0.5dB at 20kHz!  Plus nearly 86dB signal to noise!!!  And here's the spectrum analyser plot I promised. It shows the valvey (tubey) second harmonic - the one at 2kHz on the x-axis. 1kHz is the fundamental frequency from the signal generator. The more objectionable 3rd harmonic is nearly in the noise floor. Now, all this is from a 12AX7/ECC83 preamplifier valve intended for >100V HT. But it is operating on a 48V "HT" supply. Is it plate starved? No. Plate starved usually means the grid draws lots of current often requiring positive bias. This uses the normal grid-leak biasing. However, the point is that none of this would be possible without solid-state (the constant current anode source). And for the above to be used in a product (a preamp?) requires a solid-state output stage too. But as bottles sell products, and for me to put in the effort of designing another product I'd like it to sell... therefore, and regretably, this is what's going to have to go into the preamp. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: morris_minor
Date Posted: 10 Feb 2016 at 8:28am
Er, not exactly -  - but I know the standards you set for yourself! For a technical numpty like me, can you explain simply how the valves would fit into the solid state topography needed for the product to work, please? My impression is that you need SS to "power" the valves, and SS to get the audio signal out. What, er, do the valves actually "do"? (Aside from giving some "lush" 2nd harmonic distortion!) ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Graham Slee
Date Posted: 10 Feb 2016 at 10:24am
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The valves actually amplify the signal and will do all the amplification required, which is the +8dB that a preamp would need (similar gain to the Majestic when used balanced). Looking at my sketch posted earlier it can be seen that there is a valve with its anode connected to a resistor with its top end connected to HT. The output signal is from the junction of the valve anode and that resistor. The valve pulls down on negative going cycles, and the resistor pulls up on positive going cycles. The input is going positive to make a negative going output, and vice-versa, but the valve is increasing the amplitude by 8dB - making the signal 2.5 times larger (it will do much more than that but local negative feedback is used to control the gain to what is required). Because the anode resistor is such a large value it cannot pull up sufficiently to drive a low impedance load, although the valve can pull low a bit better. By replacing the resistor with a current source the signal becomes symmetrical - it can pull up as well as pull low by the same degree. It can therefore drive an impedance similar to that of the original anode resistor, and the AP input impedance was set to 200 kilohms so as not to load it too much. For the circuit to be able to drive today's input impedances, such as the input of a power amp, it will need some form of buffer stage, and that "transforms" the high valve output impedance to the required low impedance. It takes the small current of the valve circuit and amplifies it - a buffer stage is a current amplifier. The buffer will need to invert the signal inverted by the valve to maintain absolute phase. This is best accomplished by a unity voltage gain "summing junction" op-amp circuit IMO. So, by combining a tube with SS we can have tube amplification - the valve is not there for show or to introduce distortion. However, the distortion is an order of magnitude more using a valve than it would be using an op-amp to do the same voltage gain. A solid state solution would use two op-amps arranged as "summing junctions" (summing junctions/virtual earth amplifiers have the property of reducing common-mode distortion... and sound good). The hybrid solution replaces the first op-amp with the valve (it is arranged as a summing junction). The addition of the current source to its anode makes its distortion acceptable - that, along with the bias tweaking which it also needed to obtain the above results. And used post-volume control, the input signal to the valve circuit will be in the order of 70mV if the volume is set at 12 'o clock - which it most probably will be, or just a bit under. At that input signal voltage it has been demonstrated that the distortion is not so disimilar to that of a SS preamp. At that level the harmonics are almost lost in the noise floor (the excellent 85dB S/N) so maybe you will never hear any lushness at all?? Perhaps that defeats the object for some, but I suppose knowing that there is a valve actually doing the job will be enough to make it sound better?  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: morris_minor
Date Posted: 10 Feb 2016 at 12:33pm
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Thank you Graham! I understand a lot better now . . . ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Richardl60
Date Posted: 10 Feb 2016 at 1:39pm
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Again interesting! In theory could a transistor amp be created to mimic the abbributes of a valve amp and should it sound like a valve, transistor or combination? Richard |
Posted By: morris_minor
Date Posted: 10 Feb 2016 at 4:22pm
