Problem solved!

Tony was on the right path in this case ...
I had been using the AC adapter that came with the player (Model No: NP12-1S1210), made in Taiwan with an output of: DC 12v DC 1A and + center, so following Tony's advice I dug up a different adapter which still met the spec. In this case a POTRANS "brick" unit (as he described), made in China with an output of 15v DC 1.7A and + center. That was the ticket ... zero hum with NC active. Maybe it is a better quality power supply, or maybe the added voltage plays into it, but it works and that is all that really matters. You have all been very helpful, thank you so much. ...Mark

P.S. The RCA to 1/8” plug adapter is not from Radio Shack. It came bundled with a PC sound card ( Sound Blaster AWE32 Gold ), and appears to be of reasonable quality. I have yet to see a similar adapter for sale, short of using a gender changer. If anyone knows of a source I would be interested in a spare. This one is bound to break as a result of its daily use. Ideally I will get around to building a sled based box in the future.