Grounding
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Improper grounding can create a lethal hazard. Even if you avoid danger, ground loops are the most common cause of AC line frequency hum in sound systems. So it pays to learn about grounding, and use what you learn.
What is a ground loop?
A ground loop occurs when there is more than one ground connection path between two pieces of equipment. The duplicate ground paths form the equivalent of a loop antenna which very efficiently picks up interference currents. Lead resistance transforms these currents into voltage fluctuations. As a consequence of ground loop induced voltages, the ground reference in the system is no longer a stable potential, so signals ride on the noise.The noise becomes part of the program signal.
Can ground loops be eliminated?
Sometimes, in poorly designed sound equipment, ground loops occur inside the chassis. Even thought the equipment has balanced inputs and outputs. In this instance, little can be done to get rid of hum.
You should avoid unbalanced equipment in your system. The exception to the rule would be with equipment that will be very close together, connected to the same leg of the AC service.

Figure 1 illustrates a typical ground loop situation. Two interconnected pieces of equipment are plugged into grounded AC outlets at separate locations, and the signal ground is connected to earth in each of them. The earth ground path and duplicate signal ground path form a loop which can pick up interference. If the equipment is not properly built, these circulating ground loop noise currents (which act like signals) travel along paths not intended to carry signals. The currents, in turn, modulate the potential of the signal carrying wires, producing hum and noise voltages that cannot easily be separated from the program signals of the affected equipment. The noise is then amplified along with the program signal.
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What can you do to avoid ground loops?
There are four basic approaches to dealing with grounds within an audio system: single point, multiple point, floating, and telescoping shield. Each has specific advantages in different types of systems.

Figure 2 illustrates single-point grounding. Chassis ground in each individual component is connected to earth. The signal ground is carried between components and connected to earth at one central point. This configuration is very effective in eliminating line frequency hum, but is easier to use in permanently installed systems. Single point grounding is how TSC normally wires studios and is common practice in most other studios as well. It is also effective in wiring individual racks for theatre and church installs.

Multiple point grounding (Figure 3) is what you find with unbalanced equipment in which chassis connect to signal ground. It is very simple in practice, but is not very reliable ... particularly is the system configuration is changed frequently. A good example of multiple grounding would be a guitar rack. One advantage to the rack system is that the chassis are on the same rack rails which in turn referenced the whole rack as a single point ground.
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Multiple point ground systems that employ balanced circuits with properly designed equipment present no special noise problems.

Figure 4 shows the floating ground principle. Note that the ground in completely isolated from the earth. This system is useful when the earth ground carries significant noise. It does, however, rely on the equipment input stage to reject interference induced in cable shields, so the input amp better be good.
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Figure 5 illustrates the principle of telescoping shields. This scheme is very effective in eliminating ground loops. When noise enters a shield connection only to earth, that noise can't enter the signal path.
Implementing this approach requires balanced lines and transformers since ground is not carried between components. There is still debate about the use of transformers in modern audio equipment though.

Grounding Safety
The main reason we ground a sound system is for safety. Proper grounding can prevent lethal shocks. The next reason for grounding a system that includes AC powered equipment is that proper grounding may reduce external noise pickup.
The AC power cord ground (the green wire and the third pin on the AC plug) connects the chassis of electronic equipment to a wire in the wall power service that leads to an earth ground. The earth ground, required by electrical codes everywhere, can contribute to ground loops.

Don't break the AC ground!!!
With just one path to ground, there can't be a ground loop. Can there be a ground loop with one audio cable joining a console to a power amplifier? Yes! A ground connection through the AC cables and the chassis of the two units campletes the second ground connection.

One way to break this ground loop is to lift the AC ground on one piece of equipment, typically the power amplifier. This removes the safety AC ground. The system now relies upon the audio cable to provide the ground ... a practice that is hazardous!!! You also put at risk your multi-pair snake, console, post rack equipment, and most important the client. I do not endorse the use of AC ground lifts for any system ... anywhere. Don't do it.

In certain situations you can lift the shield at one end (usually the output) of an audio cable and eliminate the most likely path that carries ground loop currents. This is the way all TSC amp racks are wired and is seen as standard for most tour type rigs. This method takes into account that the shield doesn't carry audio signals. It does protect against static and radio noise. With one end lifted however, it continues to reject static and other interference into the audio path. Note: don't cut the shield of a mic cable that carries phantom power or you'll cut the power to the mic.

Maximize safety while avoiding ground loops
Try not to lift the safety ground on any piece of equipment. NEVER defeat the AC safety ground on your console or any piece of equipment connected directly to your mics. Mics take priority in grounding safety because the performers holding them may touch other grounded items, including a wet stage ... and then .... !!!
Always try and plug your equipment into the same AC service leg. This includes FOH, amp land, monitor land, and band back-line. This not only reduces the potenial of a ground loop, but also reduces the danger of electrical shock.

Always connect lighting, air conditioning, rigging motor, and so on to a completely different phase or leg of the main power distribution.
Remember to plan ahead and always think safety.
Written by Devin DeVore;  modified by S. Pankratz
TSC / TRINITY SOUND COMPANY