Alternator Whine, Ground Loops, Noise
The Ten Commandments
I. The best cure is prevention. I
can't overemphasize this point. If you've ever spent an entire
weekend tearing an installation apart in order to eliminate some
noise, you know what I mean. Take the time to sketch the system out
before you begin the install. This graphic representation of the
installation will help you to avoid introducing ground loops and
will serve as a road map for eliminating noise if it is present.
II. Don't introduce ground loops. Ground
loops are created whenever an audio ground is established at more
than one location. Theoretically, the only place the audio ground
should be connected to the chassis ground is at the source unit. In
my experience, I've found that in systems that have noise problems,
a ground loop is the culprit nine times out of ten.
III. Never run signal wires alongside power
cables. This is especially true in installations where high
powered amplifiers are used. Large amplifiers are capable of drawing
large currents. These currents vary with the musical demand of the
program material as does the electromagnetic field surrounding the
power cable. The more current that flows through the wire, the
bigger this field becomes. If audio cables are located in close
proximity to this fluctuating electromagnetic field, noise could be
induced into the system.
IV. Always use 100% shielded audio cable.
This will insure maximum protection against induced noises by power
cables and other sources of electromagnetic interference. Good audio
cables are not cheap. If you prefer to make your own cables, I would
recommend using a wire with a foil shield surrounded by a drain
V. Never use the ground wire in the vehicle's
OEM radio harness. This wire usually makes a very poor ground
due to it's length, small wire gauge, close proximity to other power
wires, and unknown termination point. Instead, ground the source
unit directly to the chassis or firewall.
VI. Make sure the amplifiers have a good audio
ground reference. In order for the amps to function properly,
the audio ground must be referenced to chassis ground at the source
unit. If it is not, the amplifier could oscillate. To check for a
good ground reference, take a volt-ohm meter (VOM) and measure the
resistance between the chassis of the radio and the shield of the
RCA line level outputs of the radio. This reading should indicate a
direct short. If this is not the case, grounding the shield of the
RCA line level outputs to the chassis of the radio will probably be
VII. Keep amplifier power ground wires as
short as possible. The longer a wire, the more resistance it
has. When a current flows through a resistance, a voltage drop is
produced. Because of this, the ground reference at the amplifier's
circuit board is no longer the same as that at the chassis of the
vehicle. This ground potential differential can lead to noise and
improper operation of the amp.
VIII. Don't connect all of your amplifier
ground wires under one bolt. Contrary to belief, this is not
required if the rest of the system is installed properly. If you do
connect more than one power ground wire under a single bolt, you run
the risk of amplifier ground modulation. This is caused by the
current demands of, for example a woofer amp, modulating the power
ground wire of a tweeter amp. This results in a squeaking noise that
can be heard over the tweeters whenever bass notes hit.
IX. Make sure all levels are set correctly.
Level setting is a critical part of the installation process. If
done properly, maximum system signal to noise ratio can be obtained.
Keep in mind that you want to drive the audio cables that feed the
amps in the rear of the car as hard as possible. To do this, reduce
the gain of the amplifiers to minimum. Turn up the volume on the
source unit to 80% of maximum. Now adjust the input sensitivity of
the amplifiers upward until the maximum intended loudness is
X. Noise filters can only reduce noise,
not eliminate it. A noise filter is just that, a filter. And like
any other filter (crossover network, etc.), it works by
modification, not elimination. Some installers rely on filters
heavily. In some instances a filter may prove necessary, but I
believe that if the system is installed properly, a filter is
usually not required. About now most of you are probably making
strange faces and saying "Now you tell me!" Well, all is not lost.
If you have already installed your auto sound system and are
unfortunate enough to have some noise, here are a few suggestions on
where to look and what to do.