home-made vca

[Tony Clark]

>      You know I just proto'ed the Analog Devices circuit up and it works 
>      fine! Tony - check your wires! Sorry to break it to you like this... 
>      don't feel too bad, I hook stuff up wrong all the time.:)

   Just after I'd sent that out, I went back and took some readings.  I'd 
initially used an unpolarized cap for the input and it was screwing up 
the waveforms.  After I'd determined that the circuit was working fine, I 
stuck a 10uF tantalum in and took another look.

>      Anyway frequency response with a 47uF input cap is from below 1 Hz to 
>      above 300 KHz (remember, I left the caps out). No phase shift, no 
>      distortion, sine, square, sawtooth, or triangle waves. Inputs over 
>      around 6V p-p will start to distort.

   With the 10uF, I got the exact same results that they had, 20Hz to 
20kHz (approximately).  This is great if you've gotten far better signal 
response with the 47!  Now, I only tested it with 2Vpp signals so your 
information is enlightening.

>      Max gain is around 2. CV range is from between -4 and -3 volts 
>      (minimum gain) to just above ground (maximum gain). I shifted this 
>      range up to be between ground and +5 by adding a 27K resistor from V- 
>      to the 4.7K/330 ohm junction on the control voltage input. However to 
>      get a 100K input impedance here you'd need to add a buffer (hey 
>      there's an extra one in the output TL082...).

   Neat.  :)  I'm going to work on that stuff next.

   Tony

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