The A-138a is a four input mixer which is intended to be used with control voltages. Each of the four inputs has an attenuator, and there is also a master attenuator on the output stage for additional control.
The A-138a has linear controls and is most suited for control voltages (the A-138b has exponential controls for audio purposes).
The A-138a has a DC coupled input stage, so that it will add static DC levels as well. Input 1 control works as a DC offset generator (about 0...+5V) provided that no patch lead is plugged into socket In1. If this feature is not required it can be deactivated by removing a jumper on the pc board