Simple VCA

[WeAreAs1 at aol.com]

In a message dated 7/14/00 5:15:52 PM Pacific Daylight Time, 
paul.maddox at ndirect.co.uk writes:

<<   Im after a simple VCA, nothing fancy , 1 or 2 transistors/fets. 
   Its to scale EGs under CV control, so no audio is involved.. >>

Hello Paul,

I agree with Jim Patchell's suggestion to use an LM13700.  If you plan on 
using those EG signals to modulate the pitch of any VCO's (a very common mod 
routing), then any DC offset introduced by the attenuating VCA will be a real 
problem.  Even if you're just using the EG to control volume or filter, 
you'll still want as little offset as possible.  Discrete transistor VCA's 
tend to be more "thumpy", have more offset problems than OTA's, and are 
harder to trim.  I think it will be easier to create a low-offset attenuator 
with a modern OTA (although you probably still will need to have an 
offset-nulling trimpot).  If you're just looking to save space on your 
polysynth boards, you can get the 13700 in the surface-mount package!

Michael Bacich