AW: Re: Analog Devices VCA ???

[Haible_Juergen#Tel2743]

> what I was really after. I've made dual control 13600 based panners,
> and the inversion of control voltage complicates things a lot.

Try the following (found in SSM2024 data sheet, but applicable to
other OTAs as well):

A simple pnp pair (not even matched, if you add a trimpot) distributes
a current to two OTAs, so the sum of these currents is automatically
constant.
The circuit is like this:
C1 (i.e. collector of PNP #1) goes to Iabc of OTA1.
C2 goes to Iabc of OTA2
B1 goes to GND by a 1k resistor
E1 is tied to E2, and goes to +15V with a 33k resistor (for approx 500uA).
B2 is used for panorama control. Feed in a CV by a 22k/1k resistor
divider. If you like, you can add a 1M resistor and a trimpot between
the supplies for center trimming. That's all and it works great! The
control law is the sum of two exponential functions, something like
a tanh if memory serves.
I use this all the time in my modular, and it's great for panning with a
triangel LFO.
BTW, if you feed  a voltage controlled current into the common
emitter, you can have a stereo VCA with PAN, with only one VCA
in the signal path for each channel. Optimum SNR.

JH.