Clean up your power act before it cleans your clock.
Electronic musicians should be intimately familiar with AC power; after all, it is required by most of the equipment we use to make music. However, most of us are woefully uninformed about this critical subject, and that can lead to all sorts of problems, from mysterious hums in the audio to outright electrocution. Clearly, it's important to understand AC power if you want to create a clean audio signal and live to enjoy the fruits of your labor.
The information presented here will help you understand AC power, but be forewarned: you should have a professional electrician make any significant changes in your home's electrical system. Mucking around with household current can be very dangerous, so leave it to a pro.
Down to Basics
The electrical signal that reaches your home from the power company is an alternating current (AC) with a sinusoidal waveform. In this country, the signal's frequency is 60 Hz (in Europe, it's 50 Hz), which is very tightly controlled to a tolerance of 0.01%. Many time-based devices are designed to sync to this frequency because it is so stable. However, it is also in the audible range, so it can be heard if it gets into an audio path. (On the plus side, it makes a very nice B-natural tuning reference!)
FIG. 1: Electrical power enters your home as a 240 VAC sine wave, which is divided into two 120 VAC sine waves that are 180 degrees out of phase with each other.
Because the signal is AC, its voltage is measured with the root mean square (RMS) method (see "Square One: Watts & Volts & Logs, Oh My!" in the December 1995 EM). The power signal arrives at your home with a voltage of 240 VRMS (which is normally called VAC). Once this signal reaches the distribution point in your home, it is divided into two signals of about 120 VAC each, which are 180 degrees out of phase with each other (see Fig. 1). The exact voltage of these signals is not as carefully controlled as the frequency because conditions at the power company can fluctuate as overall demand for power changes. A few heavy-duty appliances, such as electric stoves and clothes dryers, use the entire 240 VAC signal, but most powered items use one of the 120 VAC signals.
The distribution point is normally a metal box with fuses or circuit breakers arranged in two vertical rows. Each row distributes one of the two 120 VAC signals to the outlets and light fixtures in your home. Typically, each distribution point in the box sends power to several outlets and light fixtures, such as those found in one room.
In addition, each fuse or circuit breaker limits the amount of current that can be drawn from all the outlets and fixtures it feeds. This limit is typically 15 or 20 amps. If the combined current exceeds this limit, the fuse or circuit breaker "blows," disconnecting the power from all its outlets and fixtures. This prevents the wires in your walls from overheating and possibly causing a fire.
Speaking of the wires in your walls, the 120 VAC signal is sent to each outlet through a single wire called the hot wire. Of course, all outlets have at least two holes; newer outlets are "polarized," and the hot wire is connected to the smaller hole on the right of the outlet, so make sure that the device's hot wire is connected to the correct hole.
When you plug something into an outlet, the power signal flows from the hot wire through the device and back out to the other hole (which is the larger one on the left of a polarized outlet). This other hole is connected to the neutral wire, which goes back to the breaker box, where it is connected to a ground point. Because the power signal makes a round trip from one of the distribution points and back to the ground point in the breaker box, it is said to complete a circuit. Each distribution point is often called a circuit, as well.
Ground represents 0 VAC, and it is often established by connecting the ground point in the breaker box to the cold-water plumbing in your home. Some homes use plastic plumbing, in which case one or more metal (preferably copper or copper-clad) stakes are driven into the ground on which the home is built, and the ground point is connected to it. This approach, which is often used in professional studios, is commonly referred to as a true earth ground. (I'll return to this point in a moment.) Basically, the ground provides an infinite sink for electrons, which always take the easiest path to ground they can find. Once the power signal leaves the device, it makes a beeline to ground via the neutral wire.
Three-prong outlets include a separate ground hole and ground wire, which is sometimes called the safety ground and is also connected to the ground point. The safety ground provides protection in case the hot lead is accidentally connected to the chassis of the device being powered, which is called a short circuit. If this happens in a device with a 2-prong plug and you are touching the chassis, you become the current's path to ground, which could give you a real jolt. With a 3-prong plug, however, the current has a much easier path to ground, which protects you from electrocution.
It is so important to keep these wires straight that a convention has been established in the electrical industry. The insulation of the neutral wire is white, and the ground wire in a 3-prong outlet is either green or has no insulation at all. The insulation of the hot wire is any color other than white or green, with black being the most common.
Appliances that include motors, such as air conditioners, dishwashers, and refrigerators, can sometimes put a momentary strain on the side of the circuit-breaker box to which they are connected when the motor turns on. It's best to make sure the outlets in your studio are connected to the other side of the box from any such appliances. Otherwise, the power might fluctuate when these appliances start their motors, which can cause problems with studio equipment (e.g., it can scramble the memory of synths and other devices).
