The MIT-129 is for impedance matching of a high impedance ¼" phone equipped source (like and electric guitar) into a low-impedance (mic-level) XLR type input.

To run a low-impedance XLR terminated line (like a pro microphone) into a high-impedance ¼" phone input (like a guitar-amp input), it's often necessary to use a line matching transformer like the MIT-435.

This reversible transformer matches low- or high-impedance microphones to high or low impedance inputs. The transformer permits use of very long microphone cables with either low or high impedance microphones.