JAMMA Arcade: How to properly wire audio

Question

I’m trying to wire build a Jamma arcade cabinet from scratch. I have a question on how to properly wire audio. The Jamma standard have two pins related to audio (+/-). I have connected + to a phobo plug middle and - to the phono shield. The phono is connected to my OSSC upscaler via a phono to jack adapter. This setup causes weird behavior and restarts.

If I just connect the middle and leave the - disconnected everything seems to work ok. I suspect this only works because the devices share a common ground in the power supply. I want to make a system where I also have the option to connect my home theater amp, so the outer jacket needs to be terminated properly.

Have access to both Neo Geo and CPS/CPS2 jamma games. Problem applies to all of them.

Any suggestions/real knowhow on how this is supposed to be wired? Should the jamma - pin be left out and connect the outer phono jacket to ground instead? Do I need to add any components like resistors or similar?

nabulator · Answer

Actually the audio output pins at L/10 output amplified audio signals which are meant to directly power an unpowered speaker, usually located in the arcade. Your capture card is probably expecting line out which is preamp signal levels (less than 1 pp V). I suspect this may overwelm the input of OSSC causing it to restart.
From Arcade Otaku:

Speaker +/- "10/L" is mono. It does not need an external amp. The speaker +/- is for post-amp output directly to the speaker. Audio +/- "11/M" is for pre-amp outputs but is hardly ever used.

The solution is to use a line level converter that can reduce the power of your signal. You could build one yourself to with a voltage divider if you have the resistors that can give you the correct gain.

JAMMA Arcade: How to properly wire audio

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