------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Graham Slee
Date Posted: 10 Feb 2016 at 5:27pm
An op-amp (which is a configuration of several transistors) can be reduced in open-loop gain which widens its open-loop bandwidth and also because of it, causes distortion to rise. And, whilst keeping distortion within greatly acceptable reason, is what the ultra-linear technique does. This explains to us sat here in Slee workshop why the valve gizmo doesn't sound all that much different to the Majestic. The main difference however, is the differing amounts of second order harmonics which, with the valve gizmo has the effect of altering the tone of the highs - making them less pronounced. This is due to the mixing of the fundamental with its second harmonic causing a new low frequency low amplitude "alien" which masks the higher harmonics . . . if you get my meaning. We could use a single transistor to introduce the non-linearity that it requires, but a £5-£10 valve can save us the trouble. PS: provided we can get a 48 volt supply to it. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Richardl60
Date Posted: 10 Feb 2016 at 6:21pm
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Yes I like the tube distortion knob idea!!! Richard |
Posted By: morris_minor
Date Posted: 10 Feb 2016 at 7:08pm
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"Valve Gizmo" - a nice product name methinks. ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: tg [RIP]
Date Posted: 10 Feb 2016 at 9:42pm
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Given that the 12AX7 is a dual triode and only one half is being used, one imagines that one would suffice. Presumably 2 are being used from considerations of noise reduction or elimination of possible "crosstalk" or interchannel noise contamination ? Whatever, seems a rather good result given that you have elected to design with one hand tied behind your back WRT the limit on B+ voltage. ------------- Tony G |
Posted By: BAK
Date Posted: 10 Feb 2016 at 10:01pm
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Graham, have you thought of putting both halves of the same 12AX7 in parallel to increase current drive in your valve-based voltage gain stage? This would keep each 12AX7 connected to only one channel; one for left and one for right. ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: BAK
Date Posted: 10 Feb 2016 at 10:10pm
How about "Valve Booster" or "Booster Valve" or "Slee Valve" ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: Graham Slee
Date Posted: 10 Feb 2016 at 10:35pm
Seems a waste of the other triode but as you observed it leads to better crosstalk. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 10 Feb 2016 at 10:39pm
I wonder if it would simply be a case of doubling the grid current or are there other factors at work? Performance is grid current dependent. It would also double the miller capacitance which may affect the bandwidth. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 12 Feb 2016 at 4:07pm
Having tested the idea I can let you know it damages both triodes which now produces every harmonic equally. I guess the guys who post such ideas online don't have access to a ten grand analyser to show them what it does? Mind you I've seen circuits from product manufacturers who use this technique. I'd have thought they'd have pangs of guilt??? ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BAK
Date Posted: 12 Feb 2016 at 4:26pm
It was just an idea. I would still "dummy up" the unused 1/2 of the 12AX7; maybe ground the unused grid and cathode, and put a cap to ground on the unused plate... say .001 uF? just to remove any stray signals inside the valve. ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: Graham Slee
Date Posted: 12 Feb 2016 at 5:18pm
It's possible that would work. Similar treatment works on a particular converter chip I know. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Fatmangolf
Date Posted: 14 Feb 2016 at 9:30pm
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Wow, this has moved on whilst I've been away. Tantalising and very informative thread, thanks Graham and all contributors. ------------- Jon Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC. |
Posted By: Graham Slee
Date Posted: 14 Feb 2016 at 10:53pm
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The 12AX7 had a serious side effect. When I eventually remembered to measure the plate voltage I found it perilously close to the supply. This meant very little headroom, so it was game over for the 12AX7. My interest then drifted toward the 12AU7 (ECC82). I did a lot of reasoning about it being suitable for 48 volts HT before having a go at juggling component values, and on connecting it to the AP was really surprised by the results - excellent! Obviously the results will worsen with usage - all valves wear out. But the results covered all the bases and at rated output of 0dBu 1kHz THD was a mere 0.02% (it won't last!). So is the 12AU7 suitable for 48 volt operation? I uncovered this data sheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AU7A.pdf and on page three the average characteristics show an Eb of 50 volts. I was happy to see I'd got the calculations right. Sounds pretty decent too - almost exactly the same as not having it in the system (apart from the 7.7dB gain). ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BAK
