This simple module allows a balanced stereo signal pair to be summed to a balanced mono signal output. This can then be fed to a mono reference monitor amplifier/speaker to allow checking of mono compatibility of audio mixes.

The module circuit and PCB diagrams are provided in the pdf files here. The circuit is relatively simple and starts with a pair of 5532 op amps creating an unbalanced signal from the balanced left and right inputs. This is then summed and the mono balanced output created using an SSM2142 device. The module is powered from a bipolar power supply (+/-12V to +/-16V is ideal). A 6dB pad link is provided on board to attenuate the summed signal if required, depending on what it will be driving.

The XLR connectors are mounted on-board, however if the particular connectors cannot be sourced then shielded cables can be run from the pads to a chassis connector if desired.