A client purchased this AM4 10x4 mixer already recapped, cleaned, with a new power supply, and generally working quite well.

Wishing for greater functionality, he asked Eisen Audio to install some additional summing. First we wired some drop-on resistors and cable so that eight unused pots and toggle switches could be activated as an 8-channel monitor return for listening to 8-track playback. Then we added a transformer-based passive network and insert loop so that busses 3 and 4 could dump into busses 1 and 2 for subgrouping and parallel processing. While we had the frame emptied, we also fortified the grounding and power supply decoupling in order to lower the overall noise floor.