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Subject: Reducing CV feedthrough on the MOTM-110

From: Roy Tate <roytate@...
Date: 1999-03-30

I was playing with the MOTM-110 tonight, and I noticed a fair
amount of feed-through from the Y audio input. The following
patch worked very well.

Modules: 2 VCOs, Ring Modulator (RM), Audio Mixer, Low or
High Pass Filter, VCA, 2 envelopes (EGs), one MOTM-940 or other
mult panel, and one CV inverter.

Patch VCO1 to RM's X input, VCO2 to a "mult", mult goes to Y and
inverter, inverted signal goes to audio mixer, RM out goes to
audio mixer, mixer out goes to VCF. One EG (ADSR) goes to
VCA, other EG (AD) goes to VCF and VCO1 freq mod (using a mult.),.

I'm ignoring pitch and gate patching info since a ring
modulator could sound good with none, one or both VCOs
patched up to a keyboard or CV converter.

The most important detail is that the inverted signal is mixed
at about 25% with the ring modulator signal. This inverted
signal "masks" the bleed-through of the Y audio input. You
can think of the Y feedthrough as +3 and the inverted signal
as -3, and add them together for "zero." In reality, the
feedthrough is reduced to a very small amount that is not
noticeable. Personally,I only heard a "flanging" of the
ring modulator output, and couldn't detect the original
"Y" VCO at all!

I suppose if I had 2 inverters, 2 audio mixers and alot of
patience, I could reduce feedthrough of the X input, but
it wasn't as noticeable since it was changing (because of
the envelope).

I realize that the MOTM line-up doesn't have all of these
modules, but I'm sure it will eventually. Actually, in
about 3 months I could probably patch a similar sound up
with only one non-MOTM module: the CV inverter. And maybe
the simple mixer will have an inverting output, so I could
use a mixer as an inverter.

If there are any math majors out there, don't criticize
my explanation unless you can think of a "simpler" way to
explain this (without explaining harmonics, FFT, etc.)

Roy Tate