Here is a brief discussion
about grounding as related
to the performance stage.
THE FOLLOWING MAY NOT BE REPRODUCED
WITHOUT PERMISSION FROM THE AUTHOR ©
Audio Interference: Sometimes after setting up equipment, a loud buzz will appear in the audio system that comes and goes with the raising and lowering of light levels. Usually an audio tech will point an accusing finger at the lighting system, but the fact is that in most cases the fault lies with bad audio cables or in the audio system's grounding method. Cables can be easily swapped out to locate the bad one, so this article will discuss the latter.
Now, it is true that the dimmers are generating the buzz and the problem does go away when the lighting system is off. However, dimmer packs have filters that reduce the buzz by `n' decibels and that should be the end of it. Yet although reduced, some level of interference is still being radiated. This would normally be shunted to ground by the audio system's cable shielding, but ground faults may cause the shields to be sitting at some point above actual ground.
Voltage Divider: A signal at some point above actual ground is the equivalent of voltage or signal at one end of a resistor with the other end attached to ground. That signal will be detected and then amplified. Think of this resistance as a voltage divider with some of the unwanted signal being bled to ground and some appearing across the resistance where it can be picked up by the audio system. That resistance occurs naturally in the ground wiring in a given venue.
Ground Line Resistance: After bad audio cables, the most common reason for interference being picked up is a ground loop. This is usually caused when the house audio console is plugged into an AC outlet in the audience area and the power amplifiers are plugged into power at the stage. Each device is safety grounded through the third prong on the plug. However, actual ground is not at the outlets to which they are attached. Their ground lines must run through the building wiring to the main electrical input at the building's service entrance. There, it is attached to a cold water pipe or a ground stake/conduit.
Accumulated Resistance: Electrical resistance is encountered because of the long run the building ground wiring takes. This places audio circuit shields at some potential above actual ground. There is resistance in the ground line from each piece of equipment to their common point and then there is resistance from that common point to actual ground. Therefore, the audio equipment's shields are not actually fully grounded, so any interference signal being induced into the audio cabling is never completely shunted to ground.
The Loop: Pieces of audio equipment (the console and amplifiers) are attached through the building wiring. When they are again attached together through direct connection by a shielded cable, a loop is formed. The loop runs through the shield to the chassis of one piece of equipment, to its line cord's third prong, through the building's wiring to a common point, back through the building's wiring by another path to the second piece of equipment's third prong, and back into the shield.
Amplified Noise: Unwanted signals induce voltages into the microphone cable shields causing a higher potential at the house console's `ground' as compared to the on-stage equipment because the mics are all connected to the console. The resistances of the shield wire, added to the resistances of the equipment grounds to their common point, plus the resistance to actual ground, form the aforementioned voltage divider -- which, because of the differences in voltage potential, allows some current to flow in the loop rather than being shunted to ground. That flowing current is amplified as unwanted "buzz".
Breaking the Connection: One solution to this problem is to disconnect the shield at one piece of audio equipment to sever the loop. This is often implemented via a "Ground Lift" switch. Since this is only done at one piece of equipment, the shield is still grounded via the other piece of equipment. Another idea is to lift one line cord's third prong from its ground connection. Although this second method will also break the loop, please do not do it -- this grounding prong is required for safe use of electrical equipment.
A Common Point: The better and safer solution is to attach all the audio equipment line-cord ground prongs to the same point and then attach only that point to the building ground. The building ground you attach to should be as close to actual ground as possible. The same goes for the lighting. Run the house lighting console power to the point where the dimmer packs derive their power and ground. That way, all equipment grounds will be at roughly the same potential. Although there is still resistance to actual ground, at least individual equipment grounds are closer to the same potential, thus limiting current flow within the loop. Be sure to use a heavy gauge power cable for the house consoles so that the wire resistance will be reduced to a minimum. Also, it's best if the audio system employs balanced lines so that no side of the signal line is attached to the AC power line or to ground. In a balanced audio system only the shields are grounded.
UNDER NO CIRCUMSTANCES SHOULD
A LINE CORD'S THIRD PRONG
BE REMOVED OR BYPASSED.
This is a *safety* ground that
protects users from electrical shock.
Perhaps sometime in the future, a balanced AC power system could be implemented which would remove the neutral wire connection from ground and allow it to float. This is similar to a balanced microphone input where no signal carrying conductor is attached to ground or to the AC line. Such a method could limit or eliminate unwanted signals from travelling back up the power line where they could be introduced into an audio system.
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