PLAN B analog blog

Added by Peter:

The M12 is a standard AState variable filter.  Instead of using transconductance Amplifiers to open and close the stages, we use a vactrol.  The led is not part of the adio path, pnly the light sensitive variable resistor is...the vactrol is working as a light-controlled pot.

Hello,

What is the basic architecture of a vactrol based filter?  Do you put
a fixed voltage through the LDR and run the audio signal through the
LED?

I know that in VCA mode the audio goes through the LDR and the CV
controls the light, causing smooth attenuation, so this is my best
guess for how to make a filter out of it.

Thanks,
Monroe

OK I think I found my answer.  You just make the same circuits as
usual but use vactrols in place of some pots.  Correct me if I'm
wrong.

Monroe

On Sat, Feb 21, 2009 at 9:59 PM, Monroe Eskew <monroe.eskew@gmail.com> wrote:
> Hello,
>
> What is the basic architecture of a vactrol based filter?  Do you put
> a fixed voltage through the LDR and run the audio signal through the
> LED?
>
> I know that in VCA mode the audio goes through the LDR and the CV
> controls the light, causing smooth attenuation, so this is my best
> guess for how to make a filter out of it.
>
> Thanks,
> Monroe
>