[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

> Does your 2164 VCF sound like the LM13600 based Korg circuit?

Hey, Stewart, I'll let you know as soon as I build it!  I'm planning on
getting it done Monday night.

It is essentially the same topology, only done with 2164 VCAs and
integrators rather than 13600 OTAs and buffers.  This required some fresh
thinking, and the actual schematic doesn't look very similar to the original
at all.  However, the transfer functions are identical.  Also, I've retooled
the resonance sections.  The originals had anti-parallel triple diodes in
the feedback loops of the resonance amps.  I replaced these with
back-to-back 3.3V zeners and panel pots.  These should give sine waves at
self-oscillation which can be varied from about 3V to 15V (peak-to-peak).
Also, at high resonance, one should be able to vary the volume of the
ringing relative to the input signal.  I don't know how this will actually
sound, but it's another parameter to play with.

And, of course, the filter should be very quiet, with very low CV
bleedthrough.  I'm hoping it's going to be better than the original, but
then again it might totally blow.  We shall see!