Ground loop

 The situation which arises when two pieces of equipment, each having an established chassis ground internally connected to signal ground, are then connected via a shielded cable. This forms a relatively large loop from chassis ground to signal ground (shield), shield ground to signal ground, and chassis ground back to chassis ground. Because the electrical pathway formed in this manner has a finite impedance, a difference in potential may occur from one end of the loop to the other, allowing an AC-frequency signal (usually at AC line voltage–50Hz or 60Hz, depending on where one is) to form in the circuit. This signal will manifest itself as a hum which can, in extreme cases, be louder than the audio signal. The solution is to break the screen connection between the two devices, ideally at the end of the cable that is plugged into a receiver, such as a mixer or amplifier with a ground lifter.

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