Stereo amplifier sqeals at idle, distorts on one channel? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 21st December 2010, 08:20 PM   #1
nac134 is offline nac134  Canada
diyAudio Member
 
Join Date: Sep 2008
Default Stereo amplifier sqeals at idle, distorts on one channel?

I have a pair of Luzman M120a stereo amps that I run as dual bridged monos. The Left channel on one of them has started to emit a mild squealing sound, even at idle, and even if no source is connected. It also sounds very distorted when I play music through it.
I pulled a bunch of transistors off of the input stage, and tested them - all fine. I put them back and the problem went away for a couple days, then suddenly came back. Does anybody have an idea what might be causing this?
  Reply With Quote
Old 21st December 2010, 08:27 PM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
Squealing is oscillation. This is probably a sign the units need servicing for their age, probably needing complete replacement of all electrolytic capacitors.
  Reply With Quote
Old 21st December 2010, 08:44 PM   #3
nac134 is offline nac134  Canada
diyAudio Member
 
Join Date: Sep 2008
I should have mentioned that the caps were replaced about a year ago. I have 4 channels all together in 2 amps, but only one is doing it. Should I be looking for a faulty replacement cap?
  Reply With Quote
Old 21st December 2010, 09:55 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
A compensation capacitor gone open circuit?
  Reply With Quote
Old 21st December 2010, 10:00 PM   #5
nac134 is offline nac134  Canada
diyAudio Member
 
Join Date: Sep 2008
Im sensing a "capacitor" theme here. Can you test caps without taking them off the board?
  Reply With Quote
Old 21st December 2010, 10:21 PM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Not easily. You could perhaps check a compensation cap by temporarily adding an extra one in parallel - about the same value or a little smaller. If this solves the problem then swap the cap for the correct value. You would need a circuit diagram and the ability to read it. Typically there will be Cdom across the VAS stage, and possibly a lead-lag network at the input stage collector and a Zobel network at the output.
  Reply With Quote
Old 21st December 2010, 10:49 PM   #7
nac134 is offline nac134  Canada
diyAudio Member
 
Join Date: Sep 2008
Thanks for your help. I do have the schematic and service manual. However, my knowledge of schematics is rudimentary. For example, I don't know what a Cdom is. It will probably be easiest for me to just get out the soldering iron and start pulling of caps and testing them.
  Reply With Quote
Old 21st December 2010, 10:50 PM   #8
nac134 is offline nac134  Canada
diyAudio Member
 
Join Date: Sep 2008
Actually got a 3rd degree burn last week doing that with the transistors.
  Reply With Quote
Old 21st December 2010, 11:00 PM   #9
Banned
 
Join Date: Jan 2005
Cdom is a capacitor that goes between base and emmitter of the VAS (Voltage Amplification Stage)
  Reply With Quote
Old 22nd December 2010, 07:52 AM   #10
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Are there any switches, connectors or any other form of contact in the path?
  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
hight idle current HARMAN amplifier ericb Solid State 0 2nd March 2008 04:09 AM
Automatic idle current amplifier senicz Chip Amps 3 1st March 2006 01:17 AM
Right Channel not working on Stereo reciever nicku1302 Parts 1 29th November 2005 02:32 AM
question for those using subs with 2 channel stereo Jim85IROC Subwoofers 15 26th March 2004 02:38 PM
LA4743B Car stereo BTL 4-channel (45 W x 4) power amplifier IC emuman100 Chip Amps 2 15th March 2004 06:21 AM


New To Site? Need Help?

All times are GMT. The time now is 12:33 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