MOTM

Has anyone else tried using the ring modulator portion of their MOTM-110 as
a VCA.  I am trying to make a VCA myself out of a SSM chip and I am reading
up on VCAs.  Obviously, some of my reading source material was the MOTM-110
assembly guide.  In the theory of operation section, Paul explains how to
use the ring mod as a basic VCA.  I remembered reading this when I
assembled the 110 (wasn't that the first one?), but I had really forgotten
about it since.

Anyhow, I gave it a try today and sure enough Paul was right as usual.  I
wondered if it scaled like the VCA side, so I ran one VCO into the VCA and
one VCO into the ring mod X IN.  I applied the same CV to both the VCA CV
in and the ring mod Y in.

Here is what I found.  

Start with Unbalance control set to zero
X level acts like audio attenuation (obvious), so set it to 10
Y level duplicates the CV mod control with the same scale as the CV mod
control.
Y in switch to DC
X in switch (Paul's instructions say AC, but it worked better for me at the
audio level cutoff point on DC)
Gain on the VCA set near zero (the cutoff point you normally use)

I took the VCA out to right side of headphones and ring mod out to left
side (both through headphone amp).

When I ran the control voltage up from zero both signals varied in level
equally as best I could tell by my ear.  So, the ring mod really does make
a nice VCA if you do not need any gain out of it (like you can achieve with
the VCA 1/2 of the MOTM-110).  I think 99% of the time, I use my VCA for
final envelope attenuation without gain.

Anyhow, onto the question.  When I do this, I find some interaction between
the two halves of the module functions on the "gain" control.  As I am
biasing the CV to get the audio through the VCA to cutoff (right at that
point) the audio through the ring mod starts to leak through.  Fortunately,
a slight tweaking of the unbalance control just above the zero point
silenced the leak.  I am wondering if this slight interaction might be a
factor of my "Y" trim adjustment.  Maybe I didn't get it just right.  Or
maybe it is just the nature of the beast.

Larry (off work with time to play) Hendry