How to get rid of transformer hum?

While it may not be a feasible solution for some, try moving. The transformer in my DAC hummed audibly (hearable from ~2 feet away) and it was almost on its way to the factory for repair. There was also a seemingly unrelated hum through the speakers that floating the preamp's ground removed, though the overall sound was better with the ground enabled.

Well, I moved last week. Bigger and much more modern place. Especially the wiring. Previous place was a unit in a '50's era fourplex; new one is a "condo" (though NO shared walls) that's about ten years old.

When initially set up in the condo the DAC still hummed and the pre-problem was actually more pronounced. A couple of days later the DAC hum is completely gone and the speaker hum caused by the pre- is only audible from a scant few inches away.

Can't explain it, but what a relief!

I'm not sure this was a ground issue, Liguy. Not sure it wasn't, either. The grounding at the old place was originally suspect, but am pretty sure it is more than adequate now (long story). Given the neighborhood it may well have been similar , but different to your situation. This area is a lot more residential; the last one had a lot of commercial property nearby. Lord knows what kind of shape the power was in.

Yes, I'm rambling and don't have an answer. Just thought the ruminations might be of some help. Good luck getting it resolved!