Stereo amplifier sqeals at idle, distorts on one channel?

nac134 · 21st December 2010, 08:20 PM

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?

jaycee · 21st December 2010, 08:27 PM

Squealing is oscillation. This is probably a sign the units need servicing for their age, probably needing complete replacement of all electrolytic capacitors.

nac134 · 21st December 2010, 08:44 PM

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?

DF96 · 21st December 2010, 09:55 PM

A compensation capacitor gone open circuit?

nac134 · 21st December 2010, 10:00 PM

Im sensing a "capacitor" theme here. Can you test caps without taking them off the board?

DF96 · 21st December 2010, 10:21 PM

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.

nac134 · 21st December 2010, 10:49 PM

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.

nac134 · 21st December 2010, 10:50 PM

Actually got a 3rd degree burn last week doing that with the transistors.

Tekko · 21st December 2010, 11:00 PM

Cdom is a capacitor that goes between base and emmitter of the VAS (Voltage Amplification Stage)

Elvee · 22nd December 2010, 07:52 AM

Are there any switches, connectors or any other form of contact in the path?

21st December 2010, 08:20 PM	#1
nac134 diyAudio Member Join Date: Sep 2008	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?

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21st December 2010, 08:27 PM	#2
jaycee 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.

21st December 2010, 08:44 PM	#3
nac134 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?

21st December 2010, 09:55 PM	#4
DF96 diyAudio Member Join Date: May 2007	A compensation capacitor gone open circuit?

21st December 2010, 10:00 PM	#5
nac134 diyAudio Member Join Date: Sep 2008	Im sensing a "capacitor" theme here. Can you test caps without taking them off the board?

21st December 2010, 10:21 PM	#6
DF96 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.

21st December 2010, 10:49 PM	#7
nac134 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.

21st December 2010, 10:50 PM	#8
nac134 diyAudio Member Join Date: Sep 2008	Actually got a 3rd degree burn last week doing that with the transistors.

21st December 2010, 11:00 PM	#9
Tekko diyAudio Member Join Date: Jan 2005	Cdom is a capacitor that goes between base and emmitter of the VAS (Voltage Amplification Stage)

22nd December 2010, 07:52 AM	#10
Elvee diyAudio Member Join Date: Sep 2006	Are there any switches, connectors or any other form of contact in the path?

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