Isolating Digital Noise, need help.

I believe this proves that the ARC CDP is introducing noise into the shared dedicated line and needs to be isolated.

Yes, but per my previous post perhaps the extension cord itself would provide sufficient isolation, when plugged into the outlet that is normally used. Seems worth trying, on both the CDP (the source of the noise), and the preamp (which is probably where the noise is causing the trouble).

The amp is now known to have a grounding issue, but is it possible that that this low-level hum could be due to the fact that the second line’s grounding is unknown?

That is very conceivable. Jim (Jea48) has previously provided us with a link to a paper by Bill Whitlock (the Jensen Transformer person, although he may be retired now) in which it is explained that ground loop hum is usually caused or contributed to by coupling of EMI between the hot & neutral wires and the safety ground wire in the house wiring. Which occurs very minimally with Romex, due to its symmetrical design, but occurs to a much greater degree with wires in conduit. Is there any way to determine what kind of wiring is used between that outlet and the breaker panel?

Also, Jim (Jea48) makes a good point about jumpers. Given that there is no balanced/unbalanced switch on the amp, and that it has an unbalanced internal signal path, I suspect that jumpering pins 1 and 3 together on each XLR connector might be helpful, with respect to hum and/or sonics. Check for continuity between the center pin of each RCA connector and pin 2 of the corresponding XLR connector. Also check for continuity between the ground sleeve of each RCA connector and pin 1 of the corresponding XLR connector. I suspect you’ll measure close to zero ohms in both cases. If so, I would suggest that at some point you obtain some solid core wire of a gauge that would provide a snug fit and use it to jumper pins 1 and 3 on each XLR connector.

BTW, "OL" is most likely derived from either "out-of-limits" or "overload," and when you are measuring resistance signifies that the resistance is greater than the 20 million ohm upper limit of what the particular meter can measure. In other words, an open circuit.

Best regards,
-- Al

thanks Al. Will do tonight.

Thanks, Jim (Jea48). Note, though, that the transformer coupled outputs shown on the schematic you linked to are the digital outputs, not the analog outputs which Lowrider is using.  Although that schematic is for the CD2, rather than the CD3 MkII he is using.

Regarding overload, the UV-1 doesn't appear to have an input overload point specified, but I'd be surprised if it couldn't handle 2.7 volts.  And even more so given that it provides a 6 dB input attenuator.  And in any event, if it were overloading I'm sure Lowrider, who is clearly a very perceptive listener, would have noticed the distortion that would have resulted.

Regarding Mr. Crump's statement, I would add that in the case of most power amplifiers, at least, one certainly does NOT want to attenuate everything above 60 Hz. As has been pointed out in past threads by Ralph/Atmasphere among several other members, since most power amplifiers draw current in narrow high amplitude spikes, rather than continuously throughout each period of the 60 Hz waveform, their AC supply needs to be provided in a manner that is not bandwidth limited below something on the order of tens of kHz. That requirement being one of the reasons upgraded power cords can often provide superior performance compared to stock power cords, for power amplifiers at least, because many of them provide significantly greater bandwidth than most stock power cords.

Also, I just Googled "inductance of Romex" and found a not necessarily authoritative post in a thread elsewhere stating that it is about 1.45 uH (micro-Henries) per foot.  That is indeed high, but for run lengths that are not particularly long I suppose is probably low enough to be compatible with the kinds of bandwidths Ralph refers to, perhaps depending on the particular amplifier.  While at the same time being high enough to provide significant attenuation of digital noise, which typically has a good deal of its frequency content in the MHz range and higher.  

Best regards,
-- Al