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Phono Preamp Project

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Sylvain View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2021 at 11:14am
' Genera ' is highly treasure.
Initially purpose to transfer Vinyl to FLAC files .....but since Vinyl has returned the main source and instead of tweaking the DAC.....for a more analogue sound .....

I remembered the Graham Slee 2 which SE caused a Storm when tweaked to SE ...at my  'Audio Venue'' or 'Audio Seduction''  

But very grateful for teh above.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2021 at 2:41pm
If we know slew rate and output current we can establish the capacitive load at the frequency where beta equals one - to preserve the advertised op-amp phase and gain margins.

C = IT/V = I/V/T to get current (I)/slew rate (V/T).

So if an opamp has slew rate 250V/uS and output current 50mA, then 50/250 = 200pF.

If the opamp has slew rate 16V/uS and output current 6mA, then 6/16 = 375pF.

So, which op-amp drives more capacitive load? Yes, the one which most forum indoctrinated types consider to be bad.

Should the ratio between lower and upper frequency active-EQ capacitors be 3:1, then

1/0.375 = 0.75/0.375 + 0.25/0.375 = 1/0.5 + 1/1.5

So the series values of capacitance are 1500pF and 500pF, these being connected in series between op-amp output and inverting input.

They are not a load to ground in an exact sense, because of the divider resistor (R8 in our Genera circuit), and so the capacitive load appears in series with it.

Therefore we can argue that there is no real capacitive load provided the output can drive the divider resistor.

However, the "feedback" capacitors are not a purely resistive load - obviously. Suppose a perfect squared pulse appeared at the output. It could not maintain its shape to the inverting input of the op-amp, and therefore overshoot might occur. In that case, it may as well be a capacitive load.

The above may partly happen (refer to overshoot by Otala) so some lesser part of the value of R8 might be seen as the resistive load.

The output current is also required to drive the subsequent stage or amplifier, so there needs to be sufficient left over after the EQ.

If we look at the op-amp datasheet, the 16V/uS, 6mA op-amp is rated to drive 2k Ohms in parallel with 100pF while remaining stable. It now occurs to us that we might need some additional resistance to rescue the op-amp from instability.

The R8 value with the capacitor values shown, will be somewhat higher than in the Genera, and will introduce noise, so we really ought to reduce the value of R8 too.

In the end, we find ourselves having to increase the output series resistance both on the op-amp's output pin, and in series with the "feedback" capacitance, to ensure the op-amp's phase and gain margins aren't compromised.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2021 at 5:08pm
..Here, we could expand the subject of the phono preamp project to the whole system. Perhaps we should.....

Thank you ...But it will take me a few days to digest the 'applied mathematics ' and the rationale of Function applied. I understand that all the components on the 'GENERA'' boards been meticulously evaluated and balance to corresponding components ....  ''Genera'' is a 'Genus' platform configuration of ' multiple stage amplification' and I believe YOU can produce more specific audio enhancement applications from the ''Genera'' Platform
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2021 at 5:54pm
Walt Jung appreciated the sonic qualities of natural slew-rate J-FETs such as the LF411, LF351, TLO71. The LF411 and LF351 share an almost identical specification, but the LF351 is the better sounding, in my opinion.

The TLO71 isn't too bad either, but its signal symmetry lets it down. I'm sure some might consider its sound more exciting, and the dual version, the TLO72, is a drop-in replacement here. The slew-rate is 20V/uS instead of 16V/uS, which might make it ring slightly - that's where the excitement might come from.

Slew rate and output current are the real problems in getting the NFB impedance lower, to get the divider resistor smaller, and shave a few dBs off the noise.

If you want to go to really high slew rates, you need lots of output current for stability. Walt Jung's solution included a unity gain - non-inverting analogue buffer on the output but included in the NFB loop.

Since publication in the 1980s, the buffers were discontinued, and today's variety do the opposite of what's needed. Another op-amp arranged as a voltage follower won't do because the output current to slew-rate is wrong.

The so-called "diamond buffer" is also too fast, resulting in the bright sound of instability. The only remaining option is the transistor assisted op-amp configuration, which might track the op-amp better. The gain is sub unity, and we have to be careful not to lose the phase and gain margins. I will try and include some illustrations next post.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jan 2021 at 6:03am
Idea for a higher current - same slew rate gain stage, to lower EQ impedance, and hence R4 to reduce noise further. The op-amp then becomes the main noise source.

buffered RIAA preamplifier
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