Welcome to the Graham Slee Audio Products Owners Forum Open to all owners plus those contemplating the purchase of a Graham Slee HiFi System Components audio product and wishing to use this forum's loaner program: join here (Rules on posting can be found here) This website along with trade marks Graham Slee and HiFi System Components are owned by Cadman Enterprises Ltd |
Phono Preamp Pt2: MC |
Post Reply | Page <1 678910 11> |
Author | |
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
Perhaps you'd like to read my lengthy article on phono preamp design here before making comments like the above: http://audio-forum.gspaudio.co.uk/phono-preamp-project_topic745_page1.html I read with interest you quoting at me how other designers do things. Unfortunately those designers don't understand much about electronics - they like to play with the notion that they do. I think if you read all my writings in detail you will realise that I know about the effects of dust and pops and clicks. Also try reading some reviews, especially those saying how much lower our surface noise is. If you don't like my ways you can always follow some other manufacturer. There is a saying about not teaching the grandmother to suck eggs... |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
rokoko
New Member Joined: 30 Jul 2013 Location: US Status: Offline Points: 9 |
Post Options
Thanks(0)
|
Graham, thanks so much for your explanattion. I own your Reflec C, like it so much, do not have any complaints about it, only extremely postive feedback, all I can say. It has extremely low surface noise, I also notice that the clicks and pops are almost invisible, almost gone vs. to what I used before for the phono amp. It is amazing. It amazes how so simple and elegantly designed device can peform so well. My questions only from the desire to learn, from curiocity.
|
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
Good. I'm so often taken to task over what I do that the guard goes up immediately. I don't know why people insist on doing things the way they do - there are text books that explain how things work. But, I suppose some people believe those who make things up as they go along (and I'm pleased you don't?). I have known rich people take over old high fidelity companies and do exactly that - they even believe it themselves! If you get the chance, take a look into the input characteristics of bi-polar and FET transistors. Bearing in mind that the output of a magnetic cartridge rises in proportion with frequency, you will note that at 20kHz the cartridge outputs ten times what it does at 1kHz. For an average MM cartridge that's 50mV rms, and 250mV rms at +14dB. The bi-polar transistor clipped way before 20kHz at normal output for a MM. The FET isn't far off either. And this is regardless of what voltage the power supply is! In digital 0dB is actually +14dB because the peaks have to be accomodated. Therefore true 0dB is actually -14dB (except for loudness wars CD). These digital outs work on 3.3V. In analogue +14dB is 14dB above 0dB. 14dB is x5 so if we take 0dBu as 775mV then +14dB is 3.875V. Analogue therefore requires 5 times the digital supply: 5 x 3.3 = 16.5V. I know from doing it myself that the easy way out is to make phono stages the passive interstage way. If you do that you definitely need the higher voltage supply. If however you do it the hard way - the active way - which if you get right is far more accurate in "focus" - then, provided your propagation speeds are right, you don't need high voltages, because the active RIAA feedback instantly corrects it all. The problem has been that few understand how to achieve a good active design, and, in time honoured manner, rubbish anything they don't understand. If propagation speeds are low then the spikes generated by muck and damage will clip the supply and then it takes time to recover. That's responsible for the elongated "scratch" noises. The reason why you don't hear those noises so much, or at all, are because of the propagation speeds of the circuits we design. I could go on, but I did and that was at the link I provided. Best regards, Graham |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
BAK
Senior Member Joined: 14 Mar 2010 Location: Kentucky, USA Status: Offline Points: 1744 |
Post Options
Thanks(0)
|
Well stated, Graham.
|
|
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! |
|
Fatmangolf
Moderator Group Joined: 23 Dec 2009 Location: Middlesbrough Status: Offline Points: 8943 |
Post Options
Thanks(0)
|
Thank you Graham. Not only do your explanations make sense, the resulting hi-fi lets the music come through.
|
|
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. |
|
Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
Post Options
Thanks(0)
|
One rather unfortunate problem in doing a MC phono stage based on the AC (capacitor) coupled Genera, is the possibility that with 20dB (10 times) more gain, which is required for moving coil (which is roughly 1/10th the output of a MM), could conceivably result in power supply feedback. The result of this is called "motor-boating" - so what is its cause and how can we fix it?
For those who don't have a clue what I'm going on about there's a primer here... http://www.analog.com/static/imported-files/application_notes/AN-581.pdf With such a large NFB resistor in the Genera (see diagram)... ...both opamp (OA) inputs need to see the same capacitance to ground (C1 and C6) or it will take an age to turn-on (TC = (R5+R8) x C6), thus obeying the rule that the output will do whatever it can to make both inputs the same. But Cin (in the primer) needs to be at least 1/10th the bandwidth of the bias voltage capacitor or there is a risk of power supply feedback and "motorboating". So why doesn't the Genera do that? Well, it relies on the voltage regulator doing its job properly. The voltage regulator noise is pretty small compared with the input signal but becomes more prominent with a signal 1/10th smaller and the 10 x more gain a MC version would require. The primer article suggests the use of a zener diode to overcome PSU feedback, but zeners are noisy, so instead of "motorboating" you'd have broadband noise. It took me ages to get my head round this problem, but here is the solution: we actually use a zener but we also keep the potential divider bias voltage generator (R11, R12, C9). What we do is split R11 into a 390R resistor in series with the 56k resistor - the 390R is at the positive end, and we drop the zener from that series junction to ground. We then change R12 to 68k or 75k (whichever value you can easily grab) to keep the bias voltage roughly the same. The zener gets rid of the PSU feedback issue, and C9 polishes off the zener noise. You can try this on your existing MM Genera if you'd like. It could improve its performance? More soon. Graham |
|
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
|
|
Paperweight
Regular Joined: 21 Mar 2011 Location: Lexington NC US Status: Offline Points: 84 |
Post Options
Thanks(0)
|
I was reading through your posted link for Elliot Sound Products. One of the topics is Phono Preamps for All. He described designs from John Linsley-Hood and Douglas Self. I've pawed through Small Signal Audio Design a few times and completely forgot he even had a section on moving coil amps.
It's funny how disinterested you can be about something until you need it.
|
|
Post Reply | Page <1 678910 11> |
Tweet
|
Forum Jump | Forum Permissions You cannot post new topics in this forum You cannot reply to topics in this forum You cannot delete your posts in this forum You cannot edit your posts in this forum You cannot create polls in this forum You cannot vote in polls in this forum |