How to Filter AC Line Noise

by John Papiewski

Electrical line noise from household and commercial alternating current outlets causes a variety of unwanted effects, from poor audio and video quality in consumer electronics to lost computer data and even equipment damage. Although not all AC noise is preventable, a variety of devices can help you filter noise out, improving the performance of electronic equipment and prolonging its useful life.

Sources of Line Noise

In the U.S., government regulations and careful engineering have historically given residents remarkably clean and reliable electric power that many take for granted; by comparison, less-developed countries experience local blackouts on a daily basis. However reliable it may be, the electricity is nonetheless susceptible to many outside sources of interference, including thunderstorms, nearby radio transmitters, power tools, household appliances and industrial equipment. These can add low-frequency buzzes, static, high-voltage spikes and other types of noise to the otherwise steady, regular flow of power. Vintage electronic devices are particularly vulnerable to power problems. Modern equipment typically has some power noise filtering built in, but severe line noise can compromise the performance of nearly all electronics.

Filters and Surge Suppressors

Filters and surge suppressors are simple, relatively low-cost components that reduce AC line noise. Special electronic filters eliminate AC line noise where the unwanted signals have a higher or lower frequency than 60 Hz. For example, a low-pass filter greatly reduces high-frequency noise due to radio interference while allowing the 60 Hz AC to pass through. A surge suppressor acts in a different way, absorbing the energy of sudden voltage spikes in the AC power that would otherwise fry sensitive electronic equipment. Although a severe spike might destroy a surge suppressor, the low cost to replace the component is worth it to save expensive electronics.

Uninterruptible Power Supply

An uninterruptible power supply, or UPS, delivers clean, steady AC power even through line noise, brownouts and other problems. The UPS circuit converts AC current to DC and uses the DC to charge a battery. The DC is converted back to AC, which you use to run computers and other sensitive and crucial equipment. The process of converting AC to DC and back again cleans the power signal significantly, preventing unwanted noise from getting through. Also, during a power outage, the UPS circuit goes to battery backup power within a few thousandths of a second, preventing power loss from shutting down any connected equipment.

Proper Grounding

Poor ground connections to electronic devices can result in ground loops, an electrical imbalance between different pieces of equipment. This typically shows up as a loud, low-frequency buzz or hum in audio electronics. Although every ground connection in a room should be at zero volts, slight voltage differences between outlets produce ground loops. The noise may seem like an AC power line problem, but filtering won’t get rid of it; you need to ensure that all grounding connections shared among several devices are at the same potential. To accomplish this, you may need the help of a qualified electrician or audio technician. In some instances, the fix may be as simple as plugging all audio equipment into a single power strip.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."

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