Bob Simon wrote:
> I had forgotten that there is a high-level connection from the
> amp's main (speaker) outs to a powered subwoofer which was plugged
> into a different circuit. When I used a 3-wire extension cord to
> plug the sub into the power strip for the rest of the system, the
> hum was reduced to a very low level.
** With the TV still de-earthed ?
Hum loops are caused by TWO separate things.
1. Different AC circuits being used with independent safety grounds with varying levels of stray AC voltage - generally only millivolts.
2. Magnetic field injection into a ground LOOP or loops formed by the various items in a system being connected to each other as well as a single safety ground.
The mag field comes from any power transformer inside a loop, so part of the equipment.
Picking a single item, usually the main or power amplifier, and making that be the ONLY safety grounded thing is the usual fix.
With a home entertainment system that includes a TV signal arriving at the premises from outside, that is another source of ground hum just like situation #1 described above. Hence using a balun isolator is normal practice.
As an alternative, I wonder if an affordable power isolator exists for this type of issue so I won't need an extension cord (which adds resistance). Are you aware of something suitable?
** Most folk just break the ground pin off the plug or fit a plug with no such pin. Neither is safe or legal.
I know how to modify items to eliminate the problem safely, but it would not meet regulations so a device that did the same cannot go on sale.
For the curious - it involves fitting a 25amp bridge in series with the safety ground conductor. This allow the item to float up to a volt or so above ground with NO current flow.
The bridge is wired as pairs of inverse parallel diodes and can cope with a massive fault current if need be until a fuse blows or a supply breaker trips.
My workbench scope has this scheme fitted inside, for example.