[sdiy] 2164 Korgasmatron VCF

[David G. Dixon]

I think I've figured out the panel for my 2164 Korgasmatron VCF.  It's
amazing how laying out a panel sets strict limits on the functionality of a
module.

I've laid out the PCB for a three-input mixer, AC/DC input coupling switch,
independent coarse and fine cutoff controls, 1V/oct + 2 CVs (one standard,
one +/-), resonance controls and sine amplitude controls for both HP and LP
stages.  However, to keep the panel from getting completely out of control,
I've eliminated fine cutoff and standard CV controls (retaining the 1V/oct
and +/- CV controls).  I've also given separate HP and LP outputs.  This
gives 12 knobs, 10 jacks and 1 switch (!).  The panel is 3U wide in
pseudo-MOTM format.

I've been looking around at some other MS20 filter clones.  The "Analogue
Solutions" one looks similar to mine in concept (albeit without the sine
amplitude control, no mixer, and with an onboard VCA).  On the other end of
the spectrum is the MOTM 420, which only emulates half of the MS20 filter,
with a switch for selecting HP, LP, and N modes.  The website says that the
HP and LP are 12dB and the N is 6dB.  However, in the original, the HP is
actually 6dB at zero resonance, and increasing the resonance gives adds a
12dB BP response to the HP in a way which actually renders it difficult to
classify in those terms.  In any case, to emulate a complete MS20 filter
requires two MOTM 420s (which, at $339 each, makes for a pretty expensive
filter indeed -- mine will cost about $90 to build).

I'd like to hear some opinions on how useful the HP/LP/N switch is on the
420.  The sound samples are indeed impressive.  In the actual filter, one
can get an HP by turning the LP cutoff all the way up, and LP by turning the
HP cutoff all the way down.  N and BP responses are obtained by setting the
HP and LP cutoffs low/high or high/low, respectively.  N/BP widths can be
modulated by keeping one constant and modulating the other.  This gives an
incredible range of timbre control.  I'm just wondering whether it would be
worth it to make each stage multimode, or to leave them as is, as per the
original.