It's relatively easy to determine which side of the box your studio outlets and other appliances are connected to. First, make sure any computers and music gear are turned off and unplugged. Next, plug in several radios or lights around your home and turn them on. Then, trip each circuit breaker in the box and see which radios and/or lights turn off. Make a note of which breakers affect which outlets.
If necessary, have an electrician reconfigure the breaker box so that heavy-duty, nonmusical appliances are on one side and the studio outlets are on the other side. However, this might not be possible because the electrical load on both sides should be relatively equal.
By the way, if you notice that the lights dim momentarily when a major appliance such as the air conditioner or refrigerator turns on, call an electrician as soon as possible. This symptom generally indicates that you have a fault in the neutral or that one of the hot legs is not properly connected.
For maximum isolation, have an electrician establish a completely separate electrical service and ground for the studio outlets. In particular, the electrician can create a true earth ground using a dedicated grounding rod. This can be expensive, but it is the only way to be completely sure that other appliances in your home won't affect the studio equipment.
Another thing to consider is the total amount of power drawn by all of your equipment. You should make sure it isn't overloading the circuit to which it's connected. This is relatively easy to determine; most pieces of electronic music equipment specify the amount of power they require (in watts) on the back plate or in the technical specifications of the manual. However, the limit of most circuit breakers is specified in amps, which relates to current.
The voltage remains relatively constant at 117 to 120 VAC, so it's possible to convert watts to amps. Recall Joule's Law (see "Square One: Watts & Volts & Logs, Oh My!" in the December 1995 EM):
P = VxI
This law can also be stated as
I = P⁄V
Apply this formula to the power rating of each piece of equipment in order to determine the amount of current it draws. For example, suppose a synthesizer draws 90 watts. If the voltage is 120 VAC, then:
I = 90⁄120
I = 0.75 amps
Add up the current requirements for all the gear in your studio to determine whether the circuit can safely deliver the current you need to run the studio. If not, have an electrician install a higher-amp circuit for the studio. Fortunately, most electronic music equipment doesn't need much power. Tape decks, power amplifiers, and mixing consoles require the most, but even so, most home studios can easily run from one 20-amp circuit as long as there is nothing else on the same circuit. Be sure to keep studio lighting and ventilation fans on a separate circuit.
FIG. 2: In a properly wired 3-prong AC outlet (a), all three conductors are connected and color coded. In some cases, the ground is disconnected (b) and/or the hot and neutral wires are reversed (c).
It is extremely important to test the outlets in your studio. If an outlet is a 3-prong design, use an AC outlet analyzer, which is available at RadioShack and other electronic-parts stores. This will reveal whether the hot, neutral, and ground wires are properly connected (see Fig. 2). In some cases, the polarity of the hot and neutral connections is reversed, which can create a shock hazard and increase noise in the audio signal. In addition, the ground hole might not be connected to anything, which creates a potential shock hazard and defeats surge and spike protection. These problems should be corrected by an electrician.
If the outlet is a 2-prong design, use a neon circuit tester (also available at RadioShack and elsewhere) to test the ground. Touch one lead of the tester to the metal screw that secures the cover plate and insert the other lead into the hot-wire slot of the socket, which is the smaller slot of a polarized outlet. (Make sure the screw isn't covered with paint.) If the tester glows, the ground is okay. Unfortunately, the cover-plate screw is often not connected to ground. In this case, do the right thing: don't even bother grounding the old outlet, but have an electrician install a properly grounded 3-prong outlet.
FIG. 3: Use a 3-to-2-prong adapter only if the outlet faceplate screw is connected to ground.
If you must connect a 3-prong plug to a 2-prong outletand the only excuse for this is that you are playing a live gig and are stuck with the venue's lame power systemmake sure the cover-plate screw is connected to ground and use a 3-to-2-prong adapter that includes a ground lug on a short wire protruding from the plug (see Fig. 3). Connect the lug to the cover-plate screw to ensure proper grounding.
The Inside Story
Most of the problems caused by AC power in the studio arise because of improper grounding among the various pieces of equipment. Ideally, the chassis of each piece of equipment is connected to the ground of its power supply's input, which should be connected to the safety ground wire in a 3-conductor power cord. However, this connection sometimes comes loose, or the grounding for the unit may have been poorly implemented to start with.
To verify this connection, use an ohm meter to check the resistance between the ground prong of the power plug and the metal case of the equipment. (Make sure to touch an unpainted part of the case.) If the resistance is high, the connection between the chassis and the ground prong is broken or inadequate, in which case you should take the equipment to a repair facility and have it fixed.
Audio Enters the Picture
Every piece of AC-powered equipment includes a power supply that accepts 120 VAC and converts it into a DC voltage, typically ±15 VDC. The power supply might be internal or external (e.g., wall wart or lump-in-the-line). In either case, there is a ground point at 0 VDC between +15 and -15 VDC. This is called the signal ground, because it is normally connected to the ground conductor of the audio cables that carry audio signals from one device to another.
