Power amp outout stage self oscillation problem - 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 17th November 2008, 06:28 PM   #1
diyAudio Member
 
Join Date: May 2005
Location: HK
Default Power amp outout stage self oscillation problem

Hi Folk,

I am much depress on my just finished class A 50W Power AMP. Because its front and mid stage mounting on the same PCB and checked no problem. However, connect it with output stage with 6 pairs of MOSFET (Hitachi J56 / K176) per channel then the AMP have small "giisss" noise (not from speaker but it happen it some component). I think it should be oscillation by it self.

Just want to know how to solve ?

CK
  Reply With Quote
Old 17th November 2008, 06:31 PM   #2
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Got a scope????

You may need one......
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 18th November 2008, 02:01 AM   #3
diyAudio Member
 
Join Date: May 2005
Location: HK
I have a 100MHz scope and signal generator. How to find where or which thing incur the oscillation.

Just find some circuit such output stage FET drain-gate and gate-source parallel with a pico value of cap. What it the purpose of it ?
  Reply With Quote
Old 18th November 2008, 02:20 AM   #4
eyoung is offline eyoung  United States
diyAudio Member
 
eyoung's Avatar
 
Join Date: Aug 2007
Location: Sunny SC,USA 15 min south of Charlotte NC
Hello...

Could be a cold solder joint making a noise. Maybe a schematic
and some idea of localization of noise(I have heard resistors ring).
go back to step one of the build and double check placement and polarity of parts and all solder joints. check the basics for DC offset and bias.Look at the output on a scope and trace it backwards .

keep us posted.

Regards, Elwood
__________________
"Lead me not into temptation...I can find it myself."
  Reply With Quote
Old 18th November 2008, 05:28 AM   #5
diyAudio Member
 
Join Date: May 2005
Location: HK
Thanks your input and I will do this on week end.
  Reply With Quote
Old 18th November 2008, 02:27 PM   #6
diyAudio Member
 
unclejed613's Avatar
 
Join Date: Dec 2006
output inductors "sing" as well. it's a magnetic effect when large currents flow through the inductor. if this is happening with no signal applied, it could be oscillation. output inductor "singing" is usually heard while driving a signal into a dummy load.
__________________
Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net
spammer trap: spammers must die
  Reply With Quote
Old 18th November 2008, 03:12 PM   #7
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Leave one channel measuring the oscillating output, then use the other to poke around certain areas of the circuit where such an oscillation might originate. If the oscillation is very fast frequency, the second scope probe may change it in some way via the internal components and can help point you toward the source of the problem.

I have found sometimes too small gate resistors with/or no gate Zobel and poor layout can cause a mosfet amp to become a RF transmitter. I have set fire to the 10 Ohm output Zobel resistor through a small series 100nf cap a couple of times.(was using cheap carbon resistor )
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 18th November 2008, 03:23 PM   #8
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Do you have gate resistors on each mosfet? Value? NJormally it's the N-channel device that oscillation and also above a certain current level, easy seen with an oscilloscope. The oscillation is local, only the mosfet oscillates.
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 18th November 2008, 06:07 PM   #9
diyAudio Member
 
Join Date: May 2005
Location: HK
I do not use inductor at end of speaker output. It seems a resistor generate this sound. Most likely some of it is wire wound type.

I know Zobel is a type of filter. Not sure it will usually exist on the speaker output between + and ground. And Zobel is around 5 ohm in series with 0.1 u cap. But I am not sure why it exist and sometimes no need apply on some AMP.

I use 470 ohm in series with the gate of output FET. Suppose my AMP is self-oscillation then power on it again to trace the problem. Due to the fault finding can spend much time. Will it damage to the AMP ? How can I do ?
  Reply With Quote
Old 19th November 2008, 03:24 AM   #10
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Zobel is a filter that keeps RF from entering the feedback loop via the speaker wires and output node, assuming you take your feedback from the output. It is used, and sometimes with an output inductor || small resistor for stability. If your amp has a global feedback loop taken from the output you should at least have a Zobel filter at the output to help separate the noisy RF world from your little circuit.
__________________
All the trouble I've ever been in started out as fun......
  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
Quad KT88 oscillation problem chris.busch Tubes / Valves 15 26th January 2009 02:34 AM
CDM-12 Problem - Whistling / Oscillation poynton Digital Source 4 8th December 2005 08:50 PM
oscillation problem ABO Solid State 26 17th June 2005 10:26 AM
Help!! Oscillation on - input of CD Player Filter Stage primalsea Digital Source 2 31st January 2005 01:55 PM


New To Site? Need Help?

All times are GMT. The time now is 08:19 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