[sdiy] SSM2164 VCA

[David G. Dixon]

Actually now that I study Marc's circuit more closely (and now that I
actually understand what he's doing) I see that I can easily modify my
circuit to eliminate the switching glitch, without adding to the complexity
or losing any functionality.

Live and learn.


> 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".
> 
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