One of the most annoying -- but difficult to find and solve -- problems that can crop up in a home theater or stereo system, is a ground-loop subwoofer hum.
A ground loop is an electric difference of potential between various ground points throughout the system. Ideally, all ground points have zero volts between them.
When a ground loop problem exists, you'll hear a low frequency hum when you plug any audio or video components -- including subwoofers -- in the electric outlet. This hum is caused by the ground voltage potential differences among the system components, power cords and audio/video cables.
Although some people like trying to stop subwoofer hum by using a power conditioner, in most cases it doesn't help Instead, try the following steps to stop subwoofer hum.