[AMRadio] push pull output XFMRs and neg feedback

John Coleman jc at pctechref.com
Mon May 18 19:16:10 EDT 2009

        I had a chance to add the 3300 ohm resistor and the .001uf capacitor to the circuit at the grid of the phase splitter but it did not help or if it did it was negligible.  The oscillation is at 8000 to 10000 cps.  I think I had originally said 18000 but the scope says different and it is audible even for an old smell like me.  BTW I don't have any pre-emphasis or de-emphasis in the FB path it is strictly a resistive pad.

I'll be running some test soon and will let you know the results.

BTW the output XFMRs are Thordarson 22S63  5000 ohm ptp primary --  8 and 16 ohm secondary --  rating 40 watts

John Coleman, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim Candela
Sent: Friday, May 15, 2009 11:06 PM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] push pull output XFMRs and neg feedback


    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:


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:

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


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