[sdiy] vactrol?

[harrybissell]

WeAreAs1 at aol.com wrote:

> Keep in mind that there may be certain parameters that would be much better
> served by going through a OTA/VCA, such as an LM13700.  For instance, any sort
> of signal level attenuation that you would normally need to use a 3-terminal
> attenuating potentiometer to control (as opposed to a pot in a 2-terminal
> "rheostat" or "variable resistor" configuration), such as LFO modulation amount
> controls, or audio signal level controls.  You would need two vactrols to
> simulate an attenuating pot (or one of those special dual-element vactrols)

You can't use the dual element vactrols, they share the same LED so they will
not go opposite directions as you would wish.

Another problem...  Potentiometers have a very low 'end' resistance... they
go essentially to zero ohms.  Vactrols will never make it under maybe 100 ohms...
more like 500 ohms to 1K ohms for common units.   Maybe if you pump 50mA into
the LED... but them they will get hot and drift really bad.

Perchance... they don't make many Vactrols because of the limitations. At one time they
were the best way to go...

... nah - they just don't use 'em in FCPs

H^) harry