[sdiy] Quick query about 2164 VCAs

Mattias Rickardsson mr at analogue.org
Tue Mar 19 16:44:59 CET 2019

Hi Neil,

On Mon, 18 Mar 2019 at 23:53, Neil Johnson <neil.johnson71 at gmail.com> wrote:

> Hi Matthias,
> > BUT it assumes identical units! Offset voltages, offset currents and
> other imperfections of the real world would break the spell, possibly
> forcing one VCA to react much much more and gulp up much more of the input
> current than the other VCA, leaving us with a circuit more similar to a
> single VCA without higher current capability or lower noise.
> 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. :-)

> So, in the real world, even with VCAs on the same chip: How close and
> well-matched can they possibly be, in the worst case? Is it reasonable to
> believe that all the error differences are orders of magnitude smaller than
> the already tiny (but crucial for the function) non-zero voltages at the
> virtual ground inputs? What do you op-amp experts say, can you connect
> op-amps fully in parallel without worries? :-)
> 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!

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