An annoying low-frequency hum in your speakers usually means your audio system has a ground loop, a condition in which poor grounding produces electrical noise. The severity of the noise ranges from a soft buzz to a booming hum that's loud enough to damage your speakers. Fixing the buzz means tracing signals through your audio system and ensuring that equipment shares a common power ground connection.
A standard audio cable connects two pieces of audio equipment, such as a turntable and amplifier. The cable's outer shield joins the signal grounds of the equipment together, but the power supplies must also share an electrical ground; if they do not, the cable carries a slight AC voltage difference between the two devices, producing the tell-tale 60-Hz ground loop buzz in the speaker. Even a good AC electrical system has differences of a few millivolts between nearby outlet ground terminals; this difference grows larger as the distance between outlets increases, or when outlets do not share a common breaker circuit.
Correct Audio Grounding
The more complex an audio system's connections, the more opportunities for ground loops to creep into it. Ideally, all audio cables run to a single, central device, such as an amplifier or mixing console, and the power to all equipment comes from a single breaker circuit. This minimizes the voltage differences between device power-grounding connections.
A ground loop forms between two pieces of equipment when they do not share a common power ground. In many cases, you can isolate a grounding problem by unplugging all audio sources connected to an amplifier and reconnecting them one at a time; when the hum returns, that device has a grounding issue. Some types of professional audio equipment, such as power amplifiers, have a "ground lift" switch that disconnects audio signal grounds, leaving the power safety ground intact; using this can solve some ground loop problems. If this issue continues to plague you after you've examined your equipment, consider hiring an electrician to check your outlets.
The rounded third terminal on the power outlet provides an electrical ground; this is an essential safety feature that reduces shock hazards. Do not attempt to fix a ground loop by clipping off the electrical ground on the plug or by using a "ground-lift" accessory plug between the outlet and your equipment. The only exception to this rule is equipment that runs on batteries or low-voltage DC battery chargers -- these do not pose a shock hazard and do not need an outlet ground.
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