Like 2-conductor power cords, unbalanced analog audio cables include two conductors: hot and shield. The shield forms a concentric tube around the central hot wire and is attached to the signal grounds of the devices it connects. Balanced audio cables include three conductors: hot, cold, and shield. The hot and cold wires both carry the audio signal 180 degrees out of phase with each other, and the shield is connected to the signal ground. In many devices, the signal ground is also connected to the chassis ground, which can cause problems (more in a moment).
Ideally, these cables carry only the audio signal. In the real world, however, extraneous signals sometimes get into them. This can occur when a current in an audio device's AC ground is generated by the device's impedance to the power signal. In this case, the signal in the power ground appears in the signal ground because they are connected through the chassis.
Another common means by which extraneous signals enter the audio path is a process called induction. All current produces a corresponding electromagnetic field that radiates from the conductor carrying the current. Conversely, a radiant electromagnetic field can induce current in a nearby conductor. As a result, unwanted signals can be induced into the audio cable. Balanced cables are much less susceptible to induced noise because of the 180-degree phase relationship between the two signal-carrying wires within the cable.
The main sources of this induced signal are radio-frequency interference (RFI) and electromagnetic interference (EMI). RFI is caused by radio stations, cell phones, and other sources of radio energy that is transmitted through the air. EMI is caused by any nearby current-carrying conductor, such as power cords, large transformers, or electromagnetic coils (e.g., in televisions and computer monitors). When these signals are induced into the audio cables, they can become an audible part of your audio.
FIG. 4: When two audio devices are powered from different outlets and connected by an audio cable, they have two different paths to ground, resulting in a ground loop. Only the ground paths for device 2 are shown here; device 1 has two similar paths.
Thrown for a Loop
RFI and EMI are aggravated by the presence of ground loops, which are formed when your equipment is connected to ground through more than one path. This is especially problematic with 2-conductor power cords and audio equipment in which the signal ground is connected to the chassis ground.
For example, consider two pieces of equipment that are plugged into different wall outlets and connected together with an audio cable (see Fig. 4). Each device has its own ground connection through its power cord, but each unit is also connected to the other's ground through the shield of the audio cable, which is connected to the chassis of both devices. As a result, ground loops can act as antennas that pick up RFI and EMI, causing a current in the ground line that can get into the audio signal via the signal ground.
One way to reduce ground loops is to connect all equipment and outlet grounds to a single ground point, such as a grounding stake, using ground wires that are as short as possible. However, this is not always practical.
FIG. 5: Plugging both devices into the same duplex outlet helps reduce ground-loop problems. Breaking the cable shield of a balanced audio cable also helps.
Many people are tempted to use a ground lifter (such as a 3-to-2 AC adapter without the ground lug) on one of the AC power cords; some people go so far as to remove the ground prong from a 3-prong plug. In this case, both devices see only one path to ground: one device is grounded through its own power cord and the other through the audio cable's shield to the first device's ground. This is very dangerous and not recommended because it eliminates the inherent shock protection offered by grounding. A much safer alternative is to plug two devices that might form a ground loop into the same outlet, which shortens the ground wire between them (see Fig. 5).
Some equipment includes a ground-lift switch, which disconnects the signal ground from the chassis ground and eliminates the path to ground through the AC power cord. This is equivalent to using a ground lifter, but it's much safer. However, using ground-lift switches is a trial-and-error process; in general, most or all of these switches should be in the lifted position, but you must determine the best configuration for your studio by trying different combinations.
Another way to eliminate ground loops is to disconnect the cable shield at one end of an audio cable. This is called a telescoping shield, and it only works with balanced audio cables; both conductors of an unbalanced audio cable must be connected at both ends for the signal to flow. You can buy such cables or make them yourself. In general, the shield's connection should be broken at the end that goes to an audio input.
Yet another method of breaking ground loops is to use audio isolation transformers (also called iso transformers), which are available from Furman, Jensen, Ebtech, and others. Audio iso transformers pass the audio signal from their input (called the primary) to their output (called the secondary) by induction, which requires no direct electrical connection. (See "Square One: Going Direct" in the July 1997 EM for more on transformers.) This effectively isolates the audio signal from the rest of the electrical system.
Iso transformers can also be used on the power line. MidiMotor makes a rack-mountable box called the Hum Buster that isolates several AC outlets using power iso transformers.
Ground loops can form when the chassis of your rack-mounted gear are electrically connected in some way. This commonly occurs in a rack when the metal faceplates of different modules come into contact. It can also occur because the metal rack ears of each device are electrically connected by the metal mounting rails of the rack itself. In these cases, each device has its own ground connection, and it's also connected to the chassis ground of the other devices.
