Voltage control

[R.G. Keen]

     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!

Why don't you use an LED/photoresistor?