So simple impedance of the source is partially ruled out- we are OK with a relatively high impedance at the input, so long as it is balanced. What happens if we use the same source to drive the RCA single-ended?
Grannyring, are you able to use the Jensen running single ended? How you would do that is to use the RCA input of the amplifier by using an adapter with the Jensens, one that ties pin 3 and pin 1 of its XLR output together.
This is still pointing to an issue with the input wiring where it is able to pick up noise from inside the amp. I suspect that it is common mode to the inverting input and so is not picked up when the amp is driven balanced.
Grannyring, are you able to use the Jensen running single ended? How you would do that is to use the RCA input of the amplifier by using an adapter with the Jensens, one that ties pin 3 and pin 1 of its XLR output together.
This is still pointing to an issue with the input wiring where it is able to pick up noise from inside the amp. I suspect that it is common mode to the inverting input and so is not picked up when the amp is driven balanced.