The Mighty Mini Monotribe by Korg, now with MIDI Input and Output, by me.
I've seen lots of people doing this and all sorts of mods on the wonderful Korg Monotribe. I currently own one only, but, as soon as it comes up again on Hello Music for sale I'll probably add a second. And then complete the dream by adding a decent drum machine and sequencer of some sort, and a x0xb0x which will be sort of the crown jewel. I just love what people do when they run different verses, choruses, leads what have you from a variety of sequenced boxes like these, and I want to try it. Add some effects and have even more fun.
So you can buy the horribly overpriced MIDITRIBE kit from Amazing Machines, or you can buy the nicely priced one from Altitude909 on the Gearslutz page he posts on, or, well, you can hack one out yourself, which is the option I chose (using generous help from Altitude909).
I had trouble finding the right connectors, I wanted 180 degree 5 pin panel mount DIN connectors, and the only ones I could find were from Preh and I had to pay a lot to have them shipped overseas. You can be a lot less picky about this than I was, I was just set on having this type of jack on the side of my Monotribe, solidly mounted, ready to use with any standard MIDI cable.
Regarding construction, the header pins, the little protruding wires inside the white connector on the main circuit board, are a bit tight for soldering, I fanned them out a bit, but the joints were solid and I'm not worried about them bumping together either... it's not beautiful but it works. Some electrical tape crammed in would have been insurance. You could always do me a one-up and buy a jumper that fits the header and come up with a better layout. Or perhaps solder from the back of the board instead for a little extra room.
Other than that it was all pretty straightforward, once I sussed out the errors in my layout, which is now VERIFIED. I went with the adequate but not rock hard method of mounting the tiny circuit to the big PCB using double stick tape. The strength of the wires gives a bit of mechanical hold as well. I'm sure a fancier mounting scheme could be devised. Or I could break out the heavier duty double stick. It's fine for now.
I haven't truly delved into using this new feature, but I will say, it's exciting to hear those goofy ribbon pad sequences getting played out of my Alpha Juno, a synth with body and fatness, along with the pitch LFO. And of course it's exciting to play Actual Notes! on a Real Keyboard! into the Monotribe and laugh jeeringly at the little ribbon controller.
Here's my stripboard layout and photos of my installation. Total cost to me or you: $Not much.