VCA experiments continue...

[gstopp at fibermux.com]

     As you know I've been messing around with the VCA from Analog Devices 
     app note 105. A few moments ago I decided to throw together one of the 
     "standard" VCA designs from Electronotes, for comparison purposes. 
     This design is the simplest one, using one dual op-amp and a CA3080 
     with linear CV response (BTW it's the one I used in the ASM-1).
     
     There is not much difference in the output noise level between these 
     two circuits - both around 10 mV p-p with no signal input and CV at 
     maximum. The CV response of the 3080 VCA is clearly different in its 
     curve, with the output amplitude linearly proportional to the pot 
     rotation between ground and V+. The 3080 circuit also has standard 
     in/out impedances and clips in a more controlled fashion when 
     overdriven than the all-transistor design. The 3080 design is DC 
     coupled and starts to roll-off somewhere around 100KHz, so it has no 
     bandwidth limitations. I saw no noticeable difference using either 
     TL072 or TL082 op-amps.
     
     The 3080 circuit is quite a bit simpler, so really the only reason 
     that I can see to prefer the AD app note 105 MAT-04 design would be 
     for its exponential response and the fact that it can be made without 
     using up your 3080's.
     
     There is a switchable linear/exponential 3080-based VCA in 
     Electronotes, which I use in my oldest home-built machine. Something I 
     haven't seen is a 3080-based VCA that has simultaneous linear and 
     exponential control modes. Anybody seen such a circuit?
     
     - Gene 
     gstopp at fibermux.com