A deep humming sound emanating from your stereo's speakers is in many cases the result of connection problems between your amplifier and any turntables, CD players or other devices plugged into it. Although at low loudness settings it may only be distracting, at high power levels the noise can damage your speakers. In most cases you can solve hum problems without resorting to expensive repairs.
An audio problem called a ground loop is a common cause of hum problems. Slight differences in the AC voltage levels between two pieces of equipment creates an audio hum . Ground loops in home stereos typically occur when turntables or other sources are plugged into different electrical outlets than the amplifier.
Isolating the Cause
Usually, one piece of equipment causes hum in your audio system; the trick is to isolate the culprit. Run your amplifier's input source select switch through its settings to find the one that's making noise. If it makes noise in all of them, the problem may lie in the amplifier's power supply.
Grounding the Equipment
Before working on the stereo's ground connections, turn off all power to the system and its components. Use a heavy-duty power strip, preferably with a surge protector, to connect the amp and sources to the same power outlet. If your equipment has its own ground wire, as many turntables do, connect it to the grounding screw or post on the rear of the amplifier's case. This is a separate single wire connected to the equipment chassis. If it doesn't have a grounding wire, obtain several feet of 22-gauge stranded wire, strip one-half inch of insulation from both ends, and slip one end under a metal chassis screw of your input source at one end and the ground connection of the amplifier. Tighten both connections.
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Other Causes of Hum
A bad amplifier power supply may also cause hum. If you've eliminated the CD player, turntable and other devices as sources of hum, the power supply may be likely, especially for older equipment. Components called filter capacitors can dry out over years of use and no longer work properly. In this case, your best bet is to have the amp repaired by a qualified technician.