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Phono Preamp Pt2: MC

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2013 at 8:47am
Originally posted by rokoko rokoko wrote:

One more thought regarding desire to increase power supply voltage to some higher.

Just for curiosity, I was reading how other amateurs and enthusiast designing their phono amps, also examples of factory made phono amps. One common suggestion I found - to increase the power supply voltage to approx. +/-35 (or 70V rail to rail).

The 14dB over load factor you describe and keep in mind designing phono amp takes care of all vinyl disk overloads. But in the real life some LPs are recorded with out of spec, higher speed, more than 5cm/sec. The other, major factor is the dust particles, contamination or micro damages in the track. During this the needle will have big and fast excursions and the signal applied to the phono amp can increase by approx. 40dB+. In this case the phono amp will saturate. The coming back from saturation time will be long, resulting in much more hearable, pronounced clicks, pops. This is why designers are increasing power supply up to much higher level than 18V, to prevent clipping/saturation event.

Designers also saying that the power amp connected to the output of phono amp will not badly saturate even if phono amp in case of 40dB overload event will output up to 30V+ amplitude. I do not understand how this possible, but this is what some designers claim, mentioning some factory made phono amps power with 40V-60V power supplies.



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... Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rokoko Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2013 at 1:36pm
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. Star 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. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2013 at 2:46pm
Originally posted by rokoko rokoko wrote:

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. Star 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. 


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




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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2013 at 4:02pm
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!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2013 at 7:22pm
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Mar 2014 at 12:55pm
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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paperweight Quote  Post ReplyReply Direct Link To This Post Posted: 22 Feb 2017 at 1:17am
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.
 
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