Date Posted: 15 Feb 2016 at 4:40pm
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Are you thinking of making it a kit? Will it have an input selector? ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: Graham Slee
Date Posted: 15 Feb 2016 at 9:19pm
And within 24 hours the distortion had rose to 0.05%. As long as it stays below 0.1% for the life of the valves I won't mind. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 15 Feb 2016 at 11:08pm
Will people have the patience I wonder? OK, at 48 volts DC it is "safe" to build and poke around inside, but a blunder with power applied, such as slipping with a meter probe, can still cause damage and although it won't cause an electric shock, 48 volts is high enough to arc and make you jump. Perhaps it would need a book? A book to explain what's going on. How a tiny change in one component value can change the game in the wrong direction -- and that also happens with tubes at their regular HT voltages -- it's not just a low voltage thing. There is also some very good info online free of charge including the pages at http://www.valvewizard.co.uk/accf.html - The Valve Wizard and at http://sound.westhost.com/valves/preamps.html#sc - Rod Elliott's Elliott Sound Products. I quote Rod Elliott: "Although it's very easy to make a valve preamp stage that works (and might even sound alright), it's another matter entirely to get it right. . . . ." I have now added an AC cathode follower to the single triode preamp level voltage amplifier discussed above (see The Valve Wizard link above for the basics). However, I didn't have the room or patience to strap a current sink as the cathode load, and so it is rather non-linear using a resistor load. Once the load drops to solid-state input loading (22k according to DIN) THD rises to just under 0.1%. As the valves wear this could worsen. However, apart from the transistor current source for the voltage amp stage, the circuit is "all-tube". It would be nice if the AC cathode follower could be used to bootstrap the voltage amplifier plate load such that a simple pair of resistors could be used, and if it would give the same distortion. It would then have no solid-state components (and therefore the orange LEDs which form part of the solid-state current source could not be justified in just making the valves look more glowy). I would use four separate valves which could be considered extravagant for wasting the second triode in each package, but makes sense as each valve filament is 12V: and 4 x 12V wired in series = 48V, so no power wasting voltage dropper circuit would be needed for the heater circuits. Lots of "ifs" left to go. I hope it's worth all the R&D time which now amounts to nearly 2 weeks. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BAK
Date Posted: 16 Feb 2016 at 1:50am
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You have done a lot in 2 weeks! Seems longer. I admire your tenacity and skill shown throughout this project. ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: Graham Slee
Date Posted: 16 Feb 2016 at 7:57am
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Thank you Bruce, but it is really only a case of following procedure - working to the valve manufacturer's data, and the laws of physics. However, the design is not out of the woods yet... Many articles on valves warn that cathode followers easily oscillate, but don't explain why. The problem I have now, which the AP didn't make me aware of, is oscillation. A high pitched low-level tone is present on both channels. It is therefore important to use the test gear called ears! This noise might have been acceptable (being very low level) in the days of tubes, but it isn't acceptable to me. It is not only cathode followers which can easily oscillate - transistor 'emitter' followers can do exactly the same, but possibly in another frequency band. Why can this be? Simple! A basic 'follower' is a parasitic Colpitts oscillator. You see so many designs incorporating 'followers' without one jot of consideration for the possibility that it could be oscillating, but the physics says bluntly, it is doing so. It might not be heard in most cases, but that doesn't mean it isn't happening. Because it can happen, it will happen, and it must be put right - it can't be swept under the carpet. Being fed by some inductance because all circuits have inductance, that gives you the Colpitts oscillator input inductance. The capacitors can't be seen but they are there and they are real. They are the parasitic capacitances of the grid-cathode junction (base-emitter junction if using a transistor), and the cable capacitance which connects the output to its load completes the picture. Normally the addition of series resistance prevents oscillation, and this works well with bipolar transistors (sold-state), but the valve insists in carrying on its oscillation. This must be partly due to its extremely high impedance - and also its mass of metal and hence its own internal inductance. One needs to ask if the effort of tackling all these problems is really worth it?? Sensible level-headed people who by chance could be reading this, would think that valves aren't worth it - but