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.
If the subwoofer is plugged into its own outlet, plug the subwoofer's power cord into an outlet shared by the other components. Use an extension cord if necessary.
If the hum persists, and your system includes a cable TV, disconnect the cable line. If the hum stops, install an isolation transformer on the cable.
If the hum persists, disconnect the remaining component cables one at a time until the hum stops. Install a coaxial isolation transformer on that cable.
If the hum persists, install a line-level ground loop isolator on the subwoofer's line-level feed cable. This usually solves most hum problems.
If nothing helps, disconnect all the audio cables from the subwoofer. Leave the subwoofer plugged in and switched on. If the subwoofer hums, it points to a defective unit. Contact the company's customer service to technical support