[sdiy] moog high pass flter

[Grant Richter]

FWIW, you comment knocked a few loose nuts out of the junk pile...

The Buchla 200 doesn't have a resonant lowpass filter. At first I thought
Don just didn't like resonant filters, then I realized he designed it to
synthesize a resonant lowpass by putting a tracking 291 bandpass in parallel
with the 292 lowpass and synthesizing the resonant "hump" with the bandpass.

So we could call it a "compound" filter where the total response is a
product of two or more filters in series or parallel.

The Moog 904 A,B and C is compound filter where the filter coupler is used
to place the lowpass and highpass in series or parallel to form bandpass or
band reject filters.

The ARP Odyssey is a compound filter with the 24 dB/Oct lowpass followed by
a passive highpass.

The Moog "Satellite" synth has a compound filter composed of a state
variable and a 12 dB/Oct transistor ladder lowpass.

In your typical exponential converter, one of the transistor bases is
grounded while the other is modulated with the 18 mv/Oct voltage. Jurgen and
I have both played with expo converters where the transistors are "stacked"
and the individual base voltages modulated to give offsets from the common
input. In a 4 pole lowpass this allows you to slide the other poles around
to give variable slope rolloff.

A "real" compound filter would have a common exponential input with variable
offset for each filter section. Then each section could be set to lowpass,
bandpass or highpass and the sections switched into either series or
parallel routing.

I'm sure someone has already done this and is just waiting to show us the
schematics ;^)