a couple of changes for clarity....
On July 28, 2018 at 08:20, highfigh said...
Did you disconnect the coax at the demarc box? If you do that and the hum stops, call the cable company and have them fix their system.
If you want a quick fix, go to the closest car stereo shop and buy an audio ground loop isolator. The second link in kwkshift's post is just that and you should have at least one in your van, anyway. A company called PAC has them and they even provide a small boost, to make up for losses along the way. Not that 70' is really 'long', but for an unbalanced line, it's pushing the limit without a line driver of some kind.
The need for an isolator (or the more expensive fix of changing the sub circuit's assignment in the breaker panel, so it will be on the same phase, (which might or might not fix the problem) is easy to determine- unplug the cable (probably meaning cable company cable but unclear) and if the noise stops, you need one but first,
wait wait wait wait wait wait...
unplug it and connect a test light between the shield of the cable with the shield of the sub's input- if it illuminates, you have a ground loop or leakage. Repeat by connecting to the center lead and input + on the sub.
What voltage test light? 12 volts for car use? Neon? Plain old 120V light bulb in a socket? It's possible to have current flowing through a ground and creating hum, but with too little current to light any actual bulb. More details are needed to this right.
