The GS1-O is based on the classic Curtis CEM3340 VCO IC. The circuit can use either a CEM3340 (the Rev G re-release is readily available, and of course if you have some older NOS you can use those also) or the Alfapar AS3340 clone IC.
The IC provides Triangle, Square and Sawtooth right from the IC. There have been many classic synths over the years that used this IC, from the Jupiter 6 to the SH101. There are many reference designs and this module borrows from all of these.
The sync out can be a little hit and miss direct from the IC, so here we actually use the Square output as the Sync output. But don’t worry, we use the Triangle to waveshape a Pulse Width modulatable square, and from that the slightly rounded square that we have loosely called a Sine! The final waveshape reverses the Sawtooth and provides a Ramp instead.
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