The Moog Voyager has a couple of jacks that are really useful for dirtying up the sound of the filters and can be abused quite easily. The mixer section on the Voyager has five inputs. Four of these, oscillators 1-3 and the noise are hard wired internally, however the fifth is the external audio in. By making a (very simple) custom cable you can take the output of the mixer and feed it back in to the mixer section, effectively giving you a feedback loop that can make the filter very very crunchy indeed, as you can massively overload the signal going into it.
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This is the first addition you should make to a Voyager; well before going out and buying an external control processor or interfacing it with modular gear. A simple trip to your local electronics store and a $10 spend on a custom cable can make the filter have that extra edge. Lets have a look at the cable.
The Crunchifier Feedback Cable

The cable has a stereo jack at one end and a mono at the other. Basically it is a mono to stereo lead. The wiring of it connects the tip and centre of the stereo jack with the tip of the mono, the earth or ground is just a straight connection between the two jacks. I have taken a few photos of the wiring.
The Mono Jack

I am using a three wire standard stereo cable, soldered to the tip is the blue and white cables, and the ring is soldered to the earth.
The Stereo Jack

The stereo jack image is a little hard to see, but the cable is just a standard stereo to mono deal, so soldering up the connections is trivial. The white cable is soldered to the tip, the blue to the first ring and the earth to the final ring (or ground). Lets have a look (and listen) to it in action…
A word of warning, I don’t know what the maximum overload you can apply to the filter section is. I run it pretty hot but am unaware if there is a protection circuit to protect the internals. However I have had mine for nearly two years and have been overdriving it and have never had any hassles.