AW: Re: AW: Re: Analog Devices VCA ???

[Christopher List]

> But why would you think that you couldn't shift the current distribution of 
> a
> long tailed pair from 0% to 100%? Isn't this just a question of CV signal
> strength at one of the bases? I am sure that with +/-1V you can completely
> turn either of the transistors completely off. For reasonable attenuations
> (80dB) this CV would even be much smaller.

Hmmm, I was just playing with this last week to try to get it to work a little 
better. I laid it out on the bread board with a 30K resistor between +15v and 
the two emitters (for experimenting). The Pan-CV in went through a 15K/680 
voltage divider to the base of the tranny. This should give a +/- 210mv range 
for +/- 5v in. That's what the data sheet calls for. What happened was when the 
CV in was at +5v, one VCA would be a max output, and the other would be off. 
When it was at -5v, one would be at max output and the other would be at about 
5 or 10%. If I put the CV in to -8 or 9v, one VCA would DROP to about 90%, and 
the other would shut off.

I used a 10v p-p input, a 30K/200 divider on the input (as per the data sheet) 
and a 47K resistor in the output op-amp to get V-in = V-out for a .5mA control 
current.

If yours behaves differently, I'd love to hear what you did.  

- Chris