Single-stage active RIAA correction with second- or third-order Butterworth high-pass included

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#121
MarcelvdG said:
Regarding the mixed JFET-bipolar transistor pair, are you sure you used the same transistors as I did (J310 and 2SC2547) and if so, that their noise is properly modelled?
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No, of course I used JFE2140 @6 mA DC with en=0.9 nV/sqrtHz, while the J310 datasheet indicates en=10 nV/sqrtHz ( @10 mA DC , page 2 of OnSemi datasheet-March2006). My bipolar is BC550C biased @ 6 mA (and Rb=0 in MC model), and you use 2SC2547 @ 1.9 mA. I am interested in the reason for using a second bipolar in your differential cascade.
 
#122
The output impedance of the feedback network was such that I could get a lower noise with a 2SC2547 than with a J310 as the right branch of the differential pair, as described in the article. If that doesn't answer your question, then I don't understand your question. The graphs in the Siliconix J310 datasheet showed far less than 10 nV/√Hz at 1169 Hz, by the way.

The reason for not going for a single-ended J310 input stage was one of convenience. I didn't find a convenient way to bias the circuit that way.
 
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#123
MarcelvdG said:
the open-loop output impedance of your amplifier must be about 20 kohm, so it is not >> the feedback network input impedance at low frequencies. That probably explains the loop gain increase you see between 40 Hz and 10 kHz in post #116.
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In fact, the output resistance of the common emitter cascade (open collector) is more than a megaohm, and the reason for the increase in the loop gain at medium and high frequencies is the lowering of the impedance of my T-shaped RIAA at frequencies near 7950 µs. I attach a loopgain prooflink with stepping of 7950 resistor (R19), - without it a dimple @ 20 Hz disappears.
PS. Thanks for your bi-fet input explanation, I agree that it is better -3 dB noise penalty of differential pair but without a headache of single-ended input power supply. Your original circuit design is something between a differential stage and a single stage, but without the show-offs with the power supply
 

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#124
If your amplifier has a 1 Mohm open-loop output resistance and the feedback network input impedance is 180 kohm, the loop gain should drop by about a factor of 16 (24 dB) when you connect a 10 kohm load. I don't understand why you only see a 9 dB drop in post #116 then.
 
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#125
MarcelvdG said:
If your amplifier has a 1 Mohm open-loop output resistance
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1 Meg not amp but naked output common emitter stage without feedback loop. With FB loop closed 10k load (with series 470 output resistor) shunts resistors in T-shaped RIAA (68k +8k2) appox. on 18 dB, but DC servo with not too big RC restores about half. I use Microcap12 [ https://archive.org/details/mc12cd_202110 ] to simulate precisely. See here [ https://photos.app.goo.gl/JdQiD3SfqamUgYzD8 ] how DC servo works @ < 20 Hz
 
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#126
Nick Sukhov said:
1 Meg not amp but naked output common emitter stage without feedback loop.
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I thought so, that's why I referred to it as the "open-loop output resistance".

Nick Sukhov said:
With FB loop closed 10k load (with series 470 output resistor) shunts resistors in T-shaped RIAA (68k +8k2) appox. on 18 dB, but DC servo with not too big RC restores about half.
Click to expand...

I thought you were referring to this circuit:

Screenshot_20250324-183930~2.png


Are you? If so, what DC servo?
 
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#129
MarcelvdG said:
I thought you were referring to this circuit ( If so, what DC servo? ) :
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No, newer version from here, 3rd picture [ https://imrad-com-ua.translate.goog..._sl=ru&_x_tr_tl=en&_x_tr_hl=ru&_x_tr_pto=wapp ] , you may remember previous, in which the DC servo was connected to the output stage [ https://c10.patreonusercontent.com/4/patreon-media/p/post/64952825/60afdf1a555b4d63814dd26f093959e2/eyJxIjoxMDAsIndlYnAiOjB9/1.jpg?token-time=1744156800&token-hash=MgX-ppgt8QHq_7pzYghRdfTTKwspYHhLhgtxWJ-Fna0= ] and had a limited range of offset adjustment. You then recommended (a couple of years ago) that I move the servo output to the input jfet.
 
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#131
MarcelvdG said:
I've just added an updated schematic to post #1. It's not suitable for op-amps.
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I also refused from microcircuit op-amps in the phono preamplifier, firstly because the JFE2140 with 0.9 nV/rootHz is much better than any of the fet-input op-amps, and secondly, not a single op-amp has a transadmittance output @ class A with enough current to drive both RIAA FB network and load
 
#132
As an aside, who is actually using MM cartridges in this group? I haven't used one in seemingly 50 years. I am now using MC's connected in the current input mode.

I have built numerous phono stages for use with MM cartridges for friends over the years, some reasonably good, hence this is interesting. Yet can MM's get near the absolute performance of a MC? Surpass them?
 
#137
Hierfi said:
As an aside, who is actually using MM cartridges in this group?
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I am.

Hierfi said:
I have built numerous phono stages for use with MM cartridges for friends over the years, some reasonably good, hence this is interesting. Yet can MM's get near the absolute performance of a MC? Surpass them?
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I have no idea. I do know that some people (Hans van Maanen, Steven van Raalte, Bob Cordell, Nick Sukhov, billshurv, possibly Hans Polak, and many others) have done experiments with non-standard cartridge loading and/or reverse resonance filters (biquads) to improve the high-frequency behaviour and transient response of moving-magnet cartridges.
 
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#139
A very low impedance cartridge still has the geometric losses due to stylus non-zero dimensions and the vinyl compliance x ETM resonance (plus other cantilever wobbles) in its transfer curve, and with no easy method of (at least partial) correction.

I suspect that MC cartridges appeal to us in our mid years, when the sparkling top octave helps to make up for some of our age related hearing losses. Beyond those middle years it doesn't seem to matter anymore IME. Just a hypothesis.

All good fortune,
Chris
 
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