This circuit provides a way to get sounds other than the FatMan's oscillators into the FatMan's filter. It is a simple opamp preamp circuit that provides some gain and level control.
Note that unless you've done the Separate VCO Level Controls mod, you will hear the oscillators anyway. This mod provides 2 level controls instead of a pan control.
Remember that the FatMan must be triggered on to use the filter this way. I used a GR-50 Guitar synth. I plugged the analog guitar pickups thru distortion and into the FatMan. Because it is a MIDI guitar synth, the MIDI signal from the GR-50 triggers the FatMan. You can use anything you like. Even just triggering with a keyboard and holding a key down. (I've used a stack of coins.)