Last updated 14 November 1996


The information presented below is just that, information. Use it at your own risk.. You (the reader) are responsible for anything foolish that you might do as a result of reading this article. You assume complete and total responsibility for your actions! The information presented below is believed to be technically correct (however mistakes occasionally are made, this being the "real world"). In simple terms, if you do something foolish (as a result of reading this article), don't blame me!

AC Power Explained


Most people plug a device into an AC outlet without everthinking about how it works. Most of the time, everything works fine and no problems are experienced. However, there are a few things that are helpful to know about AC power as far as disc jockeys are concerned.

The "power" that is supplied by a typical wall socket is 120 volts AC (often written as VAC). AC is an abbreviation for alternating current. In addition, some outlets are wired for 240 VAC (these are typically used for devices requiring a large amount of power, such as electric ranges, dryers, etc.). Devices requiring 240 VAC power have different plugs as compared with devices intended for "standard" household outlets.

There are two common types of power, single phase and three phase. Most disc jockeys will never have to deal with three phase power directly. There is virtually no equipment that a DJ will ever have to use that requires three phase power, as it is used only where very large amounts of power are required. Three phase power will be briefly discussed below. Since the vast majority of equipment that a mobile DJ uses runs off of standard 120 VAC, it will be the focus of these discussions.

All power outlets have a current rating, the most common ones being 15 amperes and 20 amperes (amperes is almost always abbreviated to amps). The amp rating of an outlet specifies the maximum amount of current that may be drawn from the outlet without tripping a circuit breaker (or, in older systems, blowing a fuse). The primary reason for circuit breakers and fuses is to limit the amount of current that may bedrawn. If too much current is drawn through a wire, overheating of the conductors may occur, leading to a possible fire.

The amount of power that may be safely drawn from an outlet is the product of the voltage and the current rating, and the units are watts. For 15 and 20 amp circuits (assuming 120 VAC), the power ratings are 1800 and 2400 watts, respectively. The total power draw of the devices connected to an outlet should not exceed these values (or else the circuit breaker may trip).

What are the three "prongs" for?

You may have noticed that many appliances, tools and most pieces of professional audio gear have a cord with 3 prongs on the plug. The power outlet of course has 3 corresponding female receptacles (unless it is an older type,which has only two slots).

The longest, often rounded prong is the ground conductor; the primary purpose for this conductor is for safety. There is also a hot conductor, and a neutral (or sometimes called return) conductor. On a wall outlet, the wide slot is connected to the neutral conductor, and the narrow slot is connected to the hot conductor (the ground conductor is connected to the "half round"hole). On some cords with a ground prong, the hot and neutral prongs are often the same width. Most 2-prong cords today are polarized; one prong is wider than the other (allowing it to be plugged in only one way). You should never defeat polarized cords, they are made that way for a reason! Also, note that in 120 VAC wiring, the black wire is the hot wire! (often, black is used for ground in DC electronics). The hot and neutral conductors are the ones that actually carry the power to and from the device of interest.

The hot conductor is so called because it is the conductor that actually has the voltage on it. The hot conductor has 120 VAC on itwith respect to the neutral conductor. The voltage on the hot conductor is of nopractical use without the neutral conductor, because for useful work to be done current must flow through a device; the neutral conductor provides the path to complete the circuit. Ever wonder why birds can sit on power lines (at thousands of volts) without getting shocked? Despite the high voltage on the line, no current flows through the bird since there is no return path conductor. The reason for electric company warnings about staying away from downed power lines is that a return path is provided by the ground (or ladder) you are standing on (basically, YOU become the "neutral" conductor). If you were to jump up and grab a power line while touching nothing else whatsoever, you would suffer no shock (however, DO NOT ATTEMPT TO TRY IT).

