Early Threshold 400a Please Help! - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 16th January 2006, 01:58 AM   #1
diyAudio Member
Join Date: May 2004
Location: Toronto
Default Early Threshold 400a Please Help!

Hi,Happy New year to Everybody!
Almost two years ago I purchased a Threshold 400a from ebay with the intentions of restoring it since the seller said that it would only power up.As a very close friend of mine and I started to examine the inside of the unit and replacing old worn out parts with new ones we came to a full stop.To our suprise the were alot of parts missing from the boards in both channels.We then searched in the pass labs forums and found a Threshold 400a shematic that was posted sometime ago!We got happy for a while as the project took life again
.But again we stopped as the shematic did not match the specific amp.
We soon realized that my 400a is a very early Threshold model and is very different from the later units.I wanted to post some pictures of my 400a for some time now but I could not figure out how to attach them here in the diyAudio site.However there is a Threshold 400a on an ebay auction right now which is identical to my 400a(only a few serial numbers apart).Item number from the auction is 5853092668.Check it out.The seller has taken some nice pictures of the unit inside out.
Please does anyone has any information about tis early of an amp?A shematic for this specific unit would be so much appreciated!!!!!
Please guys help me bring my 400a back to life!Any help would be so much so much appreciated!
Thank you for your time and a happy new year!
  Reply With Quote
Old 16th January 2006, 02:32 PM   #2
diyAudio Member
Join Date: Mar 2004
Location: Everett, WA
Send a message via Yahoo to RetroAudio
Since you're into rebuilding, why don't you take some initiative and come up with your own schematic. While it's apart, it's easy to trace the components and see what they are doing. I've done this with an "early" version as well as with the "later" version and can tell you that schematics off of here do not line up with reality. It seems the version you refer to is yet another version, looks like there are at least 3 at this count. Another advantage of taking the initiative is that you'll more likely overcome any misinformation found in these forums. I was wondering myself what was up with that 400a listed since the supply wiring looks non-stock. Please don't take this advice in a negative way, it's just that help really is limited and what little is available is not fully accurate. If you wait on assistance you'll more than likely never get the amp done. I'd help if I could but that version is also new to me.
  Reply With Quote
Old 17th January 2006, 09:32 PM   #3
anatech is offline anatech  Canada
diyAudio Moderator
anatech's Avatar
Join Date: Jun 2004
Location: Georgetown, On
Hi kekropas,
Try giving these people a ring. I have found them to be very helpful in the past.

  Reply With Quote
Old 14th March 2006, 05:20 PM   #4
diyAudio Member
R-K Rønningstad's Avatar
Join Date: Jan 2006
Location: Oslo
Default Threshold 400A schematics

I believe this to be the early version of the 400A schematics. I got it on the paper it is scanned from, cirka 1979/80.
I even have a letter from Mr. Pass saying the idle current should be 900 mA.


Attached Files
File Type: pdf threshold_400a.pdf (51.0 KB, 277 views)
  Reply With Quote
Old 14th March 2006, 06:08 PM   #5
The one and only
Nelson Pass's Avatar
Join Date: Mar 2001
That would be the earlier version. You can see a later
cascoded version on www.passlabs.com/np
  Reply With Quote
Old 18th March 2006, 04:51 AM   #6
diyAudio Member
R-K Rønningstad's Avatar
Join Date: Jan 2006
Location: Oslo
Default CAS1, 400A and 4000 and variable bias

Seems the Threshold 4000 was a kind of "mix" of a CAS1 (schematics attached) and a 400A (early version), with its cascoded input and class A output design. Later the 400A got the 4000's input circuitry(?).
I understand from your writing, that Threshold dropped the sliding bias class A concept and went on to the Stasis concept partly due to Far East plagiary, with much to low bias current. Nowadays we don't see this scheme at all. But maybe it could be worthwhile to re-explore.

Attached Files
File Type: pdf threshold cas1.pdf (74.1 KB, 204 views)
  Reply With Quote
Old 20th March 2006, 12:44 PM   #7
diyAudio Member
R-K Rønningstad's Avatar
Join Date: Jan 2006
Location: Oslo
Default 400A mains switch

I am at the verge of changing the defect mains switch on my 400A. What is the manufacturer of the original switch, and what is the type number? Are there possibly some direct replacement?

Best regds
  Reply With Quote
Old 23rd March 2006, 04:23 PM   #8
diyAudio Member
R-K Rønningstad's Avatar
Join Date: Jan 2006
Location: Oslo
Default 400A mains switch

Seems I can reply to this myself. I opened up the amp, and the switch is made by Heinemann Electric Co. in Trenton N.J., U.S. of A. The switch is labeled "Heinemann Electric Co. Trenton, N.J. Cat No JC1-A8-BB-A 240 VAC 50/60Hz T.D.#20 HOLD 9A TRIP 11.25A"

Seems a present day rocker circuit breaker is the # JC1SA3L001010F. I'll inform you if I get a response from the manufacturer (Eaton Electrical).

  Reply With Quote


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
Threshold 400A Airedale Pass Labs 13 8th January 2016 02:21 PM
Threshold 400a Rolexmonger Pass Labs 1 22nd March 2007 08:03 AM
Threshold 400A HELP !! Airedale Pass Labs 2 16th February 2005 10:56 PM
Threshold 400A deafbykhorns Pass Labs 1 29th August 2004 06:50 PM

New To Site? Need Help?

All times are GMT. The time now is 12:38 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2