[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

I got the other half of my 2164 Korgasmatron wired up lastnight, and gave it
a go.  It all seems to work swimmingly.  I got lots of formants with both
filters connected in series.  My synth was screaming "yeah yeah yeah" for
half an hour!  All I have to do now is trim each side for 1V/oct and stuff
it into my cabinet.  However, I'll definitely want to normal filter A's CV
inputs to filter B, and also normal filter A's output to one of filter B's
inputs.  However, I need to order the switching jacks.

I also did a simulation of a standard MS-20 filter clone design based on
LM13600 with its built-in Darlington buffers, and the standard resonance
circuit with its two antiparallel chains of triple diodes.  This simulation
gives virtually identical results to my filter, with two exceptions:

1.  The Darlingtons add a lot of DC bias, definitely requiring a coupling
cap on the output -- mine doesn't.

2.  The standard resonance circuit doesn't give a lot of resonance in LP
mode unless the signal input level is turned very low (this is not an issue
in HP mode).  At "normal" signal levels of +/-5V, with square or saw input,
the signal swamps the resonance, so that all you get at full resonance is a
little "horn" right after the rising edges -- no siney wiggles.  I can
simulate this with my filter by turning the adjustable zener voltage
("Amplitude") all the way down.  However, if I crank it up, I get robust
resonance at normal signal levels, and even clipping distortion at high
signal levels.

Is this lack of resonance in LP mode perceived as a problem in MS-20 filters
and clones?