The only way to get the stray AC/DC interference upstairs, is by it traveling though the house wiring, or telephone line. I don't use the regular phone lines, so I can't say. So, that even when using batteries for the input source, you still may be getting interference from your ground line, which you say is still connected to the house AC ground, somewhere. But, how do you know this to be true, about the ground line being connected to the water pipes, which are also grounded to the AC grid ground? Just wondering how you know.
NickZ
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