Isolating Digital Noise, need help.

@jea48,

Beats me how you can get a ground loop hum from the AC mains safety equipment grounding conductor if only one AC mains system equipment ground is involved. It takes two tangle.

There's been a miscommunication. When I referred to the ground-loop problem, I left out some of the back-story. When I first installed the Atma-Sphere preamp, it was using a 3-prong grounded plug (tried both stock and aftermarket). The amp was using a 3-prong stock cable and both were plugged into the same 20a dedicated line. The result was a 60 Hz ground-loop hum.
I understand the principle of ground-loop in this case because they are connected by an unbalanced line.
The only way to eliminate the hum was to run the preamp with a cheater plug.

The strange part is that my previous preamp (Rogue Audio), was hooked up this way with no hum. Stock 3-prong on amp, Audience 3-prong on preamp, unbalanced interconnect.

In that thread, either you or Al directed me to the Whitlock paper and I learned that the concept of using 2 dedicated lines may not eliminate ground-loop when using unbalanced cables.

There is no cable TV and during this period, the power conditioning was removed.
 I'm going to look for my multimeter tonite.

@jond, the output of the ARC is high; 2.7V unbalanced. But the Atma-Sphere has a switch for -6dB attenuation on the input.

With the Audience PC, had the cheater on and off; still had the high-pitched noise.
Stock cable, no cheater plug.

I understand the principle of ground-loop in this case because they are connected by an unbalanced line.

Please explain it to me, because I don't understand it.

jea, I hope that's a rhetorical question, because I'm going by what I read in the Whitlock paper.
Ralph thinks my Sunfire amp may have an atypical grounding design.

Have you tried a cheater plug on your digital-without changing anything else?