[sdiy] Later MS-20 filter with 2164

[David G. Dixon]

Hey Team!

I think I've figured out how to clone the later MS-20 filter (the OTA-based
one on the famous "KLM-307" board) with 2164 VCAs (and thus, no expo
converters).  The parts count is very similar to the original: 4 VCAs
(supplied by one 2164 chip) and 6 opamps (4 for the integrators (yes,
integrators!), and 2 for the feedback amps, which are inverting in my
design).

The only difference is that the output signal is inverted relative to that
of the original filter.  I don't think that this matters at all, but I
wanted to hear your opinions.  It would be easy enough to add an inverter
between the two filters, although this would necessitate another chip, which
I'd like to avoid.

Another thing about this design is that the capacitors are no longer
floating, which should obviate the need for all those pulldown resistors in
the original, as well as the ac coupling cap between the two filters, and
probably much of the output circuitry as well (I'm guessing).  And, of
course, the signal-to-noise ratio should be much higher without those
attenuation factors of 455 for feeding the OTAs.

I'd like to breadboard (or just etch a PCB and build) this design, but I
won't be able to for at least two weeks.  When I do, I'll be looking for a
volunteer with an MS-20 to do some comparison tests.