Ground hum filter

Well.. There isn't such thing as filtering ground. Ground is supposed to be the lowest impedance connection and therefor it is used as the current return for filtering other signals..

If you have hum, there is a reason for it, and the reason is usually that the ground on the device that is causing the hum is insufficient to handle the return currents, is somehow broken, or is otherwise not designed well. All solutions besides fixing the problem are merely band-aids to the actual issue, and in cases of isolating grounds, is dangerous to do as it requires lifting earth ground and using an isolating transformer.

When I had a major ground loop, I found that it was between the recording computer and the other devices in the racks. It took me a couple weeks but I found that the chassis of the computer wasn't built very well and the various metal panels were not grounded together. I took the computer chassis apart into the various panels, sanded all the connection points and then riveted it back together.

No more hum. What was happening was the sound card was trying to ground through a cable to the converter box in the rack.

If you have hum, you probably have something similar going on. Moving backwards from your converters, unplug and replug various things until the hum either goes down or goes away. Now you know where to start troubleshooting.

First thing is first, make sure you are all on one AC circuit. Using more than one AC circuit is a huge problem in ground loops.