Is it save to convert XLR to RCA?

Granny, wow, that's mystifying. Sounds like you've tried pretty much all of the logical things.

Cheater plugs would have broken a ground loop, if that was the cause of the problem. And the Lightspeed wouldn't be susceptible to ground loop problems, because its power supply is optically isolated from the signal path, as I understand it. And also because it has a two-prong AC plug, with no safety ground connection.

You've probably already tried this, but if not try connecting the preamp and amp to the same dedicated line, and try doing that on each of the two dedicated lines.

I assume, btw, that nothing is connected to the outputs of the amplifier other than the passive speakers. I see that the Atlas is fully balanced, so if for instance a powered sub were connected to its outputs in the wrong manner, hum problems (or worse) could result.

The only other thing I can think of is that I see there is no switch on the back of the amp to select between the RCA and XLR inputs. Perhaps therefore the RCA input is wired directly to the same circuit point as pin 2 of the XLR connector (the non-inverting XLR input signal), in which case a jumper might be needed between pins 1 and 3 of the XLR connector when the RCA input is being used. Perhaps the manual addresses that. Perhaps the EMI environment is different now that the powered electrostatic speakers are no longer present, resulting in a difference in the amount of hum that is picked up by the floating input on pin 3.

Just some wild guesses, but I can't think of any other possibilities.

Regards,
-- Al

I tried that and no help. I have sub on the second outputs of my preamp. When I remove the sub from the system, still the same buzz.

I wonder if that shorting jumper will work? Nothing in the manual. This Jensen transformer is said to help eliminate this sort of thing? My amp may also sound better thru the xlr input?

I see in John Atkinson's measurements that the Atlas has extremely high input impedance, which adds credibility to my theory that the problem may be due to hum pickup that occurs via the unused XLR inputs.

If you happen to have a multimeter, check for continuity between the center pin of an RCA input and pin 2 or pin 3 of the XLR connector for the same channel, and let us know.

If you don't have a multimeter, but you have a piece of wire of suitable gauge, try jumpering pin 3 to pin 1 on each XLR connector. Obviously, the jumper should only be inserted when the amp has been turned off for at least a minute or so, to assure that a transient won't be injected into the speakers. And the wire gauge should be such that its contact with the pins is secure, so that the connection won't be intermittent when the amp is turned on.

Alternatively, for experimental purposes you could order an RCA-female to XLR-male adapter, which would short XLR pin 3 to pin 1 (ground). Markertek, B&H Photo Video, and Musician's Friend are suppliers of that kind of thing, among many others. You could connect to the RCA input as usual, while having the adapter connected to the XLR input but in turn connected to nothing. Alternatively, for experimental purposes you could connect your RCA cable to the adapter, although depending on the design of the amp it's possible that could result in no signal going through one half of its balanced signal path, which would greatly reduce power capability and probably also have some adverse sonic effects.

If a jumper or adapter eliminates the hum, then yes, a Jensen transformer would be a good permanent solution, as it would provide a balanced signal pair to pins 2 and 3 of the XLR connectors.

Regards,
-- Al

I shorted pins 1 and 3 and still the same old buzz. I appreciate your help on this Al.