As stated, the ground conductor is basically a safety feature. Under normal conditions, it carries no current. It is designed to protect the case of a device (like a washing machine for example) from being raised to a high voltage. For example, if a device (like an amplifier) was accidentally dropped, internal damage might occur which could possibly cause the hot wire to come in contact with the outer metal case. If no ground wire were present, upon plugging in the device the outer case would be raised to a voltage of 120 VAC (basically the case becomes "live"). The operator would be in for a big surprise when touching the unit! However, if the unit has a ground wire, a short circuit results, and trips the circuit breaker back at the panel box, removing the voltage from the outer case. So, follow this advice: NEVER BREAK OFF THE GROUND CONDUCTOR FROM ANY CORD WHICH HAS ONE!. Note: I've seen a number of audio installations whereground plugs are broken off. The reason is often the same: "My system has 60 Hz hum if I use the grounded cord!" Systems that have 60 Hz hum usually suffer from a condition known as ground loops, (to be covered in another article) which is basically a result of careless or improper interconnection of components. If you carefully adhere to proper wiring techniques, your system will not have 60 Hz hum when grounded cords are used. Car stereo systems that have engine noise fall into the ground loop category (most of the time) also.

Note: Many older facilities have power outletswhich have only two slots and no ground connector. Adapters are available at hardware stores which allow a 3-prong device to be plugged into such an outlet. The problem is that most people do not connect up the ground lug! For these adapters to serve their purpose, the ground connection MUST be made (usually by a wire or lug which screws to the center hole on the outlet wallplate). On some occasions I have found outlets like this where the outlet box was not grounded, so connecting the ground lug on the adapter still does no good. Such outlets should be avoided if at all possible! Outlets like these are most likely to be in VFW, American Legion, and church facilities (which all tend to be older facilities with limited budgets for upgrades). Venues such as banquet halls, hotels and schools are usually up to code (but there are exceptions!)

120 Volts? Usually but not always...

Virtually everyone plugs equipment into an outlet without ever giving a thought to the quality of the power (which can and does vary significantly). The voltage present at a typical household outlet is nominally 120 VAC. However, the value may vary considerably for a variety of reasons. I've measured voltage levels ranging from around 108 to over 130 volts at various venues! Fortunately, the frequency of the power (60 Hz) is tightly regulated by the power company and is not normally a concern. However, extreme voltage levels (low and high) can cause erratic operation or have detrimental effects on your equipment.

Low AC Line Voltage

Low voltage is less likely to damage equipment than high voltage, but it can "starve" certain equipment. If the AC line voltage is too low, some units may not operate properly (since internal circuits may drop out of "regulation"). Power amplifiers may not be able to put out their full rated output since their internal power supplies may be dependent upon AC line voltage. If your system seems like it is unable to put out high level bass that you can normally get, it may be due to low line voltage.

High AC Line Voltage

If AC line voltage is high, equipment will operate but it will be under more stress. Units such as preamplifiers, tape decksand CD players utilize special circuits called linear regulators. Linear regulators (as compared with switching regulators) are used in such devices because they are best for preserving low level signal quality (i.e.minimizing distortion). One inherent characteristic of linear regulators is that they generate heat as a by-product of the job they perform. When the AC line voltage is high, linear regulators have to work harder (i.e. generate more heat). Excessive heat is in general unhealthy for any electronic device. Also, the life of things like light bulbs decreases quickly with increasing line voltage (particularly so with many of the high intensity bulbs used in DJ lighting units).

Power amplifiers may be able to put out more than their rated output under high AC line voltage conditions, but again this will place the unit in a more stressful operating condition. Most well designed units have enough margin to tolerate operation under such conditions, but lesser designs may overheat and possibly fail (or shut down if they incorporate the appropriate safety features).

"Clean" and "Dirty" Power

The terms clean and dirty power refer to the quality of the AC power waveform. Ideally, the power consists ofa pure 60 Hz sine wave (known as clean power), but in practice this is rarely the case. Dirty power is power that has been contaminated with noise and distortion. Dirty power is caused by a variety of reasons, including non-linear devices (computer power supplies for example) being connected to the line, motors, and lightning, to name a few.

