Musician Tips: AC Advice
Without electricity there would be no rock ‘n’ roll, so it seems safe to say that electricity is a necessary evil. In order to understand how AC affects your sound system, it needs to be understood how the basics of electricity function.
Picture a garden hose with water running through it. Electricity is measured in amperes (amps), much the same way water is measured in gallons. The pressure at which electricity flows is measured in watts. For instance, on the back of a power amp, it may have a tag near the power cord that states 120VAC/300 watts. THIS IS NOT THE POWER OUTPUT OF THE AMPLIFIER! It is merely a method of determining the amount of power that the amp will require to operate properly. The way that this translates into the number of amps that a power service needs to have is as follows: using the 120V AC/ 300 Watt amp, we need to determine the number of amps that the unit requires for full power. Divide the number of watts by the voltage (300 Watts divided by 120 volts = 2.5 amps). Some amps list the amperage but not the wattage, so if this is the case, simply multiply the voltage by the amperage (120 VAC x 2.5 amps = 300 Watts).
Electric systems (112-125 Vac) usually use a three wire system. The wires are white (neutral) black (hot) and a ground wire. The ground wire is usually either bare, or covered with green coating. At the breaker box, the white is grounded to earth along with the bare or green wire. This assures a grounded circuit in the event that the outside ground is lost. Looking at the receptacle, the white wire is connected to the wide socket, the ground, to the center socket, and black to the narrow socket. Black and white, or black and ground should never be connected at any point. Further, if the device being used has a ground plug, yet the wall receptacle has no ground socket (many older systems have no ground) DO NOT BREAK OFF THE GROUND PLUG! If it is removed and the wall socket is not properly wired, the result may either shock or kill the user. This is not a lesson to be learned through personal experience. Find a ground lift plug at any grocery store, hardware store or music store (about 99 cents each) and use it on the end of the power cable. This ground plug may also help eliminate hum in the sound system when used properly. It will allow you to change the polarity of the system without using earth ground, or to lift the ground in a system that has many grounds. Most importantly, this small, inexpensive device will prevent you from being shocked when used properly.
Transmission and shield grounds are two of the other grounding systems that musicians need to be aware of. In unbalanced (usually high-impedance) systems, the side of the cable is known as the transmission ground. The shield is the foil that is wrapped around the pair of wires. The shield helps to control RF (radio frequency) noise that may create hum or static in the cables. Using high quality cables, and utilizing balanced lines wherever possible will eliminate nearly all RF problems, yet with cash registers, video games, and cheap triacs in the light dimmers all around the sound system, noise still may be a problem.
There are a few safeguards against noise and shock that can be used before the system is turned on. First, check the AC at the wall. Testers are available at most Sears, Radio Shacks, and most music stores for under 10 dollars. These testers will let the user know if the system is properly wired. Second, have a couple of direct boxes with ground lifts on them. These are available at any music store, and are used to transform high impedance signals into low impedance signals. Do not attempt to send high impedance signals through a snake, or use a high impedance cable that is longer than 25 feet in length. Cables of long, high impedance runs generate noise problems, so keep the cables short. Third, if a rack of equipment is being used and the system is humming or buzzing, isolate the individual units from each other by using nylon washers between the screws and the rack nails. If this does not work, then ground all units together by running a wire from chassis to chassis on each device.
Once again, use the ground lift plug adaptors as a last resort. Use a volt-ohm meter to determine the possibility of shock. Do not wrap cables around table or chair legs to get them out of the way. This is a quick way to generate noise as an antenna will be created. Finally, to cut down on noise in the fastest manner, make sure that the mixing board and the power amps, crossovers and outboard processing gear is all on the same electrical circuit. This one small thing will save many headaches and keep the system quiet. Being sure that the system grounds are good will prevent shock and noise problems. One final tip: purchasing an inexpensive power supply from the local music store will keep unwanted problems from creeping up. Power supplies have ground lifts, noise suppression, RF protection and constant voltage supply built right in and will speed up the system.
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