The MIDI electrical specification was updated to version 1.1 in 2014 to include 3.3V signaling.
The familiar 2 x 220Ω resistors found in 5V MIDI transmitter circuits are reduced to typically 33Ω and 10Ω in 3.3V circuits as detailed in the attached specifications.
2014 MIDI V1.1 Specification.pdf (240 KB)