There are two variants on this circuit I found (though the circuit build is self explanatory, given how the IC does everything for you.)
There's an AD725 chip (this one off a Neo Geo RGB mod page):
http://web.njit.edu/~jal0737/GFX/Neo-Geo/RGB_schematic.jpgAnd an AD724 (this is off a SCART>svideo mod page):
http://www.tvterminal.de/TVT_composite_AD724.gifNow it looks like these are basiclly the same chip, I bought a 725. They are not cheap sadly (£11) but they go down to £6 if you buy 100, I don't have £600 to throw away on a chance though.
I need to buy more caps but I have the IC and the NTSC crystal which form the core of the mod, and I guess you integrate said mod with a PCE amp.
I will review the circuit when I can put it together. Has to be better than native composite. I have a TV which takes raw NTSC through composite so I'm sure it'll accept s-video too, the IC does do PAL but PAL users have SCART so I'll review how to perforns through NTSC.