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Have you called Axiom Charles since you got the amp back?

I've been trading emails with JC.

Charles,

How many active inputs do you have connected into the A1400-8? Try to have an even number of inputs (2, 4, 6, or 8) active even though that input module may not be connected to a speaker. Also, is the hum coming from all output channels of the amp?

Right now I'm using three channels. I disconnected one so I was only powering the mains, but no change to the hum. And yes, the hum is coming through on all speakers. How's your A1400 treating you?

Did balanced vs unbalanced inputs make a difference?

Interesting that with no input signal, there is no hum.

My two A1400-8s produce a slight hum with single ended inputs but no hum with balanced inputs or, oddly, with RCA to XLR adaptors.

What did JC suggest to try?

I haven't been able to get my pre-pro back to verify that it's still the case, but if this hum is the same as it was before I sent it in for repair, which I assume it is, then no, the balanced connections didn't make a difference. I know when I first moved to balanced connections, noise did seem to be reduced.

That's interesting about the RCA-to-XLR adapters. JC was saying that if balanced connections defeated the hum before, then it would be possible for those adapters to do the same thing. I didn't see how. This would confirm that balanced connections wouldn't help, though, since I'm using the adapters now, and the hum is as strong as ever.

Actually, the only thing JC suggested I try was the balanced connections, if possible, and I explained how the hum had been present when I was still using them, and it persists with the RCA-to-XLR adapters. I think the things you guys have had me try has helped me head off some of the suggestions he would have made.

He'll be forwarding the stuff I've tried to Andrew, and he'll try to help me pinpoint the issue. Based on the hum being absent without any inputs hooked up, and the amount of hum changing based on whether it was the receiver sending the signal or the portable player, Andrew said they could likely rule out the amp as being the issue. His theory was a ground issue of some sort. JC still has more details to forward, so we'll see if that changes.

If you cheated the ground using a 3-2 prong adapter on your equipment and the hum is present, its impossible that a ground loop is the problem. There is something else going on here. My guess is the unit will need servicing.

The cheater plug does not eliminate the slight hum this amp is known to have with RCA/single-ended inputs.

Charles, the supplied RCA-to-TRS adapters don't work to eliminate the slight hum, so I am wondering which adapters you used. In my stereo setup, the preamp does not have balanced outputs, so I am using the Hosa GXM-133 adapters. These eliminate the slight hum completely. In the HT setup, my pre-pro does have balanced outputs so I just use them.

Those Hosa adapters don't even attempt to create a balanced connection. "Pin #2 Hot. Pins #1 & #3 tied together," that just ties the pin that should be carrying the inverted signal to ground (which is a shared chassis/signal ground on RCA cables).

That results in a half amplitude signal entering the gain stage of the amp. Don't know why it would reduce hum. Unless the circuit integrating the differential signal is the issue. It would require you to run the input volume at a higher level to get the same output.

Using Axiom's supplied adapter actually triggers a switch inside the jack and puts the amp's input in single-ended mode.

The Hosas are what I ended up with, just to try.

I just turned everything on today, and in addition to the loud hum, there is now a scratchy sound happening quite frequently. Makes me think something wants to fizzle out and die.