You have probably heard the term voltage spikes; these are sudden (but short lived) increases of the line voltage. Voltage spikes are typically caused by inductive loads(such as motors) starting or stopping. Voltage spikes ranging upto 1000 volts are not unheard of! If you operate a sound system on a line with frequent voltage spikes you will likely hear a "snapping" sound coming out of the speakers.

Another type of contamination on power lines is the presence of high frequency noise, often referred to as line noise or "garbage" on the line. This category of contamination encompasses a wide variety of interference signals. Voltage spikes tend to come and go; linenoise tends to be more "steady state" in nature. It may becaused by a variety of sources such as motor brushes, local radio transmitters, and devices utilizing switching powersupplies. Line noise is less likely to damage your equipment (as compared to voltage spikes), but it can have detrimental effects on the performance of your system (usually in the form of degraded sound quality).

Things like resistive heating elements (toasters) and incandescent light bulbs are largely unaffected by dirty power, but electronic equipment can suffer damage or degraded operationf rom running under these conditions. Devices which use lots of digital circuitry (CD players for example) are the most susceptible to damage (from voltage spikes) or erratic operation as a result of running on dirty power. Power can be cleaned upto a certain degree by use of device called a "spike protector/line filter". Keep in mind however that these devices have limitations and are not an absolute "cure" for the problem. Being that they are relatively inexpensive (and worth the cost a sa "first line" defense), every system should have one. Most good quality power strips have the necessary protection built right in to the unit. Devices are available which truly clean up power by removing the spikes and noise (generally known as active power conditioners), but they are VERY expensive and not practicalfor DJ use.

An alternate solution (and still reasonably priced) is to use a power conditioner/control device like the Furman PL PLUS. These units provide spike protection and line filtering, power control for 8 or 10 devices, and an AC line voltage indicator. There are anumber of vendors who offer such devices.

When setting up your system, consider where you will plug in. Try to plug into an outlet on a circuit which is dedicated to the sound system. Avoid plugging in where other devices are connected, especially devices that cycle on and off (such as a well pump motor or refrigerators). Otherwise the chances for trouble are increased. Also (this should be obvious), avoid running equipment during a thunderstorm (ideally equipment should be unplugged from the outlet for the safest condition). One local lightning strike on the power grid could instantly cause severe damage to even the most robust equipment if connected to the AC outlet!

Is the Outlet Wired Properly???

Another pitfall (although relatively uncommon) is waiting to surprise you. Occasionally, you may plug into an outlet that is miswired! Usually this is due to the work of a "do it yourself" homeowner who really does not understand the proper procedures. Public and private venues (banquet halls, schools, etc.) are usually required to have wiring done by a licensed electrician, so mistakes are much less common in such facilities. Should you unknowingly connect to a miswired outlet, you may not notice any problems at all. However, in the worst case, the result could be a serious safety hazard to you and your equipment!

NOTE: I have been to one major hotel in Norwich CT where 240 VAC was wired to a standard 120 VACoutlet! This is a SERIOUS violation, totally against the code and a major safety hazard. How did I know this? Ialways verify the integrity of power outlets before plugging anything into them. There are a few inexpensive and simple tools you can purchase which will give you the ability to quickly check the integrity of a power outlet. I highly recommend them as "standard equipment" for any serious DJ. See RECOMMENDATIONS below.

"Soft" and "Stiff" Lines

You may have occasionally noticed that when your system is playing loudly (especially with loud bass notes) that the light bulbs in equipment (or the room lights if on the same circuit) dim somewhat. This is not due to a "super amp", it is due to the inability of the line to maintain proper voltage at the outlet. This is often referred to as line voltage sag or a soft line. This usually occurs on circuits which are at a great distance from the distribution box and/or when the wiring within the building walls has small conductors (AWG 14 for example). The reason for this behavior will now be explained.

