A-135-3 is a is a voltage controlled stereo mixer with four inputs. Behind a front panel with 8 HP only eight linear VCAs (voltage controlled amplifiers) and the mixer based on the VCAs are available.

The sum of the left and right signals appear at the sockets Out L and Out R. The maximal amplification is about 0.5 to avoid clipping at the mixer outputs (otherwise the outputs may distort with 15Vpp signals at each signal input and full amplifications). If another maximal amplification (e.g. 1) is required two resistors have to be replaced.
All inputs and outputs are DC coupled. Consequently the module can be used to process both audio and control voltages.