market research suggests the majority of hi-fi buyers are missing some part of that statement (sensible level-headed...). I also admit to a fascination with the silly old beasties (valves), but mine is based on an admiration of the tenacity (to use Bruce's word) of the engineers of yesteryear - they had no other choice - and as an engineer reading between the lines I think it's safe to say they often used 'suck it and see' techniques to get tubes to kind-of behave. Perhaps the problem can be cured using that other 'ugly' hi-fi phrase; negative feedback? We will have to see. All the regular cures have so far failed. If anyone has been looking at Rod Elliot's valve preamp stage like I have, it will be noticed it is DC coupled. You can read about that here: http://www.valvewizard.co.uk/dccf.html. It might be due to the fact that mine is AC coupled and using grid-leak bias. However, I was of the impression that the DC coupled variety was responsible for a lot more harmonics (distortion), but Rod Elliott's discussion tends to suggest otherwise. And the question is bound to be asked; is it because I'm using such a low HT voltage? I would like to think not seeing as the 12AU7 data gives curves for it. However, it could very well be the case. I suppose the story must continue. Seems such a waste of effort to abandon it now - although I do feel like it. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: morris_minor
Date Posted: 16 Feb 2016 at 9:19am
A bit like http://www.youtube.com/watch?v=h6XUPwTqZnM - this . ------------- Bob Majestic DAC/pre-amp Accession MC/Enigma, Accession MM, Reflex M, Elevator EXP, Era Gold V Solo ULDE, Novo, Lautus USB and digital, Libran balanced, CuSat50 2 x Proprius + Spatia/Spatia Links |
Posted By: Graham Slee
Date Posted: 16 Feb 2016 at 11:30am
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No Bob, I'm a Yorkshireman not a Yorkie The AP still cannot resolve the residual noise I'm hearing on headphones in the background, however, after applying GOBAL NEGATIVE FEEDBACK (saying it loud for all who detest the very mention), the level has been vastly attenuated. Having said the AP cannot resolve the noise in any of its measurements, it's audio monitor reproduces it. I have a suspicion it is noise from the 48 volt switched mode power supply and not the oscillation I thought it could be. Apart from the NFB all I did otherwise was to decouple the anode circuit of the cathode follower with a 100nF ceramic capacitor which I suppose will have the effect of attenuating power supply noise. I do a lot of supposing simply because electronics isn't that simple (although in my experience, it is, to some simpletons). I think a full PSU input filter is in order. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Dave Friday
Date Posted: 16 Feb 2016 at 11:50am
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Do you have the " mullard high quality sound reproduction " books?i have two from the early 60's (5/10 &5/20 power amps &2 preamps. They don't use cathode followers though! Shall send pictures to your Facebook Kr. ------------- lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4 |
Posted By: Graham Slee
Date Posted: 16 Feb 2016 at 7:30pm
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Thanks Barry, but I may already have the preamp circuits. Is one EF86 pentodes and ECC83 twin triode? I found the oscillation noise and it isn't the cathode follower - it's the 48 volt switching power supply. With some ad-hoc filtering it was greatly attenuated. With global negative feedback THD distortion was just measured at 0.004% and IMD at 0.012% for the rated output of 775mV rms (0dBu). The voltage gain is 8.12dB (2.5). The load being the 22 kilohm DIN specification. Out at 10kHz distortion is 0.01% but the residual switching power supply noise is confusing the issue and once suitable filtering is in place it should improve. S/N is better than 84dB. Upper end of frequency response is somewhere in the region of 120 - 160 kHz (-3dB) at rated 775mV out. With distortion and noise this low it might not sound like valves at all. It simply doesn't do what valves usually do - it is at solid-state levels. So, why bother with valves... ? But I just might bother. This isn't the finished circuit by far, it is an earlier version. I won't be publishing the final circuit or the values for obvious reasons.  ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: BAK
Date Posted: 16 Feb 2016 at 8:07pm
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The Yorkshireman is a master tweeker... with the figures achieved for THD, IMD, and noise. ------------- Bruce AT-14SA, Pickering XV-15, Hana EL, Technics SL-1600MK2, Lautus, Majestic DAC, Technics SH-8055 spectrum analyzer, Eminence Beta8A custom cabs; Proprius & Reflex M or C, Enjoy Life your way! |
Posted By: Dave Friday
Date Posted: 16 Feb 2016 at 10:45pm
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Yes Graham sounds like the same books,mine are from when I made the "Three three " Very interesting to see the distortion figures of those amps! ------------- lp12,oc9mk3,ca610p,krimson40watt pa,kef105.4 |
Posted By: Graham Slee
Date Posted: 17 Feb 2016 at 8:10am
