jl cooper do midi line amplifiers for doing midi over long distances.

http://www.jlcooper.com/pages/mla.html
MIDI (Musical Instrument Digital Interface) is an increas- ingly important consideration for schools, recording facilities, stadiums, arenas, hotels, amusement parks, or other facilities where MIDI messages are required to be sent a long distance, or to multiple locations.

The MIDI standard only permits a maximum cable run of less than 50 feet. The innovative MLA Series MIDI Line Amplifiers overcome this limitation.
midi is a current loop signal- driven through an optical isolator - its not voltage driven.

the capacitance of the line limits the rise time of the signal to a certain cable distance.