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John Fields

On Wed, 09 Nov 2011 20:32:36 GMT, (Don Pearce) wrote:

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. wrote:

What does U4 do?

Provides a Vcc/2 local "ground" so I can use these op amps with a single
supply voltage.

More useful to let it oscillate as a square wave generator at 100kHz
or so, and rectify the output into a negative 15V rail. That way you
can run the op amps the way they are meant to be run.

---
Amen to that!

The beauty parts are that it doesn't need to be regulated, it only
needs to be smooth enough so that its ripples don't bump into the
output's low peaks, and it gets rid of all those damned coupling caps
and their frequency response killing reactances.

I was going to suggest that, since the mixer is going to be external
to the amp, he use a couple of wall-warts to get the dual supplies,
but I like your solution a lot better. :-)

--
JF