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-   -   Simplistic NJFET RIAA (https://www.diyaudio.com/forums/analogue-source/129126-simplistic-njfet-riaa.html)

escucalin 6th January 2019 08:45 PM

Thank you for the great insights and calculations; I reached a desired behavior using a 3.3K 2W R13 and 46R for R2-3.the noise floor is black @ 1cm near the very unforgiving high-mid range speaker. It becomes audible when volume is way too high for anyone to enjoy any vinyl recording.

Thank you so much for guiding trough!

I still have a question: Is Q3 providing any voltage gain? Being open with tp1-2 @ 3.6V it provides a more linear pathway than a resistor?

Salas 6th January 2019 09:23 PM

Q3 provides that stage's voltage gain since its the common base part of the folded cascode. The common source arranged JFETS that it shields from Miller effect provide the current gain.

escucalin 6th January 2019 10:52 PM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analogue-source/129126-simplistic-njfet-riaa-post5653599.html#post5653599)
...
As you raise the R2&R3 values towards that goal the input stage capacitance also lowers because of the higher local feedback by degeneration they create
...

Is R2,3 playing a feedback role (What kind of feedback)? Or just limits the swing of the transistor?

Salas 7th January 2019 04:11 AM

See about emitter degeneration in BJTs, source degeneration is the analogous thing with JFETs. Local feedback is the term I used because its debatable:

"While this is often described as "negative feedback", as it reduces gain, raises input impedance, and reduces distortion, it predates the invention of negative feedback and does not reduce output impedance or increase bandwidth, as true negative feedback would do"

Common emitter - Wikipedia

RCruz 7th January 2019 07:05 PM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analogue-source/129126-simplistic-njfet-riaa-post5653599.html#post5653599)
Typically for 300mV RMS output at 5cm/sec velocity (-10dBV standard hi-fi level) you would only need 41dB gain with your cartridge. Due to its high coil resistance the R2//R3 parallel total value will not start contributing noise before 170R which will never need to be reached in your case anyway. That's fortunate because it kicks one sneaky noise source out of the equation.

If your system voltage gain seems needing the 52dB then the compromise is mainly in raised THD levels and lowering of headroom to clip for ticks and pops. Maybe you could sacrifice even more phono stage gain and still have good max volume in your system? That will lower the hiss accordingly even more. As you raise the R2&R3 values towards that goal the input stage capacitance also lowers because of the higher local feedback by degeneration they create. R13 may need further adjustment for proper TP1-TP2 reading (i.e. input stage DC centering) as you go down that route.

P.S. The 500R coil resistance cart has 2.8693 nV/√Hz source noise at 25C. Or 0.4056 uV for 20kHz bandwidth. That's 76.465dB dynamic range for 2.7mV output.

Hi Salas

Would you care to explain how you got to 300mv output knowing only the cart specs ?

Salas 7th January 2019 08:34 PM

Setting an output goal (line level consumer in this case) you can work out the gain when knowing the cartridge's output

RCruz 8th January 2019 11:23 AM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analogue-source/129126-simplistic-njfet-riaa-post5657054.html#post5657054)
Setting an output goal (line level consumer in this case) you can work out the gain when knowing the cartridge's output

Thank you Salas

Now I understand.... 300mV is consumer level spec.

V2 300mV
V1 2.5mV
L = 20 ◊ log (voltage ratio V2 / V1) in dB = 41.6dB

force of 1/2 30th January 2019 04:45 AM

Regulator voltage
 
Iíve built up one of Teabagís boards aiming for ~43 db.

I am only able to get 30vdc after the shuntreg on my Folded phono board with VR2X all the way open.

Thus the lowest reading I can get between TP2 and TP2 is 4.99V.
TP1 to ground is 7.93 for both channels.

I originally used the LEDs from the minikit, but these measured low, so I replaced them with a string of all green LEDs measuring 1.98Vf each for D2X-D4X. The result is that I am still getting only 30Vdc after the reg. And 4.99V between TP1 and TP2 with both VR1 and VR2X wide open.

Any thoughts? Is this just fine?
It certainly sounds pretty good. Will it sound better if I manage to raise the rail voltage?

Thanks for your help!

escucalin 30th January 2019 07:36 AM

Quote:

Originally Posted by force of 1/2 (https://www.diyaudio.com/forums/analogue-source/129126-simplistic-njfet-riaa-post5680514.html#post5680514)
I’ve built up one of Teabag’s boards aiming for ~43 db.

I am only able to get 30vdc after the shuntreg on my Folded phono board with VR2X all the way open.

Thus the lowest reading I can get between TP2 and TP2 is 4.99V.
TP1 to ground is 7.93 for both channels.

I originally used the LEDs from the minikit, but these measured low, so I replaced them with a string of all green LEDs measuring 1.98Vf each for D2X-D4X. The result is that I am still getting only 30Vdc after the reg. And 4.99V between TP1 and TP2 with both VR1 and VR2X wide open.

Any thoughts? Is this just fine?
It certainly sounds pretty good. Will it sound better if I manage to raise the rail voltage?

Thanks for your help!


This sounds as if VR2X is capped; tweaking R3x by a Kohm or so would give the coarse headroom needed to bring TP1-2 in ballpark.

force of 1/2 30th January 2019 08:06 AM

Tweeking up or down, would you say?


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