[sdiy] Unusual BP filter - Voltage Control

[Paul Higgins]

On Wednesday, September 15, 2004, at 12:20 AM, harrybissell wrote:

> Now an interesting idea would be to use the PWM to drive the LEDs in 
> the
> Vactrols... have 8 separate PWM outputs and a linearization table for 
> each
> Vactrol. Make the matching REALLY good.   The Vactrol slowness would
> probably filter the PWM WAY WAY better than the analog switch method

Just be careful about the frequency of the PWM (i.e. make it a higher 
frequency than the upper limit of human hearing).  I remember a Craig 
Anderton noise gate design that drove a CLM-6000 with a fairly 
high-level squarewave from a 555 timer; it actually bled through to the 
photocell, at least it did if you subbed a Vactrol VTL5C4 for the 
CLM-6000.  Maybe it was partly a capacitance thing, I have no idea, but 
it was audible and very annoying.  (I designed my own optical gate at 
that point!).

-PRH