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
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