All wires have an inherent characteristic known asresistance, measured in units called ohms. All wires have some measurable resistance (unless they are at absolute zero temperature!). The resistance of a given wire depends on a variety of factors, but the dominating factors have to do with the diameter and length of the wire. A longer wire has more resistance than a shorter wire, all other factors (diameter, material, etc.) being equal. By ohms law, when an electric current is passed through a wire, some energy is lost in the form of heat; the lost energy results in a voltage drop in the wire. For a given current, the amount of energy lost (i.e. voltage drop) increases as the resistance of the wire goes up. Or, for a given wire, as the current increases so do the losses. This means that when longer power cables (or extension cords) are used, less energy is available to run equipment since more of it is lost along the way in the cable.

When an amplifier is playing music, the amount of power it requires from the wall outlet is constantly changing in accordance with the musical program material. Bass notes require considerably more power than other portions of the music, so when a bass note is playing the amplifier will "demand" more power from outlet. As the current increases, the losses in the wiring increase, resulting in a voltage drop at the outlet. As th evoltage becomes lower, any lights plugged into the same circuit will dim accordingly.

The opposite of a soft line is a stiff line. Such lines will also exhibit some voltage sag, but the difference in voltage between "no load" and "full load" is typically a few volts. Under the worst of conditions, a soft line might exhibit as much as 10 volts variation (full load compared to no load). Operation of equipment on a soft line should be avoided. Damage is not a major concern, but erratic operation might result. Most modern venues have relatively stiff lines, but if you play at enough places you will encounter a soft line eventually. Softlines and/or low AC line voltage cause the biggest headaches when a high power sound system (say 1000 watts or greater) and/or many lights are used. In this case, you may have to find several outlets (on different circuits) in order to operate in a satisfactory manner.

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The first things you should obtain (if you do not already have them) are listed below:

Another item you should consider is a rack mountable power controller/conditioner which includes a line voltage monitor. An example of such a unit is the Furman PL PLUS. As mentioned previously, such units provide filtering of line spikes and interference, facilities for constant monitoring of line voltage, plus the convenience o fbeing able to power up a bunch of equipment all at once. One nice feature of the Furman unit is that it displays the linevoltage (on an LED type bar display) BEFORE you actually power up your equipment. This way you can spot trouble before subjecting equipment to it.

Recommendation #1

ALWAYS verify the line voltage of any outlet PRIOR to plugging anything into it. Following the instructions that came with it, use your voltmeter (make sure it is set to measure AC volts on a scale that will give a good reading) to make the measurement. CAUTION! Do NOT get a shock while making the measurement... NEVER touch the metal parts of the probes you are using! If you have a unit like the Furman PL PLUS, you will not have to use any probes. Note: Radio Shack used to make a simple plug-in AC voltage meter (cat. # 22-104) that makes the measurement safe, quick and simple. Unfortunately, they have been discontinued recently. If you can find one of these, GET ONE! The voltage should be somewhere in the 115 to 125 volt range for "normal" operation. If it is not in the normal range, there are a few things you can do...

Correcting for Low AC Line Voltage

Sorry, there are no simple inexpensive ways to remedy this problem. If your budget allows, you could consider another product called a power conditioner/regulator. Such units typically use a device called a "smart autotransformer" to actually bring low (or high) AC line voltage into the acceptable range. Unfortunately, units that are capable of providing adequate current are expensive ($1500 or so), and physically heavy. Furman offers two such units, one that has a 10 amp capabilitya nd one that has a 30 amp capability.

Low AC line voltage may be a temporary condition (due to heavy demand on the utility at peak times, for example). If thebuilding you are in happens to be "at the end of a line" in the distribution system, low AC line voltage may be the "norm" (but it should never be below about 110 VAC!).

