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Understanding Record Repro |
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Richardl60
Senior Member Joined: 04 Nov 2014 Location: Yorkshire Status: Offline Points: 1468 |
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agree!
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Fatmangolf
Moderator Group Joined: 23 Dec 2009 Location: Middlesbrough Status: Offline Points: 8960 |
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I think amplifying the music not the EMI/RF makes very good sense. I wouldn't want mains hum and hiss from unshielded cables. Although I appreciate the need to get back to the recorded sound the scientific explanation of the EMI and how to handle it has helped me. Thanks Graham.
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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. |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Lossy barrier pre-pre/head amp...
After studying Doug Self's approach to moving coil, which incidentally isn't a million miles away from my own, although the layman would never have guessed, I went on my usual interrogation of the internet, searching on some of the references cited in Mr Self's chapter on the subject. It is quite annoying when you get '404 pages' and simpleton results from Google, but I eventually found a couple of old Wireless World articles - one by the late JLH (Linsley-Hood). Whereas Self uses paralleled small signal transistors for low noise, the other two articles favour medium power transistors, which are the equivalent of paralleled devices. It's called base spreading resistance which simply means the device is better matched to the low resistance of the source - the MC cartridge. Some may remember me harping on about the mismatch between MC and input electronics? Unfortunately, well intentioned as they are, all these approaches lead right back to the annoying (to me at least) moving coil 'brilliance'. But why? All these ideas use the 'low noise transistor technique' as part of an overall stage with the obligatory negative feedback, which is used to quell distortion and reduce noise even further. Each idea is properly researched and the designs cannot be criticised for instability - but - as we have found, the rising acceptance of the cartridge coil's inductance allows an incredibly large amount of interference in - which is not easily controlled without removing the signal we want to keep. Perhaps I need to add the qualification: the harmonics of the signal we want to keep, along with its phase 'integrity'. Resistors become capacitors and inductors; capacitors become inductors and resistors; inductors (if used) become capacitors. All this being at high interference frequencies (or RF). And therefore such parasitics lead to instability beyond the usual stability criterion. And that is the sound I always hear - the sound an unstable circuit makes when supposedly playing music. The answer seems to be becoming obvious. Too much gain inside a negative feedback loop. Not that there is anything wrong with negative feedback! It is just that it is too much in this application, and any (ultra) high frequency stimulus is bringing on oscillations in an otherwise stable circuit. So what did I do? I decided I would just use the transistor in its own negative feedback loop - no other stage within global negative feedback - no op-amp (as per Self); no second transistor (as per JLH). This produces a virtual earth or summing node amplifier that is not a very good one. See "Example 9.7.2 Using Miller’s Theorem" here: https://wiki.analog.com/university/courses/electronics/text/chapter-9 ...and don't worry about the math! It serves its purpose at explaining that such a stage is "lossy" (or even lousy), but only in the way that it is not good for most applications. However, for moving coil it is - from my 40 odd years of experience so far - the answer! With very little open-loop gain and therefore very little negative feedback it simply cannot "take-off" (a slang phrase used to describe the inconvenience of all kinds of oscillations or ringing) and can do little damage to the wanted signal and its harmonics. Its bandwidth is considerable, but simple filtering stops interference - or all the interference sources we have at hand levelled at it. Couple it to the input of an MM sensitivity RIAA stage and you're there. It is low noise because it is a "load of paralelled junctions", which are all low resistance - ideal for the low source resistance of a low output MC. In any other application its distortion would be unacceptable, but here the signal is that small that it is of no consequence. It is around 0.01% THD at 1kHz, which would not please Mr Self (whose life seems to have been spent on achieving the lowest possible distortion) but you'd have a job to hear it. The main point is that there is much less of the annoying brightness of MC. It becomes more melodic - more involving - a more balanced sound quality. I seem to have spent too much of my life searching for answers as how to make phono cartridges do their jobs properly, and this solution isn't that far off the only solution which worked back in the disco days for ceramic cartridges (see page 229 of this PDF: http://www.americanradiohistory.com/Archive-Practical/Wireless/70s/PW-1976-07.pdf) - Marie Antoinette?? |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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The mixing of two frequencies to form another new frequency is known as heterodyning (https://en.wikipedia.org/wiki/Heterodyne).
