[AMRadio] push pull output XFMRs and neg feedback


Jim Candela jcandela at prodigy.net
Sat May 16 00:05:45 EDT 2009


John,

    I had similar issues with a stereo 6L6 amp I built many years ago. The oscillation would come and go, and it eventually killed the tweeters on my old Utah 12" 3 ways. The following article describes the problem well in a similar amplifier:

http://www.pmillett.com/file_downloads/chicago_100W_amp.pdf

See R1, and C1, and description on paragraph 1 on page 63.

I know you are experiencing the issue in only one channel. I bet if you swap transformers, the problem stays in that same channel. Adding the R1, C1 like the article might completely eliminate the problem.


Here is a more updated circuit designed by Patrick Turner. He uses a transistor current source in the phase inverter such that he gets perfect balance:
http://www.turneraudio.com.au/Integrated5050.htm

Patrick talks a bit about stability, and ultrasonic oscillations. A good read, as is his whole website.

Regards,
Jim
WD5JKO




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