AW: VC pots

Haible_Juergen#Tel2743 HJ2743 at
Thu Oct 26 18:05:00 CET 1995

>   pro's: nonlinear (quadratic?) CV response is often desirable
>               (in compressors and in phasers)
>               small component, easy to layout the pcb
>The response isn't a quadratic curve, but I don't know that it's
>anything especially useful.

This is just from trial and error (I have to look into those books
again for FET theory): Building a Phaser with linear
gain cells and sweeping it with a triangle generator reqires an
exponential function to achieve a smooth sweeping sound.
Phasers with FETs sound nice with the triangle directly contolling
the gates.

> I'm pretty sure all special VCA chips are in fact OTAs.

I don't know the exact definition for "OTA", but I think some
VCA circuits use a modified 4-quadrant-multiplier structure and
have the audio signal modulating the *current sources* of
the two long tailed pairs, routing the *control voltage*
to the bases of the pairs.
There are many VCA's which have a "normal" OTA as a
core and lots of linearisation circuits, of course.
I'd like to know more about the pro's and con's of these
two concepts. The first one is tempting, 'cause the current
sources can be modulated very linear with quite high
voltages. But there must be some drawbacks, too ...
I am not sure about that, but maybe those VCA's that have
a large input voltage range (and not a current input or
millivolt input) *might* have this structure. I have seen
this on some old DBX IC. Maybe the old SSM2020, too?
(I am just guessing, but I doubt they have integrated the
divide-down resistors You need for +/- 10V inputs on an

Just some thoughts - I'd be pleased with any further information
or correction.


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