The 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 centre position the signal is subtracted from the output sum with increasing amount (i.e. negative). Turning the knob clockwise from the centre position the signal is added to the output sum with increasing amount.
