[sdiy] SSM2164 VCA

[David G. Dixon]

> Why don't people use the 2164 for VCAs?

I was touting my Dual Log-Linear 2164 VCA here several months ago.  It's
based on the Irwin linearization scheme, and is a variation of the Gallo
log-linear circuit.  It doesn't have all the bells and whistles of Marc's;
just two identical log/linear VCAs based on a single 2164 chip (I use the
Coolaudio V2164 clone, available from either Small Bear or Magic Smoke for
$3.50, less in larger quantities).  Each VCA has Input and CV attenuators,
CV Bias control, and normal and inverted outputs.  Separate CV attenuator
and bias controls allow independent adjustment of DC level and swing.
Hence, just about any envelope or waveform can be used as a CV source for
the VCA, regardless of whether it is 0 to 5V, 0 to 10V, -5 to 5V, or
whatever.  Also, when gating the VCA with an ADSR, the CV Bias can be used
to add a little bit of output after the envelope has pinched off, which
gives a pleasing pseudo-reverb effect.

The only problem with the circuit, as Phillip Gallo pointed out to me the
other day, is that when the log/linear switch is actually being switched,
there is a very short time (< 1 ms) during which the linearizing opamp is an
open-loop integrator, which may temporarily send the amplifying VCA to
maximum gain.  This had never caused a problem for me, because I generally
do not flip the log/linear switch during use.  It's either on one setting or
the other.  However, if the switch is flipped when the VCA is in a high gain
setting, it can sometimes give an annoying "pop".  I will probably try
limiting the voltage from the linearizing opamp to some maximum negative
value with a diode clamp to limit the VCA gain during switching.  Other than
that minor detail, the VCA is a dream to use.  I'll send you the "package".