Listening to MC I have been aware of odd sounds no matter how I have constructed the phono stage preamp. These are not sounds in isolation, they are a change in the timbre of particular instruments, and sometimes differences in tempo. Could this be heterodyning? On previous ocassions I have mentioned the mixing of interference with high harmonics, and my belief that it has something to do with MC sounding much different to MM. The interference shown on scope traces earlier in this topic show that it is much higher in relation to the signal with MC. Therefore, if it is the mixing of frequencies - heterodyning (a new word in the topic which better describes the process) - then it makes sense that it will be more obvious with the low output of MC. Proving it is the difficult part. So far I have made a pre-pre or head amp which is less susceptible to mixing because it has low open loop gain. The sound has improved, but there are still sounds where the timbre isn't acceptable. How to attenuate the problem further? The only way I can think of is to use carbon resistors in place of the metal film types I normally use. Carbon film doesn't do high frequencies all that well. Here we are not talking about the audio spectrum but frequencies in excess of 1 MHz. If heterodyning is the culprit and the interference is in the high MHz, then the use of carbon film resistors in the head amp might make it more lossy. If so, then the suggestion is that heterodyning is the probable cause. I am awaiting some printed circuit boards which will allow me to build both versions (metal film and carbon film) to enable me to do comparisons. Hopefully I can report back shortly on my findings. |
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BackinBlack
Senior Member Joined: 05 Feb 2012 Location: Hinton, N'hants Status: Offline Points: 2020 |
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I'm following this line of development with great interest.
It does seem to me that there has been a general disregard that the moving coil (sound) is flawed in many iterations of phono amp. Nevertheless, this sound appears to be popular, or is it just another example of Kings New Clothes syndrome? It does occur to me that in earlier days (JLH's era) when the airwaves were somewhat cleaner and amplifier bandwidth/upper frequencies more limited, the MC might genuinely have sounded better then MM, but times have changed and not all "progress" has been good. |
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Just listen, if it sounds good to you, enjoy it.
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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Back to my pet subject: heterodyning; it is also known as sum and difference. Two frequencies mix to produce their sum, and their difference. The sum is higher in frequency; the difference is lower in frequency.
I was recently made aware of this video of Peter Ledermann... And at 19 minutes in, he talks about sum and difference regarding moving coil. The resonant frequencies of the cantilever which he discusses seem to correlate, and so it is possible that the sum and difference frequencies he talks about do exist. Since making a buffer or isolation stage which reduces the effect of the high frequency interference discussed in the early part of this topic, the audible effects of those are vastly attenuated. It works because the transistor used is a power transistor, which having a much smaller base spreading resistance is lower in noise than small signal transistors, but being a power transistor it hasn't the same high frequency gain. Added to this are input resistor/capacitor filters which do not load the cartridge, but simply attenuate the high frequency interference. Still, after many weeks burn-in there is brightness, and I can only put this down to the resonance of which Peter Ledermann speaks. Otherwise the sound is far more musical, and I suppose, for those used to hearing moving coil, will sound just right. I am getting used to this more musical brightness, which without the interference heterodyning isn't anywhere so harsh as it was. |
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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
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BAK
Senior Member Joined: 14 Mar 2010 Location: Kentucky, USA Status: Offline Points: 1744 |
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Thank you Graham for sharing. I have been following this thread of Graham's with great interest. I feel Graham has found an answer to pre-amplifying low output magnetic cartridges, MI and MC, with the low gain input "buffer" transistor stage he describes he is using currently. In his discussion about cartridge design, he mentions the higher separation between channels is a fairly good indication of the cartridges quality at reducing spurious jitters caused by the resonance of the stylus/cantilever/suspension mechanism. This makes sense to me as I have always tried to select my cartridges by this specification (and the reliability of the manufacturer to report accurate specs). Many manufacturers do not specify this (I feel) very important spec... channel separation is of course the basis of stereo sound.
Edited by BAK - 19 Nov 2017 at 5:05pm |
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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! |
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