I ran across that last night, thanks. Turns out the neogeo has a third, seldom used audio channel called the SSG(software sound generator). It's seldom used because it can generate some square waves and that's about it. I can't tell if the circuit on jamma x is amplified audio or not though. The 6.8k resistors and caps make me think that it is amplified audio... looks like a voltage divider. Surely the ssg pin is unamplified somewhere down the chain. I don't mind building the circuit, but it seems dumb to merge the channels and then reduce to line-out levels when two of the three channels are already available at line out levels. Maybe just build half of the circuit for the c-channel if I can't find it at line level?