The A-101.3 is a vactrol based 12 stage phase shifter with unique features such as free patchable feedback loops and access to all 12 stages inputs and outputs. This allows any stage output to feed back to any stage input and phase shifting is controlled manually or by voltage control.
The A-101.3 is basically two independent 6 stage phase shifters with separate inputs and outputs plus a polarizer (the polarizer is able to generate positive and negative amplification in the range -1...0...+1 with-1=inversion, 0 = full attenution and +1=unchanged signal). The phaser shift level LED is connected to the vactrol control circuit to indicate the state of the vactrol. The open structure of the module allows multiple feedback loops and even "forward" loops (e.g. from stage 5 to stage 9). In combination with polarizers additionally the feedback or the output polarity can be normal or inverted, allowing many possible filter types.