Module A-138c is a four channel mixer, that allows to add or to subtract four incoming voltages to the output signal. In the middle position of the corresponding control the amplification is zero. Turning the knob counterclockwise the signal is subtracted from the output sum with increasing amount. Turning the knob clockwise the signal is added to the output sum with increasing amount. The output control works in the same way, i.e. the resulting output signal can be additionally attenuated and/or inverted.
The module is used in the first place to mix control voltages (e.g. ADSR, LFO). It can be used to mix audio signals too but there is no difference between adding and subtracting audio signals unless they have a fixed phase relationship (e.g. the outputs of a VCO, or the input and output signal of a VCF). For audio signals without phase correlation there is no difference between addition and subtraction.
Control In1 works as a DC offset generator (about -5V...+5V) provided that no patch cord is plugged into socket In1. If this feature is not required it can be deactivated by removing a jumper on the pc board.