[sdiy] Quick query about 2164 VCAs

Neil Johnson neil.johnson71 at gmail.com
Tue Mar 19 17:11:11 CET 2019

Hi Matthias,

>> Indeed, but the mental model of the front end of the 2164 as an
>> opamp's virtual earth is just that - a convenient mental model.  It is
>> good enough for a lot of reasoning when considering just a single VCA,
>> but when you connect several of them together that model no longer
>> applies.
> Sorry, could you elaborate? I'm not sure I can quite follow where you're heading. :-)

The input stage and feedback mechanisms in the 2164 are not _exactly_
like that for an inverting op-amp, but close enough that in the
single-VCA circuit you can treat it as such, so you can consider it a
virtual summing node, just remember to add the frequency compensator,
and you're good to go.

But when you step away from that simple application the simple model
applies less and less.  So in the case of the paralleled inputs you
can do this with the 2164 but not with op-amps.

Experimenting with the 2164 is fun :)

>> For op-amps, see the work of Doug Self.  For the 2164, yes under
>> certain circumstances!
> I've only seen op-amps connected together with small resistors at the outputs - not completely paralleled!

Yes that's what I was referring to.  For example:

Although 32 parallel op-amps is a bit extreme, it does give you lower
noise and/or higher drive strength.


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