Voltage control

[gstopp at fibermux.com]

     The LED/photoresistor idea is a good one. Using this method you can 
     retrofit voltage controlled time constants onto the existing EG's in 
     your commercial gear. A while back I built a programmable semi-modular 
     around a Minimoog head, and at first I used LED/photoresistor doodads 
     on the EG's. These I made with photoresistor cells from Radio Shack 
     plus standard red LED's with series resistors, sealed into little 
     units with black heat-shrink tubing. However, the control range was 
     limited (I'm trying to remember what the problem was... well there was 
     something that wasn't quite right about them....I forget excatly 
     what).
     
     Anyway when Will Alexander and I re-built Emerson's Moog we replaced 
     these crummy photo-things on the EG's with "Vac-Teks" (sp?) and they 
     are great for this. We had some extra so I put them in my Minimoog 
     project and so it is to this day. I think you can still get these 
     Vac-Tek things, but I'm not sure where (anybody?).
     
     You can't just replace the 1M pots in a standard EG with 
     transconductance amps - they are not true voltage-controlled 
     resistors. A photoresistor is a true resistor, and if you add an LED 
     it becomes a current-controlled resistor, and if you add a series 
     resistor to the LED the whole mess becomes a voltage-controlled 
     resistor. You can build voltage-controlled resistors with discrete 
     components or multiple transconductance amps, but then you gotta ask 
     yourself how complicated you want to make things. The Vac-Teks sure 
     are easy.
     
     If you're into building a voltage-controlled EG from the ground up, 
     that's much easier than a standard EG design with VC resistors, but 
     then again you gotta ask yourself how much work you want to do. It 
     still is possible to dig deep and scrounge up a couple CEM 3310's or 
     SSM 2050's, and these are very simple and wide-range EG's.
     
     But then again if you use photoresistors, and if you mount them on the 
     outside of the synth and don't seal them against outside light, you 
     can make yourself a light-sensitive kind of "Theremin EG" body 
     proximity-controlled EG time synth (be the first on your block!).
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Voltage control
Author:  "Duane R Balvage" <dbalvage at ptdcs2.intel.com> at ccrelayout
Date:    8/29/96 9:09 AM


     
  Hello everyone -
     
     I am looking to replace 1M pots with a voltage conrolled element
     
  of sorts, and I have one question ...  HOW!?!  I know it involves
     
  the use of transconductance amplifiers, but, I really don't know
     
  how to go about it. Any one got a schematic or a good explanation on
     
  how to do this? I have been pondering this for a while - I want
     
  to build VC envelope generators!