Correcting for High AC Line Voltage

If AC line voltage is high at your site, options are still limited, but there are a couple of things you can try. First, try another outlet on a different circuit; it may have a slightly lower voltage. Second, plug in something that draws about half the amount of current the outlet is rated for (7.5 or 10 amps for 15 and 20 amp outlets respectively). The resulting load on the line may "pull down" the line voltage if the line is not too "stiff". Keep in mind however that you have used about half of the outlet capability on a "dummy" device (fog machines or high power lighting units do nicely as dummy loads). The last resort is to plug your equipment into the end of a long (100 foot) extension cord (make sure the cord has AWG 12 conductors)and spread it out, do not leave it coiled up!. This method works by dropping some of the line voltage in the cord, but the side effect is a "soft" line as discussed above. The best (but of course most expensive solution) is to use a line power conditioner/regulator as described above.

Recommendation #2

ALWAYS check the integrity of the outlet wiring using an outlet tester (available at Radio Shack and most hardware and electrical supply stores). If the tester indicates a fault, FIND ANOTHER OUTLET. In my opinion you are better off running a heavy (12 AWG) cord to a distant outlet (one that checks out OK of course!) than plugging into a miswired outlet! The most common fault (in my experience anyway) is an open ground condition. If the outlet box is not grounded, there is little you can (easily) do to remedy the problem. If the box is grounded, you can use a 3-prong to 2-pron gadapter to correct the situation but you must connect the ground conductor of the adapter. If you plan to just leave it hanging unconnected, you may as well not bother using the adapter at all.


Outlet testers are an excellent "first line" test of an outlet, but they are not capable of certifying that an outlet is properly wired! For example, if an outlet is wired such that the ground wire in the cable is left unconnected and the neutral conductor of the cable is connected to both the neutral slot AND the ground conductor of the outlet (totally inviolation of proper procedures by the way but I have seen it done), the outlet testers described will be "fooled" and indicate a "good" outlet! There are alternative methods that can be used to fully prove the integrity of an outlet, but they involve more complicated measurements which I consider out-of-scope for this article.

Recommendation #3

If you use lighting equipment (fog machines, bubble machines and the like fall into this category) do not plug them into the same outlet as your sound system. Find another outlet which is on a separate circuit for your lights.

Note: Rarely will you find a venue where anyone has a clue about the building wiring, where the circuit breaker panel is, etc. The exception tends to be higher class major hotels. You will likely have to do your own detective work to determine if outlets are on the same circuit. Here's what you can do:

1. This method assumes you can find the circuit breaker panel. You will also need two devices (drop lights, portable radios, or lights from your light show work well for this purpose) in order to do this test (DO NOT USE YOUR SOUND SYSTEMAS ONE OF THE TEST DEVICES!). Make sure your sound system is powered OFF. Plug one device into each outlet that you are considering. Then, flip the circuit breakers off and on one by one and observe the results. If both items you plugged in go off when any one breaker is off, the outlets are on the same circuit. If at least one of your test items remains on at all times, the outlets are on different circuits.

2. If you cannot use method number one, this method may betried. Set up your sound system (hopefully you have a unit like the Furman PL PLUS or at least something with an incandescent bulb in it). Power up the system but keep the volume at zero for now. Find your most powerful lighting effect (or a fog machine)and plug it into the outlet you are considering while looking at your voltmeter (or incandescent lights on your system). If the voltage on your meter drops several volts (or the lights on your system dim slightly) the outlets are probably (but not definitely) on the same circuit. Try another outlet untilyou find one that causes little or no voltage drop (or ligh tdimming). Note that this method is not foolproof!

