The schematic above depicts an addon for the FatMan which will add the following functions:
  • Sawtooth VCO with Voltage Controlled Pulse Width Modulation
  • An LFO as one control voltage source to drive PWM
  • A suboctave (subharmonic) that outputs either square and/or triangle
This circuit is actually 3 separate pieces as denoted by the colored background. The LFO is in pink and the Triangle SubOctave is in blue. The basic VCO and PWM is against white. The pink and blue parts are optional, and all that is required to make the cicuit work without them is simply to eliminate them. This means you can build as much of it or as little as you like, and if you decide later to do more, the colors make it clear what else to build.

The whole circuit is tested, but there are some minor adjustments required to the drawing. The Triangle SubOctave will output a more or less triangle wave, not perfect, because the sawtooth that is driving it is not perfect. However, my mission was not to produce a perfect triangle, rather just to get close; to change the timbre of the suboctave to something more sine like than a square wave. I do not know if this circuit is original.

Some part values have changed from René's original design due to changing the power supply from a balanced +/- supply to the odd +8 -12 that the FatMan uses. I am now doing all development and tests on a FatMan style supply.

ICs required to build the whole unit:
2...4069 CMOS hex inverter
2...TL072 JFET input opamp, dual
1...4027 CMOS Dual J-K flipflop
1...4066 CMOS analog bidirectional switch
Note that all of these parts are common and relatively inexpensive!