A small FET gainstage is the spirit of the Pass B1

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hello all,
a while ago I saw a small schematic of (2) FET's configured as what is known in the vacuum tube world as a SSRP gainstage. Here is a schematic as I remember it. Gain could be adjusted via the ratios of R1 and R4.

So, anyone tried this circuit? Remember where this might have been posted on the web?

-T
 

Attachments

  • SSRP FETa.gif
    SSRP FETa.gif
    12.5 KB · Views: 570
...anyone tried this circuit? ...

I made it few years ago with bf862 JFETs and I was not aware that somebody else published it somewhere else too (great originality is not possible with simple circuits).
JFETs should be matched in order to ensure symmetrical clipping (half of the PS voltage at the drain of Q14). Adjustments can be made by slightly changing the value of R1 or R2 (some load-lining too).
It swings almost rail to rail so it would be interesting to try it with 2sk170 which allows PS voltage up to 40V.
NFB (R6,R4) provides low Zout (100R or so).
If the gate of Q14 is leaky, input cap should be used (10u).

Sound is OK but a bit too sweet (nice for the girl& guitar type of music, somewhat mushy & muddy in complex orchestral pieces). Anyway, easy enough for a quick build & listen.
 

Attachments

  • bf862-sepp.gif
    bf862-sepp.gif
    3.8 KB · Views: 457
Juma
do you find noise to be an issue with the values of R4/R6?

Have a look at FirstWatt F3. It uses similar feedback arrangement, with similar values, and with all those big noisy FETs it shows a noise figure of only 50uV (which makes it one of the quietest FW amps).
OTOH, FirstWatt M2 shows a noise figure of 500uV and still nobody has noise concerns.
Here you have very low noise device (BF862), circuit with less gain than F3, widely known topology, and you are still worried about the noise.
Before going into circuit's noise analysis, what do you think - why is all that so ?
 
Last edited:
I presume the noise performance of the M2 has something to do with sticking a mains transformer and a signal transformer in one box and people do freak out about this.

I am not proficient in noise analysis, but I stuck my nose in a text book. Now I wonder why people rave about the noise of a 0.8nV/rtHz device that has a 13nV/rtHz resistor in front of it.

Let my try and guess why you think it is unimportant: a jfet has high input impedance so I need not worry about current noise?
 
  • Like
Reactions: 1 user
Founder of XSA-Labs
Joined 2012
Paid Member
I can attest to the BF862 in this configuration as absolutely low noise below measurement capability of equipment of mere mortals (-130dB). I have built Juna’s BF862 preamp and it sounds great. Nothing audible with no signal. Also, volume pots don’t make any scratching noise when adjusted. Pure smooth as butter volume adjustment.

Regarding M2, I have noise/hum issues well beyond specified 600uV due to mains transformer EMI coupling into small signal transformer when in same box. I would highly recommend a SMPS to get the +/-24v power supply and save yourself the hum headache.
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.