The A-138b is a four input mixer which is intended to be used with audio signals. Each of the four inputs has an attenuator, and there is also a master attenuator on the output stage for additional control.
The A-138b has exponential (logarithmic) controls and is most suited for audio signals (the A-138a has linear controls for control voltages).
The A-138b 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
