One common question I have received in Tech Support over the years with Onlyfactorydirect.com is, "why is my subwoofer humming?" In fact I had this same questions regarding a Powered Speaker, pro audio set up the other day. This information does not only apply to home audio powered subwoofer, but typically any powered audio DJ, band, or home application.
This is typically a result of a ground-loop in your stereo / home theater system. “Ground-loop,” you say, “what the heck does that mean?” Let me explain. A ground loop is caused by the difference in electrical potential at different grounding points in an audio/video system. A ground loop typically adds a loud low-frequency hum or buzz as soon as you plug in any variety of audio or video components, including subwoofers, cable-TV boxes, satellite-TV feeds, TV displays, amplifiers, A/V receivers or turntables. More simply put, a ground loop occurs in an audio system when you have two or more ground connections (three prong plugs) that are at slightly different electrical voltages. The two different grounds “fight” each other and an audible hum is the result.
The simplest way to solve this issue is to plug all theater/audio components into one power strip with a surge protector. If this is not possible, you will need to relocate some connections, we suggest starting with the cable-TV box, as it usually is the primary culprit. If disconnecting the satellite / cable TV causes the hum to disappear, you will need a Cable TV ground isolator.
To verify that you do indeed have a ground-loop, disconnect the RCA jacks or raw speaker wire connection with the subwoofer on and the volume turned all the way down. Once you have disconnected the RCA jacks/ raw speaker wire connection turn the volume back up. If it continues to hum after your increase the volume without the subwoofer being connected, then you have narrowed the cause to your home electrical wiring or the subwoofer.
If you establish that it is in fact a “ground-loop,” and you cannot connect all your components to one power strip than we suggest purchasing an inexpensive in-line ground isolator, if you have HDTV this is not an option. If the fore-mentioned is not an option, than your next step, is to look for all of the other ground points in your system. You will need to isolate the additional ground, and disconnect it; there are several options as to how to approach excluding a ground connection, using a cheater plug is one option.
If the hum is not caused by a “ground-loop” it could be caused by the cables. You want to use new high grade cables to connect your subwoofer. Old, mediocre quality, loose RCA jacks, or kinks in your cables can produce a hum by a broken connection in the cable itself.
It should be noted that ALL audio amplifiers have some degree of background hum. If you can easily hear the hum from your listening position there is cause for concern.