[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

I did the panel wiring for one of the two filters on my 2164 Korgasmatron
VCF board, and it works exactly as simulated.

The filter sounds good.  I don't have any other MS-20 clone (or the real
deal) to compare it to, but I'm pretty happy with it.  The Resonance and
Amplitude knobs do exactly what they are supposed to, and the resulting
waveforms look just like the simulations.

At moderately high resonance settings, with a sawtooth input, a little
"blob" of resonance peaks forms about halfway down the ramp.  It looks a bit
like what you'd get if you put a high frequency sine wave into a VCA and
modulated it with a triangle wave, only it's superimposed on the sawtooth.
My simulations confirm that this is exactly what is supposed to happen.
It's very different from a COTA filter (or its 2164 VCA-based equivalent).

I'm also happy to report that the zeners seem to be doing more or less
exactly what the strings of diodes would do in their place (again, according
to simulation).

Concerning the notch setting: It is just the sum of the LP and HP responses,
and that's what it sounds like as well.  It's a good sound.  You can
modulate it with an envelope like a LP filter, but the buzzy HP response
stays superimposed on top.  It's a beefier, raunchier sound than LP alone.
It also sounds really good with high resonance.

One thing, though: I can't seem to reproduce the "formant" sound with the
notch setting at high resonance like that soundclip on the MOTM 420 webpage.
All the other clips I think I can emulate.  I'd like to know exactly what
was done to get that sound, to see if my filter can do it as well.  I think
they might be using two filters.  When I get the other half wired up to the
panel, then I should be able to get all kinds of interesting timbres with
both filters in series.  However, I won't be able to do that until I return
from Saskatoon.