[sdiy] Focurite VCA

[jhaible]

It's very similar to a discrete LM1496, connected as 2-Quadrant
multiplier. Which isn't really possible with a 1496 because
of it's fixed internal connections. But if you look at it that way,
you'll understand the circuit immediately.

Basically you're converting a votage to a current with so much
DC offset that the current is always positive. Then you divide
this current into two paths with a differential pair, and throw away
one half. This current steering has an expo transfer courve, and it
id does not contribute any noticeable distortion. But your output
current will have more CV feedthru than the signal (because
of the added DC). So you simply build the circuit twice, run it
with different sign, and use the difference of both outputs.
Ideally, all the DC part cancels, and with such high precision
components and extra trimming, I'm sure the remaining feedthru
is very small.

Must be quite low distortion, but no smooth overload character,
so think twice before you use it in a synthesizer.

JH.


> Mark,
>
> Thanks for the link...it is indeed a very interesting design, right from
> the input stage, through the twin gain cells running in anti-phase, to the
> interesting side-chain circuit.  There's so much detail in this circuit
> its amazing!  Thanks!!
>
> Neil
>
> --
> Neil Johnson :: Computer Laboratory :: University of Cambridge ::
> http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
> ----  IEE Cambridge Branch: http://www.iee-cambridge.org.uk  ----
>
>