[sdiy] Linearized 2164 VCA Warning

[David G. Dixon]

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If you are linearizing the SSM2164 (or CoolAudio V2164), be aware that the
input voltage offset of the linearizing opamp will influence whether you
will be able to completely shut off the VCA or not.

If the offset is such that the output of the opamp goes to the positive rail
when both inputs are grounded, then the opamp output will approach the
positive rail and the VCA will go perfectly silent when the CV input is
grounded.  However, if the offset is in the opposite direction, then the
opamp output may be less than 2V when the CV input is grounded, in which
case the VCA will still generate audible output.

In his famous EDN article, Mike Irwin alludes to the fact that a small
negative input voltage will send the linearizing opamp output to the
positive rail, and he also encourages the use of a low-offset opamp in this
application.  However, he does not address the issue of what to do if the
offset is significant or, more importantly, of the wrong polarity.  I
experienced this problem even using TL074BCN, which is supposed to have
offsets less than 3mV.  

There are two ways to deal with the problem:

1) Hand-select this opamp for the proper offset polarity.  I suppose that
about 50% of the specimens of any given type would be suitable.  However,
this is not a very good option, especially if you are building these units
to sell to others.

2) Bias the linearizing opamp at zero CV by sucking a few hundred nA of
current from the summing node.  If you are using a -5V reference for the
linearizing VCA, then also summing this into the opamp through a 10M
resistor will give -500nA of current into the summing node, which is more
than enough to swamp any offsets and ensure hard shutoff at zero CV.

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