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What is Alternator Noise?
Alternator noise is a high pitched
whine caused by the car's electrical system. When the
engine spins the alternator around, the alternator
induces an AC voltage that is converted to DC and used
to charge the car's electrical system. It acts pretty
much as an inverted electric motor (motion is put in,
and voltage comes out). The problem is that a small
amount of AC voltage remains in the system. Frequencies
change accordingly to the engine's RPMs. If the engine
spins faster, noise frequency is higher. That is why
you would hear alternator noise coming from mids and
tweeters, but not subwoofers, since subwoofers only
play low frequencies.
What causes alternator
noise?
1. Induced noise through RCA's:
When a wire has current through it,
a magnetic field circles around it (i.e electromagnets).
Conversely, if there is a magnetic field perpendicular
to a wire, current will be induced. If you have your
RCA wires going from the radio or equalizer to the
amp running in parallel to your power wires, an AC
current will be induced and added to the sound signal.
The sound signal travelling to the amp is a low voltage
signal (in the mV range - thousands of a volt). The
induced signal will be amplified along with the music.
Avoiding this problem is very simple:
DON'T run power and RCA wires together. If there
are points in which they do have to cross, try to
place them perpendicular to each other. Run the power
wire from the battery to the amp on one side of the
car, and the RCA wires along the other side of the
car. On most cars it is better to run RCA's on the
passenger's side, and power wires on the driver's
side. Note that noise may be also be induced by factory
harnesses, car computers and other electronic equipment.
2. Ground loops:
Your car's electrical system (and
your stereo) use the car metal chassis as a ground
(there is always current flowing through your car's
metal parts). If your battery and alternator are
(typically) under the hood, and you are installing
an amplifier all the way back in the trunk, then
current flows through that power wire you ran from
the battery to the amp, and back through the metal
chassis to complete the circuit.
Theoretically the car's metal has
no resistance, and it should not matter where you
tie grounds for amplifiers, radio, battery and alternator.
They all should "look" like the same point, right?
Well, the metal in your car does have resistance,
and there is a potential difference from the front
of the car, where the battery is to the middle of
the car, where the radio is, and to the back of the
car, where most amplifiers are. The potential difference
of the grounds makes the whole system act as an antenna,
where they pick up noise. Measure voltages at battery,
amplifiers and radio. There should be very little
difference between the measured voltages. If there
is a difference more than 1/2 volt, then you might
have noise problems.
To fix this problem, make sure that
the amplifiers have a good ground first. Use at least
10 Gauge wires for the grounds (and power). If you
have 2 or more amplifiers, DO NOT go from the ground
terminal of one amp to the other and then from there
to ground, most likely you will have noise. Ground
each amplifier independently. Same thing if you have
added stiffening capacitors, go to a separate ground
for the cap.
Troubleshooting
If you installed everything using the
above guidelines and you still have noise, then try
to figure out what is causing the noise (a very LONG
and tedious process). First, double check grounds at
amplifiers, crossovers, radio, etc. Make sure AM/FM
antenna has a good ground. Try to figure out what is
causing the noise. For example, if you have crossovers,
equalizers, etc, bypass them by hooking RCA wires straight
from the radio to the amplifier. If noise went away,
you know problem is maybe RCA wires or grounds hooked
up to crossovers/equalizers. If you have more that
one amplifier and have noise only on one amplifier
try switching RCA wires around. If noise stays the
same, then problem is the amplifier, if it switches,
noise is coming from previous components up the line.
As said before, it is very hard to find out what is
causing alternator noise.
Don't get one of those noise filter
boxes unless you have completely figured out that the
head unit or equalizer are causing the noise. 99.9%
of the time you will be wasting your money in buying
noise filters.
If you have tried everything in the
world, and still have that annoying noise, contact
your nearest car stereo shop. Some of them will be
reluctant to fix something not installed by them, or
maybe will charge you a lot for something you could
not figure out that only took a couple of minutes for
them to fix, so shop around first.
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