jl cooper do midi line amplifiers for doing midi over long distances.
http://www.jlcooper.com/pages/mla.html
midi is a current loop signal- driven through an optical isolator - its not voltage driven.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.
the capacitance of the line limits the rise time of the signal to a certain cable distance.