I've been upgrading my system. I got a Marantz receiver, and a EP350 sub. They're work just fine with the coax cable between them. But since the receiver has a 12 Volt DC output, and the sub has a trigger input, I figured I'd try to save a little electricity and let the sub turn off when the receiver is off.

The trigger works. Plugging the 3.5mm, mono cable into the sub, turned it off. Connecting the other end to the receiver, and then powering on the receiver, in turn powered on the sub.

But then I noticed a hum coming from the speakers plugged into the receiver. Disconnecting the trigger wire, makes the hum go away.

So there's obviously a ground loop happening here. The receiver and sub are both plugged into the same power strip. The receiver has a two-prong plug, so no ground, and a double insulated chassis. The EP350, has the three-prong plug, with the safety ground.

I tried removing the chassis ground screw, I'm not sure if that trick is still supposed to work, but it didn't resolve the hum. The receiver does have a chassis ground screw on the back. Should I try running a separate wire between the two chassis grounds?

Any other ideas? I'd really like to be able to use the trigger.

The wire is probably your easiest troubleshooting test.
Hope that's all you need for the pesky nuisance.

I have a really bad one from my cable provider even get frequent drop outs cable and internet. So far about 4 tries and the last round a guy admitted they need to put a new connection to my building.

luckily I have an older Monster Power center with a Ground loop isolator built in works magic for the time being.

Unfortunately isolation transformers don't work for DC, so there's not a passive way to break the trigger line between the two.

I'll see if I can find a scrap piece of wire that can fit under the sub's ground screw, and do a quick test. If that works, I'll make something cleaner.