To prevent ground loops in racks, make sure the faceplates do not touch each other and use nylon washers on the front and back of the rack ears when attaching devices to the rack with metal screws. To be extra safe, use nylon washers with a sheath that fits into the devices' mounting holes to prevent any metallic contact between the rack ears and mounting rails via the mounting screws. Some people even build their own racks with wooden mounting rails to avoid ground loops.
To reduce EMI from power cords, it's very important to keep audio cables as far away from power cords as possible. Use cable ties to bundle power cords on one side of a rack and audio cables on the other side.
Despite all the precautions you might take to prevent grounding problems, the AC signal from the wall can fluctuate due to circumstances beyond your control. These fluctuations include surges (temporary increases in the voltage) and spikes (momentary but huge increases in the voltage) from lightning and other sources. The voltage can also drop dramatically in a brownout or disappear altogether in a blackout.
If your gear experiences these conditions, it could be damaged; at the very least, it could operate improperly and its effective lifetime could be shortened. In addition, the power signal, which should be a nice, clean sine wave, can be polluted with noise from RFI/EMI and other sources before it reaches your home, and this noise can get into the audio signal path.
Fortunately, you can protect yourself from most of these problems with various power-management devices. These devices typically include several AC outlets, which can be used to power an entire rack. More expensive units often include several types of protection and are available in rack-mount cases.
The simplest form of protection is a surge/spike protector. Many power strips include this type of protection, which is provided in two ways. Transverse-mode rejection guards against spikes between the hot and neutral lines, and common-mode rejection protects against spikes between the hot or neutral line and ground. Make sure the surge/spike protector you use has both types. Surges and spikes can also travel along telephone wires, and some protectors include phone jacks in addition to AC outlets.
Many power strips also include RFI/EMI filtering, which is also called line conditioning. This uses a lowpass filter with a cutoff above 60 Hz that redirects higher-frequency signals to ground before they get to your equipment. The result is a clean sine-wave power signal. However, these filters can generate extraneous currents in the ground when used with a standard 120 VAC power source.
FIG. 6: A balanced-power system divides the 120 VAC from the wall into two 60 VAC sine-wave signals that are 180 degrees out of phase with each other. Any EMI from this type of power system is effectively canceled out.
One solution to this problem is called balanced power, which is used in products from Equi=Tech, Furman, MidiMotor, and others. In this scheme, the hot and neutral wires from a balanced power supply each carry a power signal of 60 VAC instead of 120 and 0 VAC, respectively, and these signals are 180 degrees out of phase with each other (see Fig. 6). This resembles the way 240 VAC is divided into two 120 VAC lines in your home as well as the operation of balanced audio cables. The total voltage between the "hot" and "neutral" wires is still 120 VAC, so the equipment works fine, but any current in the ground is canceled out. In addition, radiated EMI from the two conductors cancel each other out, effectively eliminating any induced EMI from the power cord.
The next step up in power management is a voltage regulator (also called a line regulator or stabilizer). These are available from companies such as Furman, Juice Goose, and Tripp Lite. A voltage regulator attempts to maintain a constant output voltage to each of its outlets in spite of varying input voltage from the wall. Most can provide a steady 117 or 120 VAC as long as the input voltage remains in the range of approximately 90 to 130 VAC. (Some regulators can deal with input voltages up to 300 VAC.) If the voltage rises above the unit's maximum input range, it should trip an internal circuit breaker to prevent damage.
A voltage regulator can be very helpful in the event of a short brownout (also called a sag), but it can't protect against a long voltage drop below the unit's minimum input or a complete blackout. In most cases, the regulator's outlets are rated for a given amount of power, so make sure you match the power requirements of each device with the appropriate outlet.
The only protection against complete blackouts is an uninterruptable power supply (UPS). This device has a battery that kicks in if the power drops out, preserving the data in your computer and synth/sampler RAM and giving you time to save your work and safely shut everything down. (The amount of time before failure varies depending on the UPS and the load, but usually you get at least ten minutes.) Some units, such as models from American Power Conversion (APC) and Furman, also have surge/spike protection, RFI/EMI filtering, and voltage regulation.
The most important factor is the time it takes the universal power supply to detect a power loss and switch over to the battery. The combined detection/switching time should be under 10 ms. Some systems even include software (such as APC's PowerChute Pro) that monitors and tests the UPS and lets you schedule computer shutdowns to conserve power.
Power management is of critical importance in any studio if you want to stop hums, buzzes, and other noise from creeping into your audio signals. Armed with a solid grounding in the principles of AC power, you can now start to clean up your audio act.
EM Technical Editor Scott Wilkinson has gotten a couple of real jolts by carelessly connecting power cords. Thanks to Jim Furman of Furman Sound for his help with this article.