How to test for oscillations? - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 October 2004, 11:36 PM   #1
Jye is offline Jye  Australia
diyAudio Member
 
Jye's Avatar
 
Join Date: Aug 2003
Location: Brisbane, Australia
Question How to test for oscillations?

Hello,

I have been making a 100W amp using the parallel circuit in the Overture Series High Power Solutions application notes and fired it up for the first time last night. It worked first time and everything is fine apart from some crackling that I am putting down to a bad RCA connection and the fact that it heats up to about 80 degrees C in about 30 seconds I have a feeling it could be due to some high frequency oscillations, since there was no audible noise.

So my question is how do I test for these? And I have access to a CRO and signal generator.

Cheers
Jye
Attached Images
File Type: jpg dcp01621a.jpg (86.8 KB, 475 views)
  Reply With Quote
Old 19th October 2004, 03:14 AM   #2
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
one thing about the heat-sink you picture == they are one heck of a lot LESS efficient than one in which the fins are not "interrupted" -- and if you aren't using a fan the C/W rating is much less than you would suspect.

you can see the oscillation on an oscilloscope -- it helps if you have a scope with FFT -- then you can peg the frequency in a couple seconds -- if it is oscillating make sure that the lead lengths are as short as possible (tack the resistors directly to the chip pins), incorpoate the 0.7uH choke which National recommends, make sure that the filter caps on the power supply pins are bypassed with 100nF mylars. Bypass capacitors as seem to be shown in your photo (being several mm from the supply pins on the amp chip) are about as useful as teats on a bull.
  Reply With Quote
Old 19th October 2004, 05:30 AM   #3
Jye is offline Jye  Australia
diyAudio Member
 
Jye's Avatar
 
Join Date: Aug 2003
Location: Brisbane, Australia
Thanks Jackinnj,

I have placed some filter caps on the supply pins of the chip but the oscillations are still present. On the scope the oscillations are at about 150kHz and 5vp-p. One thing I did notice is if I turn the power supply down to about +/- 9v the oscillations disappear and all is well

I will try and get some chokes and try that out tomorrow. But in the mean time any other comments are much appreciated.

Cheers
Jye
  Reply With Quote
Old 19th October 2004, 06:02 AM   #4
diyAudio Member
 
Join Date: Feb 2001
Location: Silicon Valley
Send a message via Yahoo to thespeakerguy
I only see the bulk electrolytics. Any low ESR high frequency caps on the supply pins?
  Reply With Quote
Old 19th October 2004, 06:08 AM   #5
Jye is offline Jye  Australia
diyAudio Member
 
Jye's Avatar
 
Join Date: Aug 2003
Location: Brisbane, Australia
I have just placed some on the under side of the board
  Reply With Quote
Old 19th October 2004, 12:40 PM   #6
sek is offline sek  Germany
diyAudio Member
 
sek's Avatar
 
Join Date: May 2003
Location: Berlin
Hi,

Quote:
One thing I did notice is if I turn the power supply down to about +/- 9v the oscillations disappear and all is well
As of the National data sheet, the chip switches off (activates it's undervoltage-protection) at +-9V. This would mean that your oscillations are entirely independent of the supply voltage. Is the output (test/audio) signal still present when the oscillation stops at +-9V?

If not, you already have determined that the oscillation is affected by the protection circuit, which could narrow the suspect components/wires down when looking at the internal block diagram.

Have you tried improved input filtering (series 1kR, parallel 220pF) to rule out any EMI induced swinging? Next would be bypassing the feedback series resistor (e.g. 100pF in parallel upon it).

Hope this helps,
Sebastian.
  Reply With Quote
Old 19th October 2004, 12:42 PM   #7
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
are you using the 0.7uH choke? You can wind 15 turns of #20 enamelled wire around a 2 watt carbon resistor -- if you don't have magnet wire you can use the regular RS, PVC insulated wire, it just won't look pretty.

you could try placing a 220pF capacitor across the input pins -- but this is mainly to keep Radio Frequency Interference out of the chip.
  Reply With Quote
Old 19th October 2004, 07:07 PM   #8
sek is offline sek  Germany
diyAudio Member
 
sek's Avatar
 
Join Date: May 2003
Location: Berlin
Erm, didn't I just write that? Except for the winding suggestion!
Edit: ah, we've been editing in parallel. So the suggestion must be very good.
  Reply With Quote
Old 19th October 2004, 07:37 PM   #9
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
we've been editing in parallel -- sometimes I leave the message open while I see how something is working. sorry to be duplicative.
  Reply With Quote
Old 20th October 2004, 12:33 AM   #10
diyAudio Member
 
BlackDog's Avatar
 
Join Date: Feb 2004
Location: Wellington, New Zealand
Quote:
Originally posted by jackinnj

you could try placing a 220pF capacitor across the input pins -- but this is mainly to keep Radio Frequency Interference out of the chip.
Not really related to this post, but I can confirm that the 220pF cap across the inputs works a treat if you live next to the regional radio transmitter, like me!

Steve
  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
oscillations IN OPAMP help ???? prorms Solid State 3 30th September 2007 12:17 PM
Getting rid of oscillations in the preamp engels Tubes / Valves 11 10th June 2007 09:23 PM
Oscillations in P3A ?? Stormo Solid State 3 20th January 2003 09:34 PM
amplifier oscillations lowfi Pass Labs 33 29th May 2002 07:32 PM
Aleph 3 oscillations Koy Pass Labs 4 22nd April 2002 04:56 PM


New To Site? Need Help?

All times are GMT. The time now is 06:10 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