[sdiy] Adding offset to vactrol??

[Aaron Lanterman]

On Jun 3, 2008, at 7:48 AM, Dan Snazelle wrote:

> 1. they dont start reacting until a certain voltage ( is it 2  
> volts?)-is there a way around this?

Trick is to shove a known current through it. Quickies/dirtiest way is  
to put a CV through a resistor to a negative terminal of an op amp,  
and put the postiive terminal to ground, and then put the vactrol in  
the feedback loop. If you look at the Buchla LPG circuits you'll see  
that basic idea with a few other resistors and clamping diodes and do- 
dads, but that's the basic idea.
>
> 2. they stop reacting when being driven by a high frequency from a  
> vco or lfo. (not that high a frequency either. the led just STAYS  
> on. for example is there a way to add an offset so that the vactrol  
> thinks 1khz is actually 350 hz -this is just an example but i'd love  
> to be able to actually have the vactrol send out a cv that can  
> follow a vco from very low to very high.

I'm not sure I follow you there - vactrols just don't respond very  
fast; that's part of their charm. Also, for different audio input  
frequencies, the response time seems to be different, so as you close  
down on the vactrol you can get an interesting ringing effect. At  
least that's what it sounds like to me. I think Peter Grenader has  
written about that, and if you search for the sound file examples of  
his Plan B version of the Buchla LPG you can hear examples of it.

- Aaron