RCA outputs spurious signal - solved

[Gary]

I am using the 'play -n -c2 synth 10 sine 1000' app to create the signal. When I turn the output pot all the way up, there is a spurious signal riding on the output signal. It is 0.3-0.5Vpp and approximately 520kHz. It persists after the signal stops for 4 seconds or so. I just need to turn down the pot slightly for it to not show up. On the 'scope it looks like a full-wave rectified signal. It was due to use of a long shielded cable plugged into the RCA outputs. Looking directly at the unloaded RCA outputs, there was no spurious.