Adapters for RCA

Like CFB stated, in DAC/CD a single RCA output/input=coax.

What connectors do you have? AFAIK, Audio Research used RCA or BNC coax, XLR (AES/EBU) and an optical connector. I doubt that they have no RCA connectors unless this is a strange variant.

BTW, the coax you are using must have RCA or BNC connectors which is used by many cable manufacturers.

I assume you are using a BNC coax cable. These are in many ways better than RCA coax cables (both are used to transmit identical 75 ohm s/pdif signals, but BNC is a better shielded connector). While RCA digital cables are more common ALL of the better digital cables should be available in BNC vrsions. Do not go to RCA if you don't have to. Great digital companies like Muse don't have any RCA connectors, just BNC or XLR (and their fancy I3W).