Nakamichi PA-7 Hum

I don't think lack of line filtering would have anything to do with the problem. I would focus on the following possibilities:

-- Interconnect cables that might have an open shield connection somewhere, or that are excessively long, or that are low quality (specifically, that have excessive resistance across the length of the shield).

-- Oxidized plugs or jacks. Use contact cleaner if you have it, or if not just remove and reinsert the plugs a couple of times; the scraping action will often help.

-- Interconnect cable plugs that fit loosely, so that the ground connection is not being made securely.

-- Try plugging the ac power cords from all of the components into the SAME power strip or surge suppressor or outlet receptacle, if that is not already the case.

-- If any of the components have two-prong non-polarized power plugs, try reversing their polarity.

Hope that helps,
-- Al

With time the internal power supply of the amp may become noisy due to the lack of DC filtering. If this is the case you'll need to inspect PCB for the loose soldering joints and replace power supply electrolytic capacitors.

Very true, but in this case he said that the problem does not exist when he selects a cd player with his preamp, while it does exist when he selects other line-level inputs.

Which tells me the problem is unlikely to be internal to his power amp, or to his preamp for that matter. I think it's more likely to be an open return path (shield connection) in an interconnect, or one of the other things I mentioned above.

Or, as others have said, a ground loop with the catv system IF the problem only occurs when there is a connection to something within that system (which I suspect is not the case based on the wording of his original post).

Regards,
-- Al