The A-138u is a simple, low-cost mixer with two units. Each unit is equipped with three inputs and one output. The upper unit has for each input a trimming potentiometer available that allows to adjust the amplification in the range 0...+1 for each input (factory setting is 1 for all inputs). The inputs of the lower unit all have the same amplification +1.
The output of the upper unit is normalled to the first input of the lower unit. That way the module can be used also as a mixer with five inputs: for this the inputs 1, 2 and 3 of the upper unit and the inputs 2 and 3 of the lower unit are used. The sum output for all five signals is then the output of the lower unit. As soon as a plug is inserted into input 1 of the lower unit the module becomes a dual 3-in-1 mixer again.
Inputs and Outputs are DC-coupled, i.e. audio and control signals can be mixed.