Owner`s manual

Minimizing Noise in Autosound Systems

Now that we’ve established some of the causes of noise in any audio system, let’s apply them to a

typical autosound system. First let’s make some assumptions:

1. The manufacturer did their design homework and the unit represents the best that they could

1. do, considering their design constraints.

2. The unit meets its published specs.

Identifying noise sources

All the noise filtering in the world won’t help unless you can identify the various potential noise

sources in the system.

Alternator Sounds like a whine whose pitch varies with engine speed. It can be

caused by poor or careless grounding, ground loops, poor wiring prac-

tices, or bad level matching. Bypassing the alternator output with a

capacitor may help.

Ignition A ticking sound that changes with engine speed (the clicks get closer

together). It can be caused by poor or careless grounding, ground loops,

poor wiring practices, or bad level matching. Resistor spark slugs may

help, especially when tuners are affected.

Turn Signals A clicking sound synchronized with the turn signals. Make sure that the

auto body ground connections to the lamps are secure. Bypassing the

input and/or output to the flasher unit with a capacitor may help. Other

possibilities include poor or careless grounding, ground loops, or bad level

matching.

Dash Lamp Dimmer A buzzy whine whose pitch varies with the dimmer setting. Try bypassing

the dimmer input lead with a capacitor and/or different grounding points

for the autosound system.

Horn A buzzing sound, synchronized with the horn (how did you guess?).

Again, a bypass capacitor should fix this too.

Amplifier Power Supply

High-powered amplifiers sometimes cause problems because the DC to

DC converter inside of them radiates EMI (electromagnetic interference)

into the electrical system of the car. This type of noise sounds like a nasty

buzz, that isn’t affected by engine speed and is usually affected by some

or all of the system controls. Sometimes (especially when the units are

mounted on a board in the trunk) the manufacturer will leave the chassis

of the amp floating (not connected). This turns the amp chassis into an

effective antenna. Connecting it to the auto body ground will turn the amp

chassis back into a shield, which is what it should be.

Figure 4 shows how to bypass the alternator output. It’s important to put the bypass capacitor as

close as possible to the source of the interference. Make sure that the ground connection (the

vehicle body is the place to use) to the capacitor is solid and clean. If you have an ohmmeter,

measure the resistance between the capacitor and the vehicle body. It should be less than 0.3 ohm.