Why is my subwoofer and/or speakers humming?
There are a few possible sources of hum in your system. The sections below are an attempt to describe these issues as well as specify their solutions.
Ground Loop:
What is ground loop interference?
Ground loop interference causes the vast majority of hum related complaints on HTF. This is caused by a difference in electrical potential between your cable TV or satellite cabling and your audio system. Although these long cable runs are supposed to be properly grounded, very few installers take the time to install the system properly. The cable should be grounded to the house ground, not a pipe or small ground tap outside. The result is that this ground potential causes a 60Hz hum on the audio portion of your system. This audio range is usually reproduced by a subwoofer so people blame it on their sub. The problem usually appears when the user starts using components with 3-prong AC power cords. Without any 3-prong (grounded) devices, a ground potential will not exist through the cable. Note that 'floating' an existing ground plug (
by using a 3 prong to 2 prong adapter or by cutting the ground prong off) is usually not safe!
How to determine if it's a ground loop issue:
Unplug the cable going into your satellite or cable TV box. If the hum disappears, then you have identified your problem. Other possible culprits are radio antennas or long runs of cable (especially if they are not shielded properly). Go through the system and determine which part is causing the problem.
Solutions:
For a cable ground loop you will need a part that breaks the grounding path of the cable into your system. Recently Radio Shack has begun to sell
ground loop isolators for RCA interconnects. A Radio Shack 75ohm->300ohm converter screwed into a 300ohm->75ohm converter will also perform this task. Radio Shack also sells FM traps and Ham radio filters that may be useful if you live near broadcast towers. Using quality coax cable will also help. I like to recommend "Quad Shield RG6 coax" which can be found at your local home improvement store. The Quad shield helps to keep the signal conductor inside the cable from picking up interference.
 | NOTE: Using a 'cheater' plug (3-prong to 2-prong adapter) is dangerous and not recommended! Unless the manufacturer specifically recommends the use of a cheater plug (also known as a "ground lift"), do not install one in your system. If you disconnect the ground and something inside the component's case shorts out then you could become the path to ground (i.e. electrocution). While these devices will sometimes reduce ground hum- you should understand the risk involved. |
Transformer hum:
A transformer is a device that takes one voltage level (incoming power) and transforms it into another level (power for circuitry). This is achieved though the principles of magnetism and electricity. Because of this, the transformer can literally vibrate (called 'lamination rattle') and needs to be securely fastened to the amplifier enclosure. If you hear a serious vibration/rattling sound from your amplifier then get your local technician to tighten the fasteners holding the transformer.
Servo controlled subwoofer:
Some subwoofers are servo controlled, meaning they use a small device to actively monitor the speaker cone's motion. The feedback of this device will result in a
very soft hum (i.e. you can only hear it if your ear is close to the speaker). This is a drawback of the servo design and there is no solution (beyond getting a non-servo design). It should be mentioned that many people will accept this in order to gain the potentially increased accuracy of a servo design.
Bad cables:
It is possible that some cables could have a slight short due to shoddy construction or damage. Additionally, poorly shielded cables can potentially pick up EM noise (which can result in unwanted noise during playback). Note that adequate cabling is not expensive... Radio Shack Gold and AR brand cables are two examples.
Noise on the house power circuits:
It is possible for some devices to inject a large amount of noise into your HT system via the power wires. Examples of these noisy devices include dimmer switches and refrigerator compressors. If this is causing issues with your system, relocate either the noisy device or the system onto a different circuit. Some forum members choose to install dedicated circuits for their large HT systems. This should be done or supervised by an electrician. Another solution is to install a filtering device or regenerate the AC on the circuits going to your HT. These devices vary in cost and some include surge protection.