Just fired up an Aleph J circuit with some ebay jfets - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

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 15th May 2011, 02:45 PM   #1
diyAudio Member
 
Join Date: Jan 2002
Location: Cleveland ohio
Default Just fired up an Aleph J circuit with some ebay jfets

I just fired up an Aleph J channel...built on Brian's excellent boards...

The matched Jfets are off of EBay...from a guy in isreal. Are there any in- circuit measurements that I can make to determine if the jfets are genuine?
My correspondence with the supplier makes me feel confident that the 2sj74 bl are genuine.

The circuit biases up...is drawing the correct amount of current. offset is .3 volts (my aleph 30's are about the same offset)

anything else I can check??? I have an O-scope, a signal generator and a good fluke VOM.

Thanx
  Reply With Quote
Old 15th May 2011, 03:29 PM   #2
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
in circuit is fine, you'll load the cct a bit with the meter, so the parts after it may not get the exact current, but that doesnt matter so much for this. sounds to me like they are ok, as generally the fakes are showing considerably higher idss.
  Reply With Quote
Old 15th May 2011, 03:38 PM   #3
diyAudio Member
 
Join Date: Jan 2002
Location: Cleveland ohio
QUSP...thanx for your quick response....

How do I check idss???
  Reply With Quote
Old 15th May 2011, 04:54 PM   #4
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
in circuit you can only check the current that it is putting out, or its vgs for your gate current, but to check idss you really need to pull it out. for p-channel jfets check idss by connecting gate and source together and through a 9v battery, or a 10v supply and connect the drain to ground through a 100r resistor and measure the voltage drop across the resistor.
  Reply With Quote
Old 15th May 2011, 04:55 PM   #5
qusp is offline qusp  Australia
diyAudio Member
 
qusp's Avatar
 
Join Date: Oct 2009
Location: Brisbane, Australia
if you are asking for matching, its more useful for most applications to match vgs at the same gate current you will use

an excellent article by the jfet king Erno Borbely on jfets and matching them

Last edited by qusp; 15th May 2011 at 05:00 PM.
  Reply With Quote
Old 15th May 2011, 05:10 PM   #6
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Quote:
Originally Posted by audionut View Post
The circuit biases up...is drawing the correct amount of current. offset is .3 volts
This should be measured after an hour or so, as the circuit needs to warm up
to its final offset figure. If you need to adjust it, best place is the resistor
off the Drain of the diff input pair. You can put a 500 ohm pot in place of
the 392 ohm resistor.

  Reply With Quote
Old 15th May 2011, 11:59 PM   #7
diyAudio Member
 
Join Date: Jan 2002
Location: Cleveland ohio
Thank you Master...

I measured the Idss of some one of the other pairs. Came out to 8.7 ma...so I think that means these are probebly genuine...
  Reply With Quote
Old 16th May 2011, 12:06 AM   #8
anilva is offline anilva  India
diyAudio Member
 
Join Date: Mar 2004
Location: Bangalore
Quote:
Originally Posted by Nelson Pass View Post
This should be measured after an hour or so, as the circuit needs to warm up
to its final offset figure. If you need to adjust it, best place is the resistor
off the Drain of the diff input pair. You can put a 500 ohm pot in place of
the 392 ohm resistor.

Papa, aren't we putting a pot in place of R8 (1K) to adjust the offset? That's what I read in the Aleph J thread and implemented....
__________________
DIY F5, F5T, F6, Aleph J, Fostex 206 based BLH, Altec Onken with 515B LF & 288H HF, Technics SP10 MKII & EMT938 turntables
  Reply With Quote
Old 16th May 2011, 02:30 AM   #9
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
You can do either. Adjusting R8 has the slight advantage of not having
any effect on the open loop gain, but I was too lazy to look up the part.
I remembered the 392 ohm resistor instead, which is where I put the
pot on the J2.

  Reply With Quote
Old 12th January 2012, 09:15 AM   #10
Thago is offline Thago  Belgium
diyAudio Member
 
Join Date: Nov 2006
Location: Asse, Belgium
Hi

My Aleph 2 functioned flawlessly for over 3 years, but now I have a problem with one of them: the 392 ohm resistor (drain of the input pair) just disappeared into smoke. Does this mean that (one of) the IRF9610's is broke? Or do I have to search elsewhere?

Last edited by Thago; 12th January 2012 at 09:17 AM.
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Guidelines for switching N-channel JFETs to P-channel JFETs (and vica versa)? Test Electrix Construction Tips 9 24th January 2011 06:22 PM
Fired up my Aleph 30 for the first time tonight... hifinerd Pass Labs 2 30th August 2010 10:47 PM
JFETs in series in what looks like a common source circuit? teemuk Solid State 1 6th June 2008 03:06 PM
Dual JFETS on EBAY Andypairo Swap Meet 7 11th December 2003 11:04 AM


New To Site? Need Help?

All times are GMT. The time now is 01:09 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2