The PCB

Assembly was done using manually applied soldering paste and reflowing it with a hot air rework station.

The Plugs

The 1/4" phono plugs are just standard solder lug plugs where I have cut the lugs down and bent them in towards each other.

Marking the placement of the plugs.

The pads for the plugs were extended along the whole length of the PCB edge so that different plug spacings could be used if they were different between mixers. However, it turns out, (at least with the two mixers I have with me, which are different brands) that spacing might be standard. I'll have to check a few other mixers when I get a chance, but it would be pretty convenient if that were the case.