This is a DIY mixer designed to save space in a portable cabinet. It is based on my 2U DIY Matrix Mixer. The circuit is almost identical, except that this one has DC coupled inputs and is unity gain. Like that one, the pots are audio taper. As I reflected on the panel design, I realized the benefit of making the fourth input be a non-attenuated auxiliary input, a feature not present on any of my other mixers.
Internally it is two completely separate mixers, each built on a Tellun MUUB-2 board. (You could use one MUUB-4 instead. I happened to have two MUUB-2 boards lying around.) The circuit is about the simplest possible. Download the Small Matrix Mixer Circuit.
The panel configuration maximizes utility. It is labeled L and R (left and right), as a convention when using it as a final output mix. Each of the left inputs is normally connected to ground with no plug inserted, and each of the right inputs is normally connected to whatever is plugged into the corresponding left input. As an example use of the AUX inputs, imagine the two outputs from a Blacet Mixer being patched there. Now you have a five input stereo mixer, DC coupled, with a bias pot for each channel. Patch L OUT to AUX R IN and you have a six input mono mixer. Or patch the output from a buffered attenuator to the AUX L IN, and you have a fourth channel, equally mixed to both outputs.
Download the Small Matrix Mixer FPE Panel.
Construction was straightforward, using a shortened Bridechamber 3-jack bracket to mount the two MUUB-2 boards. It was time to purchase more hookup wire, so I pinged George Mattson for the part number of the wire he uses. For a 100 foot roll of Belden 24AWG UL1061 BLK it’s Mouser 566-9984-100-10. Pricey, but excellent quality. The pots are Mouser 652-91A1A-B24-D20, a Bourns make with very nice feel. These have pins rather than lugs, but were easy to lay-solder to. I bent the pins back for easy access and soldered to them after the pots were mounted. Power to the second board is run underneath from the first board’s power connector.