[sdiy] linear vactrol driver circuits

[Aaron Lanterman]

On Jul 27, 2009, at 12:06 PM, Tom Wiltshire wrote:

> On 27 Jul 2009, at 15:56, Julian wrote:
>
>> Ive been considering using vactrols to add CV control, but am  
>> wondering what my best approach is for driving them.
>>
>> Im thinking along the lines of a CV input jack, and a front panel  
>> pot, and an opamp mixer / driver for the led, but am wondering as  
>> to the best way to get away from the log response of the vactrol.
>>
>> Im sure this must be a common issue when using vactrols, so im  
>> wondering what other people do?
>
> I don't know what other people do, but I've been experimenting with  
> driving the LED in a vactrol with a PWM signal. This is supposed to  
> give a more linear relationship between the driving signal and the  
> LED brightness. I was using a microcontroller, but you could easily  
> use a ramp or triangle wave oscillator and a comparator for an  
> analog PWM solution.

Grant Richter has a vactrol-based interpolating circuit based vaguely  
on that idea:

http://www.musicsynthesizer.com/Corpse/VCEOM.html

Grant uses a LM3914 bargraph chip to drive the vactrols, and he adds  
some "dithering" noise to that, which the vactrols smooth out.

- Aaron