Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Pass Labs This forum is dedicated to Pass Labs discussion.

A small FET gainstage is the spirit of the Pass B1
A small FET gainstage is the spirit of the Pass B1
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 4th December 2017, 11:38 AM   #1
tecnofossil is offline tecnofossil  United States
diyAudio Member
 
Join Date: Sep 2003
Location: MA
Default A small FET gainstage is the spirit of the Pass B1

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
Attached Images
File Type: gif SSRP FETa.gif (12.5 KB, 506 views)
  Reply With Quote
Old 4th December 2017, 02:21 PM   #2
WalterW is offline WalterW  Netherlands
diyAudio Member
 
Join Date: Oct 2012
Location: Utrecht
A small FET gainstage is the spirit of the Pass B1
Looks like this:
Attached Images
File Type: jpg 56082-f2248fdb52b758e14f834fabe6f50acc.jpg (16.2 KB, 474 views)
__________________
Pass F5, F5Tv3, ACA, Aleph J, DDDAC1794, DCB1, BA3-preamp, VFET CSX1, Pass F6, Pass M2-HPA, VFET-X balanced, Pass J2
'If you see me talking to myself, don't worry. I'm getting expert advice'
  Reply With Quote
Old 4th December 2017, 03:16 PM   #3
BesPav is offline BesPav  Russian Federation
diyAudio Member
 
Join Date: May 2011
Location: Russia, Moscow
Quote:
Originally Posted by tecnofossil View Post
SSRP

SRPP.
  Reply With Quote
Old 4th December 2017, 10:18 PM   #4
juma is offline juma  Germany
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Berlin
Quote:
Originally Posted by tecnofossil View Post
...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.
Attached Images
File Type: gif bf862-sepp.gif (3.8 KB, 386 views)
__________________
Please don't use private messages for question related to audio projects. It defeats the purpose of the forum.
  Reply With Quote
Old 4th December 2017, 10:59 PM   #5
aspringv is offline aspringv  Australia
diyAudio Member
 
Join Date: Sep 2010
Location: Perth, Western Australia
A small FET gainstage is the spirit of the Pass B1
Tubecad has all the articles in the world on srpp stages...
  Reply With Quote
Old 5th December 2017, 09:40 PM   #6
ElFishi is offline ElFishi  Europe
diyAudio Member
 
ElFishi's Avatar
 
Join Date: May 2008
Juma
do you find noise to be an issue with the values of R4/R6?
  Reply With Quote
Old 5th December 2017, 11:42 PM   #7
juma is offline juma  Germany
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Berlin
No. Test it yourself and try to answer why it is not the issue here..
__________________
Please don't use private messages for question related to audio projects. It defeats the purpose of the forum.
  Reply With Quote
Old 6th December 2017, 01:00 AM   #8
xrk971 is online now xrk971  United States
Got Foam?
diyAudio Member
 
xrk971's Avatar
 
Join Date: Oct 2012
Location: Metro DC area
A small FET gainstage is the spirit of the Pass B1
I think this is similar as well. I built it sometime ago. Lots of BF862’s.

Click the image to open in full size.

Click the image to open in full size.
__________________
XRKaudio https://www.etsy.com/shop/XRKAudio
  Reply With Quote
Old 6th December 2017, 07:19 AM   #9
juma is offline juma  Germany
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Berlin
Quote:
Originally Posted by ElFishi View Post
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 ?
__________________
Please don't use private messages for question related to audio projects. It defeats the purpose of the forum.

Last edited by juma; 6th December 2017 at 07:21 AM.
  Reply With Quote
Old 6th December 2017, 10:26 AM   #10
ElFishi is offline ElFishi  Europe
diyAudio Member
 
ElFishi's Avatar
 
Join Date: May 2008
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?
  Reply With Quote

Reply


A small FET gainstage is the spirit of the Pass B1Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Tube gainstage for newbie siouxiebuff Tubes / Valves 7 20th February 2015 06:07 PM
Gainstage for soundcard. MammaMumin Chip Amps 5 31st March 2008 08:14 PM
Inverting FET openloop gainstage Onvinyl Solid State 28 19th October 2007 01:07 PM


New To Site? Need Help?

All times are GMT. The time now is 10:16 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki