home-made vca

[Tony Clark]

>      Frequency response - 20Hz-20KHz flat
>      Noise level - 110dB below full output
>      Distortion - under 0.03%
>      
>      The MAT-04 transistors are said to be matched to within 200 microvolts 
>      offset voltage of each other, and the Betas within 2%. The CV range of 
>      the example circuit looks to be in the negative region but a simple 
>      level-shifter can probably move it into a more synthesizer-oriented 
>      region.

   I breadboarded their circuit Friday and got it running just before I 
left work.  It doesn't quite work like I expected it to, but that may be 
from the fact that I didn't quite have "all" the parts.  I used two 
MAT-02's instead of a MAT-04, and NTE-123AP (2N2222 equivilants), with 
TL082's instead of OP-41's (specs are close on those), and an OP-77 
instead of the 27.  Gee, could I have gotten any farther away from the 
design??!
   Anyway, here's what I was able to obtain:

   Gain of circuit:  Roughly a little better than 10
     This was a suprize as the documentation quotes a max gain of 2
   Distortion:  I have no easy way of measuring this, but looking at
     sine waves, there is some visible distortion that start at around
     15kHz.  Of course as I don't hold out hopes that my circuit was in
     any way acting "proper".
   Noise level:  Really LOW!

   Some other problems I was having, I couldn't get the voltage control 
to affect the gain of the circuit whatsoever!  The input signal would 
greatly affect the output depending on signal strength and DC level.  
According to the paper, it was designed for O-3V RMS signals, but I 
couldn't put in levels that large without causing major clipping in the 
output (but then my output gain was 5 times larger anyway).  Smaller 
input signals tended to cause the output to distort and do all manners of 
crazy things.

>      It seems tempting to try this out, but as long as I spec a part from 
>      AD perhaps it might as well be an SSM2018!

   Please do try it out!  This looks like it could be a great circuit if 
modified some!  It needs to have symmetrical operation around ground and 
the CV needs to be in a range acceptable to the synth world (I'll accept 
a WORKING CV to start with though!)
   If I get time, I'll nose around and see what I can do with the circuit 
to get it in better operation.

   Tony

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