At some point you must interface with the outside world, preferrably with 1/4” or XLR cables.
This module has TRS cables on the front panel. These jacks are wired in parallel with another set on the back panel (power supply panel).
This module is entirely made from protoboards using op amps and some discrete transistors to drive the headphones output.
There are two “aux” inputs in the upper left that can be switched into the headphones only. This can provide a cue mix to the performer that is separate from what the audience is hearing. This can be useful for preparing a patch before the audience hears it.