Schematic showing the effect of an extraneous current flowing through the ground conductor of a signal cable linking two components.  This situation, called a ground loop is a very common cause of noise, hum and interference in audio and video systems.  The diagram shows the typical circuit of the line driver and receiver in the components that interface with the cable.   The signal V1 is applied to the conductor of signal cable S by a line driver in component 1. The cable has a ground conductor which is attached to the metal chassis of each component.  At the destination end in component 2, the signal and ground are connected to a differential amplifier to produce the signal V2.  If the chassis of the two components are at different potentials a current I will flow through the ground conductor, which has a resistance R, causing a voltage drop IR along the cable ground.  This voltage will be added to the signal voltage by the differential amplifier in component 2, resulting in interference on the signal
V2=V1+IR{\displaystyle V_{2}=V_{1}+IR\,}
If I is an AC current, and the system is an audio system, a 50/60 Hz hum may be heard in the speakers.
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Chetvorno