[sdiy] ASM-1 VCA observations

[Tom May]

Finally, the VCAs in my ASM-1 have bothered me enough, and I have had
time enough, that I took a look into some problems I've had with them.

Problem 1:

I expect this problem will occur with any VCA that puts the OTA
current control in the feedback loop of an op amp.  Within a small
area around CV = 0V, there is a noticeable noise glitch in the output.
In this state, the op amp is switching approximately two diode drops
as it needs to turn one diode off and another on.  Apparently things
are not too stable in this state, and the op amp can oscillate and/or
become sensitive to noise pickup.  I put a 15pf cap between the op
amp's output and -input, this reduced the frequency of oscillation to
subaudio, and sounds ok for long releases, but with a faster release
there is still a small but audible thump as the CV crosses zero and
the op amp switches.  I'm thinking that even if I used a small amount
of hysteresis to clean up the switching, the thump may still persist.

Has anybody else noticed or solved this problem?

Problem 2:

The VCA next to the LFO ticks when the LFO's output switches.  The
problem doesn't go away when I bypass the LFO's op amp's power supply
right at the chip.  I think the solution to this is probably just not
to use the on-board LFO.  Oh well.

Tom.