The A-138c is a four input mixer with a linear response, with zero gain at the middle position. Turning the knob counter clockwise subtracts the signal and clockwise adds the signal. The output control works in the same way.
The A-138c is primarily designed to deal with mixing control voltages, as phase correlation between audio signals may not give much effect.
Input 1 works as a DC offset generator (-5v to +5v as long as no patch lead is in input 1), this function can be deactivated if not needed with a jumper on the board, although as long is input 1 is used, it can be bypassed by using input 1.