[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

Well, I finished the Korgasmatron board tonight.  The only problem is, to
get this thing panelized will require a densely-stuffed 4U wide panel in
MOTM format (16 knobs, 16 jacks, 2 switches), and even then I'll be leaving
a couple of little things off, like fine tuning.  However, I managed to fit
two identical MS-20-style filters onto a board only 2.1" x 6.6".  Like the
MOTM-420, there will be HP-N-LP switches on each (I decided that was pretty
cool after all, based on simulations and on the sound clips on the MOTM
website), and each will have a three-input mixer (which is probably
overkill).  In addition, there will also be a resonance zener amplitude pot
and an AC/DC coupling switch on each filter.

One other little change I made from the original was that I decided on an
overall gain of just less than 2 (56k/30k), with unity gain on the inputs at
100% attenuator settings.  I figured that, since the original resonance
circuit is limited to 1.8V (three diodes in series), with about 3.25x gain
on the output amp, I could achieve the same thing with my 3.3V zeners with
1.9x gain.  (Of course, I can dial in much larger zener voltages if
desired.)  The nice thing about that is that at a 50% setting on the
amplitude pot, the sine waves are exactly +/-5V at self-oscillation.  This
combination of input and output gains should also give a useful range of
variability of resonance vs input signal.

Hopefully I can get a prototype panel completed tomorrow night, and then I
can see if this thing was worth all the hype.