A-138p/o are used to built a simple performance mixer (instead of an external stand-alone mixer). Module A-138p is the 4-fold input module, A-138o is the output module that can be combined with one or more A-138p.
controls and inputs for each channel
input (3.5 mm mono socket)
signal LED (as a coarse monitor of the incoming level, the level is measured after the gain control)
gain (control to adapt different audio levels)
Level (the main volume control)
internal jumper for each channel to choose between Aux pre/post main Level control
internal connection to the output module and to other A-138p modules