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Get the Noise Out of Your Home Theater
You've heard it before. That annoying hum through your home theater's speakers.
Hopefully you've only experienced it at a friend's house and not in your theater. It can be
one of the most vexing problems you encounter. Many late nights have been spent trying
to eradicate the dreaded hum. What causes it? Most noise problems in a home theater or
any audio system are caused by grounding problems, faulty cables, noise on the mains
power line, or faulty electronic gear. What can you do to fix it?

Ground Problems
The single biggest cause of noise problems in a home theater or any other audio system
is a ground loop or a bad ground. When you connect two pieces of audio / video
equipment with an electrical connection, such as an audio interconnect, you usually tie
the signal grounds together as well. You fall into the ground loop trap when the ground is
also tied together in another place, such as the ground in the power cord or having metal
parts of the chassis touch another common conductor. This creates a path for current to
flow from one unit to the other through the ground. A difference in resistance to ground
between the two units will induce current to flow. If the current's path takes it through the
audio signal ground, you will get the dreaded hum.

Another common source of ground loops in a home theater system is  the cable TV
connection. In many cases, the cable TV system's ground will have a different ground  
potential than the rest of the system. This causes a maddening, 60Hz hum that can be
quite loud. To test if a noise inducing ground loop is being caused by your cable TV
connection, simply disconnect your cable from the back of your cable box. If your a/v
equipment is connected any other equipment that is connected to the cable, just
disconnect it at the wall where it comes into your media room. If the hum disappears,
you've found the culprit. There are ground isolation transformers available to solve this
problem. Be aware, however that not all of them have the bandwidth to work with digital
cable. Some of the older cable isolation transformers only work out to 500MHz or so, not
the 1 GHz required by many digital cable systems.

Rejecting induced noise that gets in the cables from adjacent audio and electrical lines is
why so much professional equipment uses balanced audio interconnects. Balanced
interconnects reject this type of noise. Balanced interconnects use three wires instead of
the typical RCA connection's two. There is a signal +, a signal -, and a shield. If noise is
induced into the cable from an external source, it enters the + and - signal lines equally,
and is cancelled out. It's a pity that most consumer audio video equipment doesn't use
this type of interconnect. Luckily, the cable runs in most home theater systems are fairly
short, normally less than 3 meters or so. The short cable runs mitigate much of the risk of
using unbalanced interconnects. Just the same, use quality, well shielded cables, and
don't run them for long distances in parallel with noise inducing cabling.
Another potential noise problem in your theater is generated inside your home by lighting
controls and motors such as refrigerator compressors. This noise is fed onto the power line
an then into your audio / video equipment. There, this noise wreaks all sorts of havoc with
your video images and audio. This type of noise is electromagnetic interference (EMI). In
additon, computers, fax machines DVD players, or any other device with a microprocessor
can possibly contaminate the power line and cause noise in your home theater system.
This can be difficult to trace, but in my experience, this type of noise is not as prevalent as
noise caused by grounding problems, faulty A/V equipment and bad interconnects.

If you are unfortunate enough to have a radio station or ham radio operator in close
proximity to your house, this can be a major cause of noise. The wires in your system, such
as AC line cords and speaker cables, act as giant antennas. Inadequately shielded A/V
equipment can also let in this type of RFI. You will hear it as garbled dialog or music from
your system, even when nothing should be playing. If you turn off your power amplifier or
surround receiver, the noise goes away. There is a possible cure for this maddening
problem! You can use ferrite beads on the cables connected to your equipment to choke
the RF before it reaches your components. Ferrite beads are basically just pieces of iron
that clamp around the cables and power lines. You've seen them as big lumps on computer
monitor cables. They're very inexpensive and many times will solve your problems. They're
only about $2/ea.

A good power conditioner is an essential component for ridding your system of RFI entering
through the AC mains. Don't skimp here. Not only will a high quality unit help with noise
problems, but it can clean up your video and protect sensitive, microprocessor based
electronics.

One power conditioner that works very well at protecting A/V equipment and getting rid of
some kinds of noise at a modest price is the Furman M-8DX. It's specifically designed for
use in home theaters. You can
save money on one here. Furman is prized by many touring
bands for their ruggedness and the way they protect sensitive PA and recording equipment
from the varied power conditions found on the road. Unlike most other power conditioners,
they are not sacrificial, meaning they don't sacrifice themselves to save your equipment. If
they did that, you'd have to replace them after a limited number of power surge or spike
events.
If you've got home theater video
problems, troubleshoot them here:
fix video problems
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