Modulus VCA

[Buck Buchanan]

At 12:44 PM 5/13/98 +0100, jorgen.bergfors at idg.se wrote:
>The only problem I seem to have with it is that there is a DC offset at the 
>output if the control voltage goes negative. Is this normal and in that
case, 
>any ideas how it can be avoided?

Hi jorgen and all,

For modules that have failures/odd behavior/etc. when a control voltage
goes out of range, I put a precision clipper in the CV path.  It stops the
cv from going any farther than desired.  You can clip + and/or - excursions
of CV at a specific point and the circuit has no negative effects on
circuit behavior other than requiring an op-amp for each clipper and a
follower.  I use low drift op-amps like OP_07s, LF412s, etc. so that the
added op-amps don't add too much offset drift over temp.

Look in the OP-AMP Cookbook (Jung - Sams Publishing I think...) for the
best examples.  Someday I'll get some of my designs on the net which have
examples of this circuit.

If you don't have access to this book, I'll send you a description of the
circuit.

Take it easy,

Buck