Hi Nick,
Peavy used to make a device similar to what you are looking for, but this was made way back in 1996 or so.
It's called the PC-1600.
You may find one on ebay or tucked away on a back shelf in a music store.
Info:
http://www.sonicstate.com/synth/peavey_pc1600/ Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.