Tl;Dr: I took apart a custom cable I’d soldered only to find I’d already made the modification I wanted to make. I remade the cable, and now it’s not as durable as it was previously :/ But the plus is I remade the cable with possibly better specs.
The cable is a Y-connector, which is very commonly available. But rarely wired correctly – I made a ‘Stereo-to-Mono Summing Box’. The reason for all this effort is connecting my stereo computer speaker output to a mono party speaker. The end result is nice sound. Importantly I’m not stressing my computer audio output with correct wiring.
This problem relates to this issue. Basically connecting consumer gear (line level stereo) to PA gear which is individual balanced/unbalanced channels.
When I opened the cables up they had resistors in them already, albeit different values and missing a 20k ground tie-down which I added.