At Tabby's request, and since others seem interested in the synth stuff we've been working on, here's the VCO I originally posted in the melody generator thread. I've got it working, but I'm interested in hearing what others think about it. Schematic here. I used 1n4148 for diodes.
Is this powered by a standard 9v negative center power supply? I'm new to building these synthy-ish layouts. Thank you for sharing. I just finished freppo's awesome New Wave Control Voltage Generator, along with his U-235 circuit. Lots of good fun to be had there. I'm hoping to hook up this modlunetta to it (I'm also building your Wasp layout).
"The first stage (U1c) is a voltage summing amplifier or adder which adds the CV with the offset voltage. Without a CV signal I can control the frequency with the offset control and with a CV signal I can add (you probably guessed it) an offset voltage. However it does invert so the second stage (U1b) inverts it again so that a positive CV signal gives a higher frequency. The schematic I posted is the VCO I used in my modular lunetta which is powered by a single supply, so the - is just the GND connection. However a summing amplifier does need a reference voltage to work properly which is provided by U1a.
note: I later changed the circuit by connecting the level pot to this reference voltage as well instead of - which gives a much better control. The way it is drawn in the schematic it would actually have an effect on the frequency when nothing was connected to it. "