This mod to my quad 99cdp should have been done years ago. Ever since I got the player I wondered why Quad had never fitted a a coaxial digital out to match the optical out its fitted with. So looking at the schematics its clear that the digital output to the optical totx173 sender is in fact a digital signal from the 74HCU04 chip. I've added a 75 ohm res from digital ground to signal but this has had minimal effect on the sound via my varrious dacs. Simply drill a hole above the optical out socket, fit an RCA socket of good quality connect the ground to digital ground on the pcb or pin1 of totx173 and the signal to pin 4......I used a tightly twisted silver pair in ptfe. Sound is markedly clearer than the optical out ....but then it would be as I'm using a good digital ic rather than a bog standard optical cable. I should have done this they day after I bought the thing .....but there you go.
