Module A-138m is a 4 x 4 matrix mixer with switches for unipolar/bipolar mode for each column. 

Unipolar means that the controls work as attenuators. 

Bipolar means that the controls work as polarizers. In this mode the amplification is zero in the middle position of the corresponding control. Turning the knob counterclockwise from the center position the signal is subtracted from the output sum with increasing amount (i.e. negative). Turning the knob clockwise from the center position the signal is added to the output sum with increasing amount. 

By means of an internal jumper one can select if the four upper controls work as DC offset generators - provided that no patch cord is plugged into the upper input socket. 

If this feature is not required it can be deactivated by removing the jumper on top left of the pc board. The function is identical to the A-138a/b

technical details

Width: 20HP 

Connections: 
Audio input 1 
Audio input 2 
Audio input 3 
Audio input 4 
Audio output A 
Audio output B 
Audio output C 
Audio output D