[sdiy] Korgasmatron vs MS-20

[David G. Dixon]

Yesterday, I had the chance to put the 2164 Korgasmatron Dual VCF head to
head with a real live Korg MS-20 (the older, KORG35-based version).  I'm
happy to report that they sound virtually identical.  The only real
differences are that the original has a narrower (and, for performance at
least, more user-friendly) range of cutoff frequency, and a slightly
different relationship of frequency to pot knob position.

One thing my system (not the filter per se) does not emulate very well,
though, is the warmth of the MS-20.  I think this has to do primarily with
three things:

1.  The MS-20 is very noisy, and this noise originates well back in the
signal chain and is subsequently processed through the filters and VCA.  My
system is virtually noise-free.  Mixing a touch of white noise into the
filter along with the (very precise) sawtooth wave from the 2164 Expo VCO
brought the sound of my system somewhat closer to the MS-20.

2.  The oscillators in the MS-20 are probably not very precise.  Although we
didn't look at anything on a scope, it sounds to me as if the sawtooth wave
generated by the MS-20 is a fairly round-shouldered affair.  This produces
the equivalent of a slightly LPF'ed sawtooth wave going into the MS-20
filter which has very little of the "buzz" associated with a pure sawtooth.

3.  The VCA in the MS-20 is a single-transistor cheapo affair which, again,
is probably not very precise and thus adds a bit of warmth and distortion to
the signal.  My VCAs is based on linearized 2164 VCAs, and it is very
precise.

In any case, I am very happy with how my filter stacked up against the
original.  However, one of these days, I'd really like to develop a
rack-mounted clone of the MS-20.  It is a beauty!