3. Method 3 is foolproof but THIS METHOD ASSUMES YOU ARE FAMILIAR AND EXPERIENCED WITH MAKING ELECTRICAL MEASUREMENTS. I FYOU DO NOT HAVE EXPERIENCE IN THIS AREA THEN DO NOT ATTEMPT THIS METHOD!. You will require at least one extension cord and an AC voltmeter capable of measuring up to 250 volts. Plug an extension cord into an outlet and bring the end within a foot or so of the second outlet you are considering. Set your voltmeter to AC VOLTS at a range scale of no less than 250 volts. CAREFULLY connect one probe to the narrow (hot) slot of the extension cord, and then CAREFULLY connect the other probe to the narrow (hot) slot of the second outlet. If the voltmeter reads about 240 volts, you have definitely located two outlets that are on separate circuits. You might also read something like 208 volts; if so, you have definitely located two outlets that are on separate circuits. If you read little or no voltage, it is possible that the outlets are on separate circuits, but you will not be able to prove it without doing additional measurements (which I will not cover). If you are in this situation, find another outlet and repeat the test; do this until you find one that gives a reading of 240 or 208 volts (again, these numbers are approximate and might vary by 10% or so). In any case, USE EXTREME CAUTION as you will be measuring voltages that can easily cause death if you are careless! (Note: the seemingly strange 208 volt reading mayresult if the building you are in uses three phase power).


Can I run my system on a portable generator?

Depending on the power demands of your system, youcan run on a portable generator (I'm talking about the "garden variety" 5000 watt models), however, I personally do not recommend it for a number of reasons:

1. Portable generators generally have dirtier power than you will find on a typical commercial line.

2. Portable generators have relatively poor line frequency regulation as compared to commercial power. This means that the frequency may not be 60 Hz and/or it may wander. Any device that uses a power transformer (this includes most audio equipment) may be placed in a stressful condition especially if the line frequency is too low. The line frequency may wander because portable generators do not have the rotational inertia necessary to maintain good frequency stability under changing load conditions.

3. If the generator runs erratically (due to bad gas or a poorly tuned engine) this can place the electronics of your equipmentin a very stressful situation (not to mention that CD players and tape decks will probably stop playing).

4. Portable generators are less well suited to powering reactive and non linear loads (characteristics which powerful amplifiers can have). They are best suited to running constant, resistive loads (like incandescent bulbs and toasters).

In my opinion, you should avoid running your system on a portable generator. However, if your system draws a small amount of power relative to the generator capability (for example a 500 watt system on a 5000 watt generator), you may be OK. If you do, be sure to use a "spike suppressor/line filter" and make sure the generator runs smoothly! NEVER start (or let the generator stall) with your equipment plugged in and turned on!

3 Phase Power

Virtually all commercially generated power is three phase power. In addition, most large electrical power distribution systems are also three phase.

A three phase voltage system is a system that has three voltages (ideally equal) spaced "120 degrees" from each other ( a full derivation and mathematical description of three phase systems is "out of scope" for this article... I may write one eventually). It is easier to generate voltage in this fashion, and in fact devices that operate on three phase power ore often more efficient than their single phase counterparts.

If you look at power lines running along major streets (and high tension lines) you will notice that almost always there are three wires in a group (sometimes you will see a pair of thin wires running along the very top of electrical towers; these are for lightning protection). The three wires are the three phases, known as phase A, phase B, and phase C. Three phase power is provided to facilities which require large amounts of power (such as restaurants, offices, malls, etc.). Few (if any) homes are supplied with three phase power.

As stated before, no equipment (that I know of anyway) for DJ use requires three phase power. There is one amplifier that Crown makes that uses three phase power (I believe it is a 10,000 watt mono amp). Such an amp is not intended for DJ use; even if you had one, most venues you are likely to play (unless you are doing concerts) will not have the three phase power required by the unit.

In Conclusion...

After reading this page you may be leery of ever plugging asound system into an outlet again. Be assured that MOST of the time things will go smoothly and you will experience no problems. The intent of this article is to try to give an idea of the kinds of things to be aware of and some "first line" strategy as to how to proceed in unlikely event that you do have problems. One thing is certain: if you do have problems (say for example the sound system stops playing for no apparent reason) the guests will always look at the DJ as if he/she is the cause! Guests generally do not understand (unless you happen to be playing for an electrical engineering or technical firm) that sometimes the problem is the result of bad power (or wiring) and generally out of your control. By doing some detective work ahead of time, you can minimize the potential for equipment problems and embarrassing situations!

Questions or comments? Send me e-mail:

E-mail to Joe Roberts

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