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drdna wrote:

Yes, I assume the problem was the ground path through neutral, either because of a high impedance or because of an interaction of the PP2000 with the isolation transformer.

Ah, you have also an isolation transformer sitting in the system? Where does it sit and what it does?

drdna wrote:

But if everything is plugged into the PP2000 and the PP2000 is unplugged, nothing is connected to the wall. It just loses the reference to the additional ground of the neutral AC path?

A presence of one single reference to ground is the same as having no references to ground from the hum perspective. You will have a noise if you have more than one references to ground. You can easy test it by lifting ground from PP2000 input. It would not be a good way to operate but as the hum-test it will do.

drdna wrote:

Well, I tried this first and the hum was always there unless the PP2000 was unplugged.

Adrian, I still do not see how it might be PP2000 related and I have no idea why you lifting the PP2000 output still, as you report,  it has the hum. It just does not makes since to me.

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