[sdiy] SEM LFO square wave mod...

[James Coplin]

I've been told that you can modify the SEM LFO waveshape by applying
negative voltage to pin 5 of A23 (a CA3080).  However, I'm not sure how to
implement this exactly.  Looking at the schematic that pin is a 15v supply
for the chip.  I know how to invert the VC to get a negative voltage using
a transistor but what is the right way to sum it in with the +15v supply?
Anyone done this and know what the max "safe" negative voltage to blend in
is or am I not looking/seeing something right? Thanks as always!

James R. Coplin