VCA discussion

[Tony Clark]

>      Does this mean that the control voltage input is exponential?

   Looks that way to me too.

>      By looking at the circuit I 
>      suspect that driving the base directly will result in exponential 
>      current ratios.

   Does this mean that an opamp/transistor lin/exp converter will work 
very nicely to control the circuit gain?  I hope so.

>      It is also interesting to note that although the 
>      signal rapidly diminishes to a flat trace with the scope set to 5V/cm 
>      vertical, as you "zoom in" downwards to 20mV/cm vertical you can 
>      follow the waveform as it gets smaller and smaller, in a very smooth 
>      way. (BTW Juergen, perfect waveshape is preserved all the way to 20mV 
>      - no distortion at all as the CV changes!)

   BTW, did you _actually_ get a flat trace out?  I was still getting a 
very small signal on my output.  I'd like to see the output go completely 
flat on mine.

>      There are a few points about the existing circuit that need to be 
>      addressed if this design is to be used as the basis for a standardized 
>      synthesizer module:
>      
>      1. Input impedances of signal and CV are not 100K, but rather 4.7K for 
>      the CV and somewhat less than 30K for the signal.
>      
>      2. Neither input allows for summing (of course input summers would 
>      solve this as well as #1 above).

   Definately.

>      3. Signal inputs above around 8V p-p do not cleanly limit or clip, but 
>      rather get a very nasty distortion as the top of the waveform gets 
>      "cut and pasted" into the middle! This makes a sine wave look 
>      something like a strange function out of a trigonometry textbook. 
>      Maybe useful for something, but in your final signal path it might be 
>      rather rude. Some kind of input limiter function may be needed.

   What kind of input and output signal levels do you think are 
adequate?   BTW, that strange function output was what I was getting when 
I first fired up the circuit.  I guess you know why I was a little 
distressed!

>      4. When the CV travels too far in the negative direction some DC 
>      component does affect the output (a simple diode may prevent this).

   Yes, I did notice this.  Annoying.

>      Chris Crosskey had an interesting thought when he told me that perhaps 
>      it would be a good idea to split the VCA into two categories - 
>      CV-modulating VCA's, for control of DC control signals and some audio, 
>      and audio-only VCA's to allow for clean and noiseless amplitude 
>      moduation on the hi-fi side of things.

   I think this one lends itself nicely to audio only.
   What about CV-modulating VCA's.  What would you use them for?

   Tony

-------------------------------
I can't drive (my Moog) 55!
-------------------------------
Tony Clark -- clark at andrews.edu 
http://www.andrews.edu/~clark
-------------------------------