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I think the reason for the low distortion from this preamp must be because of the 'novel' plate load of the input triode U1. The 12AU7/ECC82 is 10x higher current than the 12AX7/ECC83 - but is 1/10th the gain supposing the plate load to be a normal resistor. High current = low noise / low current = low distortion. The 12AU7 allows a higher current to flow. Not much - it is only around 1 milliamp but sufficiently more than I had using the 12AX7. Therefore the noise is OK, and needs to be otherwise it masks the distortion, and then the distortion can't be read by the meter. But by 12AU7 standards, the current is quite low, and it has to be at low HT voltage or the grid will draw current. So because of the low current, the distortion is low, and around a tenth of that obtained with the 12AX7. So, the choice of valve is good, but alone it should not be capable of 0.004% THD according to my train of thought. That's because of the non-linearity of having a resistor as plate load. But here we have Q1 as plate load. Q1 is a PNP bipolar transistor. It's base is at fixed voltage due to the LED and voltage drop resistor R2, so the current flowing through it and into the plate is determined by R1 - and that current is constant, or fixed. So U1 (12AU7) plate 'sees' a high impedance load that looks infinite. It's like having something like 1000V HT and a 'long' (really high value) resistor. That effectively removes the non-linearity of having a simple resistor plate load. But if you then drive that into any old input, the input resistance would be in parallel with Q1, and would bring back the non-linearity to a degree. So, what the AP was measuring was U1 loaded by the AP which wasn't what U1 had the potential of doing. U2 (another 12AU7 triode) and associated circuit is a cathode follower. It has near 100% negative feedback so its load can be up to near its own cathode resistance before it starts pushing it non-linear. OK, the resistive load (R7 incidentally) is non-linear, but the load becomes part of R7 and so it's all down to the valve's own output impedance. And this has been brought really low because of the near 100% negative feedback. U2 grid bias is of the leak variety via R8, and its bias voltage set by R9 in ratio with R7. The junction of R9, R7 follows closely what the grid does so the voltage across R8 hardly changes at all. R8 is a large value to start with but as the voltage across it hardly ever changes, it becomes several times its value so the output of U1 (first triode) sees a massive resistance in parallel with its constant current load. It's so big as to have near zero effect on U1's linearity. This is one form of bootstrapping and is called that as it is akin to pulling ones-self up by ones own bootstraps. That's impossible for us to do, but in electronics, it works! And therefore, U1's linearity is preserved - U2 circuit effectively buffers U1 from the load and we are left with what U1 can do if only it could be isolated - and we have effectively isolated it. We can only guess at what gain U1 is doing. Its data reckons it has a gain of 10 for a resistive load, but using our "magic" load it should be at least double, and based on what the global negative feedback (R10/R5) suggests I know it's a lot less than 40dB x R10/R5 (250) which would give a gain of R10/R5. It's doing around 3/4 of that so I reckon U1's gain is somewhere between 25 and 100. So R10/R5 gives us some useful global negative feedback which just falls short of the 40dB negative feedback (NFB) Harold Black of Bell Labs discovered in 1927. So if we guess at 20dB NFB the math suggests distortion is being reduced by a factor of 10, and so the unaided distortion of U1 is around 0.04% or probably a little less - but we'll never see it as that without the rest of the circuit. The voltage gain of the circuit is somewhere between 8 and 9dB which is just about 2.5 - 2.8. A properly recorded CD played on a 2V output CD player will be doing around 400mV most of the time, and the output from a phono preamp will be doing about 300mV. Good power amps have input sensitivities of 400mV to 1V. And those with simple valve amps requiring 1.5 volts? Well, if the input impedance is that of valves, the preamp should do a gain of 2.8, and you'll be 1.2dBW down on max power - close enough! Therefore this stage stands a good chance of becoming a preamp. I've been wondering what I could use as a clean gain stage for a preamp for near on five years now. This might be it, and you might also now see the long awaited preamp. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: Graham Slee
Date Posted: 17 Feb 2016 at 5:34pm
