[sdiy] Later MS-20 filter with 2164

[David G. Dixon]

> > From his analysis it would appear that the diodes do very little in the
> > later filter, but are very important in the early filter.
> 
> Not quite. In the first filter version the diodes are at the output of the
> filter from where the signal is sent to the first capacitor and the VCA.
> In the second filter the diodes are only in the feedback path. They are
> still important in getting the right resonance sound (as they clip the
> feedback signal). The effect also depends heavily on the input signal
> amplitude.

Yes, of course.  There are three small-signal diodes in series, so that's a
clip point of about 2V.  I haven't analyzed the original filter (I wanted
to, but I haven't have much time to spare on the ol' hobby lately), nor have
I read the various analyses all that closely, but the levels through the
original filter must not be terribly high if the clipping point is 2V.  I my
"Korgasmatron" concept (catchy name, no?), the signal levels are whatever
they are on the input signal, and the self-oscillation amplitude is +/-6.7V,
as I mentioned earlier.  In that regard, my version is a much more
"conventional" filter.  All it retains of the original is the transfer
function (which is a lot, I think).

Of course, having said all that, I'm not sure how much clipping the feedback
will actually contribute to the overall sound, other than to make the
ringing a bit harsher, which doesn't sound all that appealing to me.