A while back Carmelo Azzarello posted a design for a sub oscillator, then later a revised version. He did it on stripboard as an auxiliary device to use with his Behringer Crave, but I decided to draw up a PCB for a synth module version.
A sub oscillator is a device that takes an audio signal as input and produces signals at a lower frequency, typically an octave lower than the input. This one works by generating a pulse whenever the input rises past a threshold and using that pulse to trigger a CD4013 flip-flop. The flip-flop output changes state, on to off or off to on, once on each cycle of the input, so the result is a square wave with half the frequency. There are two flip-flop units in a CD4013 so the output of the first is used to trigger the second; its output is a quarter of the input frequency. In musical terms, the flip-flop outputs are one and two octaves below the input. They’re mixed, AC coupled to remove the DC offset, and sent to the output jack. In the revised version, the separate outputs, DC coupled, are also sent to jacks: with a clock input instead of an audio oscillator, they provide /2 and /4 clock division.
Aside from replacing the battery power with synth power and a couple of other inconsequential changes (I omitted the AC coupling input capacitor Azzarello described as optional, and used a BC547 transistor instead of 2n3904) I kept the circuit as I found it. I left footprints for two “optional” Zeners but didn’t use them in my build. The PCB is about 4.5 by 6 cm, and it and a knob and four 1/4″ jacks wouldn’t fit very well on a 2.5 cm panel, but on a 5 cm panel they’d leave lots of space. I considered putting two boards behind a 5 cm panel but there’s not that much space. Eight jacks would be too many. But after all, the clock divider outputs are a bonus. Why not leave two out? If you need multiple clock dividers, build a clock divider module.
So my panel has inputs and sub osc outputs for two units, and clock divider outputs for one of them. The module uses two of the PCBs, but on one the resistors, diodes, and Molex connectors for the clock outputs can be left off. In addition the power header, filter capacitor, and protection diode for the power input can be omitted. Instead it’s just wired to the power rails on the first PCB.
I had the PCBs and panels fabbed a couple of months ago, but other projects kept bumping this one aside. With the Mega Module completed I finally put the Dual Sub Osc together. It was an easy enough build, and when I plugged it in… it didn’t work at all. Multimeter checks didn’t uncover anything, but when I looked at the CD4013 trigger on a scope it looked very small and slow. “Almost looks like the capacitor’s too large,” I thought, and looked at the original schematic where I saw it said “10nF”. My version’s said “10uF”. Nanofarads, microfarads, what’s the difference? (A factor of 1000.) I unsoldered the 10 µF electrolytic and replaced it with a 10 nF ceramic, and everything worked great.