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Because the load impedance becomes part of the cathode load (I suppose that should be obvious...) then the gain and hence critical measurements will be influenced by the load being driven. Cables too will have some influence but using something sensible (like a CuSat50 or Lautus) won't change things much at all (we use similar for analysis). So here's the specs... Frequency response: 3.5Hz - 140kHz (-3dB) or 7Hz - 70kHz (-1dB) Frequency response (compensated option): 3.5Hz - 60kHz (-3dB) or 7Hz - 30kHz (-1db) Note: the compensated option might be the one I choose. Gain: 7.7dB with 22 kilohm load; 8.7dB with 100 kilohm load S/N: 84dB 20Hz - 20kHz THD (distortion) at 1kHz: 0.004% into 22 kilohms; 0.014% into 100 kilohms THD (distortion) at 10kHz: 0.01% into 22 kilohms; 0.02% into 100 kilohms IMD (intermodulation distortion): 0.012% into 22 kilohms; 0.044% into 100 kilohms I reckon this is useable! Here's the FFT plot (spectrum analyser)  The 2.4kHz peak seen between the 2kHz second harmonic and 3kHz third harmonic is some spurious noise, and being at -90dB should be of no concern (although I'd like to get rid of it). The 2kHz second harmonic is greater than the 3kHz third harmonic, which is what you'd expect from a tube... but it's doubtful you'll hear its effect at -77dB. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: pcourtney
Date Posted: 02 May 2016 at 3:40pm
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that was a great read, and I am now more informed than ever about the Valve/Tube infidelity saga that has so many people (who should know better) saying it sounds better, but in reality they should just be saying "it just sounds a little different to solid state" , and why don't you Mr Punter listen to both pre amps and make your own mind up ! and to be honest Graham, if you could design a pre-amp that has a valve bypass switch, so that HiFi dealers all over the world could demonstrate the qualities of both, and that it would also work if the valves were completely removed, then some of the hysteria about valve amps may dissipate in time that could also solve the google SEO for GSP, not only as a new product, but if a "White Paper" could be written by Graham Slee about designing a pre amp that can have valves inserted in the gain path, and what it brings to fidelity ( or not ) the GSP loan program (at least for the UK ) would really be a very clever marketing ploy to throw into the mix as well :-)
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Posted By: Paperweight
Date Posted: 12 Feb 2017 at 6:07pm
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Good choice on the 12AU7. I have an old military R392 HF mobile receiver that uses 12AU7 tubes in the IF strip with 25-30 volts on the plates. Since their gain is reduced with the low plate voltage, they just tacked on an extra stage or two to make up the difference. 12AU7s and its ancestors have very linear plate curves. I bought a bunch of 7 pin 6C4s which are the equivalent of half of a 12AU7. They can drive a speaker at under one watt on 90 volts. Also, at quantity they are fairly inexpensive with military versions often going for $5 or so. Battery tubes have always been fascinating to me. It's amazing they could run the filaments off a 1.5 volt battery and 45-90 volts on the plates.
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Posted By: Graham Slee
Date Posted: 12 Jan 2020 at 3:00am
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Resurrecting this old topic, I've had my nephew George staying this weekend, and he's more advanced in guitar playing than my son Ryan. They're about the same age. We wanted to hear him play the Strat I'd bought Ryan, but all I had for amplification was a Proprius monoblock, which although it can drive the Fane Ascension F70 bass reflex cab Ryan built, it doesn't have the gain, or the sort of sound you'd expect. Then I remembered the valve preamp discussed in this topic, and dug it out half expecting it not to work, but to my surprise it did.  Plugging the guitar into one input and using just one channel into the Proprius improved the gain and the sound, so I decided to loop it into the other input and take that output to the Proprius. Not only was there sufficient gain, but the sound took on a signature George really liked. It also went well with the Epiphone Les Paul. Next a Zoom G2.1u effects unit was inserted and set to "Extreme Distortion" ("Ed" on the display), still on the Epiphone, and things really started buzzin' (not as in buzzing!). We were treated to as many hard rock riffs as George could remember, and it really sounded true to original. By this time I had to set the Proprius down to roughly 10 o'clock volume setting, because even with all my sound proofing I was starting to get worried. I got George to guess the power and he reckoned it sounded like a 100 watt amp, and was surprised when I told him it was just 25. Running on the original 48 volt supply, and although biased for a 72 volt supply, the preamp did a great artistic job. I think it worth building an extra two stages, making a total of 4 stages and stick a tone stack in the middle and some volume controls either end, and to possibly put in a transformer power supply. After a bit of tube rolling, the RFT ECC82 was chosen to give the best sound, with the JJ Tesla a close second. I think it will make an excellent guitar preamp. ------------- That none should be able to buy or sell without a smartphone and the knowledge in how to use apps |
Posted By: peterb
Date Posted: 12 Jan 2020 at 2:01pm
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A proper British made Valve Guitar Head Amp would be something that could fly! A good Mesa Boogie sells for over 2k! What it would need is a celeb player to be associated with it then their fans would follow! ------------- Peter --------------------------------------- Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2 |