A40 rail voltage on output - 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 18th November 2004, 03:02 PM   #1
Eric is offline Eric  United States
diyAudio Member
 
Eric's Avatar
 
Join Date: Nov 2000
Location: Central PA, USA
Default A40 rail voltage on output

Hi Everyone,

A search on this topic revealed over 1300 unrelated posts, so I'm hoping someone can tell me what is going on here...

Last night I was fiddling around with slightly increasing the bias on my a40 amp when I noticed something strage. When the amp was plugged in and turned on with nothing connected to the inputs (they were not shorted either) I got full rail voltage on the output of one channel, but not the other.

The output returned to its normal 30mV DC offset level once the input was shorted, but when the input was open, there was 32v on the output- but only for one channel.

Any ideas as to what causes this? Then channels are identical and when connected to a my CD player, both play music just fine and don't exhibit any other strange effects...

A description of my amp is here

Thanks!
Eric
  Reply With Quote
Old 18th November 2004, 08:23 PM   #2
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
You need a resistor to ground on your input. If you already put
one there as seen in the schematic , you'll find it bad or not really
connected.
  Reply With Quote
Old 18th November 2004, 09:08 PM   #3
Eric is offline Eric  United States
diyAudio Member
 
Eric's Avatar
 
Join Date: Nov 2000
Location: Central PA, USA
Thanks, Nelson! I'll give it a look when I get home. Its entirely likely that I knocked it loose while I was adjusting the bias...

Eric
  Reply With Quote
Old 23rd November 2004, 03:58 AM   #4
Eric is offline Eric  United States
diyAudio Member
 
Eric's Avatar
 
Join Date: Nov 2000
Location: Central PA, USA
OK, I've done some more poking around and still haven't yet solved this problem...

Nelson, as you suggested, I checked the resistor from input to ground. I assume that you mean R22 (10 ohms) on the schematic that connects the input ground to the chassis. My design does not have this resistor on either channel as my chassis is made of wood.

The problem seems to be related to bias also. When there is no input connected to the bad channel, there is full rail voltage on the output and the output transistors do not heat up at all. The output transistors only heat up when an input is connected, or when the input is shorted. When this happens, the output drops down to around 40mV or so and dV across R16 through R19 (emitter resistors) measures approximately 0.57V (just about normal).

I replaced the feedback and bias circuit (Q6, D3, R11, R12, and R13) but this made no difference. The output transistors themselves seem to be fine (not blown or shorted) as measuring the resistance from base to collector, collector to emitter, etc reveals them to be nearly idential to the output transistors on the good channel.

Does anyone have any other suggestions?

I guess the next step will be a fishing expedition: comparing each component across the good and bad channels, looking for an oddball resistance measurement or something...

Eric
  Reply With Quote
Old 23rd November 2004, 04:13 AM   #5
jam is offline jam  United States
diyAudio Member
 
jam's Avatar
 
Join Date: May 2001
Location: Auburn, CA, USA
Eric,

Look at R2 and R3.

Regards,
Jam
  Reply With Quote
Old 24th November 2004, 02:55 PM   #6
Eric is offline Eric  United States
diyAudio Member
 
Eric's Avatar
 
Join Date: Nov 2000
Location: Central PA, USA
Thumbs up Problem solved!!

Nelson and Jam,

Many thanks for your help! Jam, thanks for beating me on head with what Nelson suggested! I went back and re-checked R1, R2, R3, and Q11 (which, I suspect, is exactly what Nelson meant - I was just too slow to figure it out...).

As it turns out, there was a bad solder joint somewhere in the mix. I am not sure exactly where it was because I just re-melted each solder joint looking for a cold solder. The bad channels is now behaving better and the DC offset is also much reduced- from 60mV down to 2mV. Both channels now get toasty warm with or without an input connected!

Thank you for your help and have a Happy Thanksgiving!
Eric
  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
Counterpoint SA20 Mosfet output rail voltage quinnling Tubes / Valves 0 21st November 2006 04:50 PM
supply rail voltage on 3875 output ? Buhl Chip Amps 10 9th November 2006 10:44 AM
Rail Voltage VS Output Power ???? Zero Cool Solid State 10 18th October 2004 09:38 PM
Is the output power tied to the rail voltage Shoog Pass Labs 2 15th August 2002 11:26 PM
Rail voltage compared to transformer voltage SteveG Solid State 4 23rd July 2002 12:50 AM


New To Site? Need Help?

All times are GMT. The time now is 10:35 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