Syntherjack shared details on his site for adding an LFO, suboscillator and more to the Korg Monotron Duo.
While the Monotron Duo mods don’t transform it into a great synth, compared to a stock Monotron Duo, it offers a lot more sonic potential. Or as he describes it, it’s “A big upgrade for this small device.”
You can view a demo of the Monotron Duo mods via the embed above. Details and schematics are available at the Syntherjack site.