Jim
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: [email protected] [mailto:[email protected]]
On Behalf Of Jim Candela
Sent: Friday, May 15, 2009 11:06 PM
To: [email protected]
Subject: Re: [AMRadio] push pull output XFMRs and neg feedback
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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