If all you want to do is use the Amiga's own sound facilities, you can get plain old MIDI controller keyboards with no on-board sound quite cheaply - visit your local music store and you can probably get a velocity/aftertouch-sensitive 61-key keyboard for $50-70. Combine that with an Amiga serial-to-MIDI adapter and software, and you're good to go. Alternatively, you can keep an eye out on eBay or your local Craigslist for some neat old synthesizers and control them from the Amiga...but be warned, it's a money pit! :lol:
