Joonas has written up documentation on his MIDI to USB adapter with Teensy LC:
Thankfully, I had a MIDI connector and a high-speed optocoupler at hand, and with these I could implement a MIDI in rather easily. After some investigation with Arduino Uno, it seemed quite simple to receive the serial MIDI bytes and dump them over Arduino serial (I’ll write another post about this later).
However, Arduino cannot become a USB MIDI device very easily, so here comes the really nice part: Teensy LC can, and the Teensyduino add-on included a working USB MIDI and also serial MIDI libraries!
More info at Code and Life site.
Via the contact form.