[sdiy] vactrol?

[WeAreAs1 at aol.com]

In a message dated 8/20/05 11:47:36 AM, philmacnutt at mac.com writes:

<<  Thanks for all the info from everyone.

Quick question, does the performance (speed, etc) of the vactrol matter much 
if i am just using them across potentiometers with a preset unit (programmer)? 
 that is, the values are only changed once when setting up the programmers, 
then they stay put.  I'm a little out of my knowledge-zone on this, so forgive 
me if i am not making sense... >>

It sounds like that wouldn't really be a very critical application, so the 
vactrol response speed would not be so important.  Just know that when you 
change to a new preset, there will be a brief settling time for the new parameter 
values.  It may or may not even be noticeable, unless you're playing a note on 
the exact same beat as your patch change.  

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), and that 
would get expensive very fast -- not to mention, the response to CV would not 
be nearly as predictable and linear as with an OTA, and the response of two 
vactrols would not likely match very well.  LM13700's cost about $1.50 these 
days (sometimes even less), and each one contains two separate complete VCA's.

MB in Las Vegas (where Austin, TX seems like a cool, tropical paradise)