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Ground loop (electricity)
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==In low frequency audio and instrumentation systems== If, for example, a domestic [[HiFi]] system has a grounded turntable and a grounded preamplifier connected by a thin screened cable (or cables, in a stereo system) using phono connectors, the cross-section of copper in the cable screen(s) is likely to be less than that of the protective ground conductors for the turntable and the preamplifier. So, when a current is induced in the loop, there will be a voltage drop along the signal ground return. This is directly additive to the wanted signal and will result in objectionable hum. For instance, if a current <math>I</math> of 1 mA at the local power frequency is induced in the ground loop, and the resistance <math>R</math> of the screen of the signal cable is 100 mΩ, the voltage drop will be <math>V = I \cdot R</math> = 100 μV. This is a significant fraction of the output voltage of a [[moving coil]] pickup cartridge, and imposes an objectionable hum on the cartridge output.{{efn|In practice, this case usually does not happen because the pickup cartridge, an inductive voltage source, need have no connection to the turntable metalwork, and so the signal ground is isolated from the chassis or protective ground at that end of the link. Therefore, there is no current loop, and no hum problem due directly to the grounding arrangements.}} In a more complex situation, such as [[sound reinforcement system]]s, [[public address system]]s, [[Guitar amplifier|music instrument amplifiers]], [[recording studio]] and [[Television studio|broadcast studio]] equipment, there are many signal sources in mains-powered equipment feeding many inputs on other equipment and interconnection may result in hum problems. Attempting to cure these problems by removing the protective ground conductor creates a [[shock hazard]]. Solving hum problems must be done in the signal interconnections, and this is done in two main ways, which may be combined. === Isolation === Isolation is the quickest, quietest and most foolproof method of resolving hum problems. The signal is isolated by a small transformer, such that the source and destination equipment each retain their own protective ground connections, but there is no through connection from one to the other in the signal path. By transformer isolating all unbalanced connections, the unbalanced connections are converted to balanced connections. In analog applications such as audio, the physical limitations of the transformers cause some signal degradation, by limiting bandwidth and adding some distortion. === Balanced interconnection === Balanced connections see the spurious noise due to ground loop current as [[common-mode interference]] while the signal is [[differential signaling|differential]], enabling them to be separated at the destination by circuits having a high [[common-mode rejection ratio]]. This rejection can be accomplished with transformers or semiconductor output drivers and line receivers. With the increasing trend towards digital processing and transmission of audio signals, the full range of isolation by small pulse transformers, optocouplers or fiber optics become more useful. Standard protocols such as [[S/PDIF]], [[AES3]] or [[TOSLINK]] are available in relatively inexpensive equipment and allow full isolation, so ground loops need not arise, especially when connecting between audio systems and computers. In [[instrumentation]] systems, the use of differential inputs with high common-mode rejection ratio, to minimize the effects of induced AC signals on the parameter to be measured, is widespread. It may also be possible to introduce narrow [[Band-stop filter|notch filters]] at the power frequency and its lower [[Harmonics (electrical power)|harmonics]]; however, this can not be done in audio systems due to the objectionable audible effects on the wanted signal.
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