How to Filter DC Noise

By John Papiewski

Car radio
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A whining sound in your car’s radio is likely to be DC noise from the alternator, especially if the pitch of the noise changes with engine speed. Uncommon in most modern cars, the noise is generally due to deteriorated alternator parts or problems with the car’s electrical wiring.

Alternators and Noise

A car’s alternator produces direct current used to charge the battery and power all the electrical accessories while the engine is running. The alternator itself consists of magnets that spin inside a set of copper coils; the moving magnetic field generates a current in the wires. The current is AC, however; a diode rectifier circuit converts the AC to DC, which powers the car’s electrical system. Because the DC comes from rectified AC, some electrical noise is present in it. When the noise is excessive, it can work its way into your car’s sound system and become a nuisance.

Noise Filter

Auto parts stores sell electronic filters designed to remove alternator noise. Most filters of this type are heavy-duty capacitors wired between the alternator’s positive terminal and ground. The capacitor sends the noise current safely to ground and passes only clean DC to the car’s electrical system. The filters are inexpensive and easy to install.

Ground Connection

Poor electrical ground connections may be to blame for DC noise in your car. A loose connection may be difficult to trace, as the wiring systems for modern cars have become complex with many hard-to-reach parts. If your car has other electrical “gremlins,” such as erratic power windows or turn signals, a broken ground may be the culprit behind DC noise.

Alternator Repair

Normal alternator wear and tear can damage the rectifier circuit or other electrical parts, leading to DC noise in the sound system. Other symptoms of alternator failure include dead or poorly charged batteries, headlights dimming excessively when the car is idling and dashboard warning lights for the battery. This is a serious issue, as a dead battery can leave you stranded. To fix this problem, you must have the alternator repaired or replaced.