No matter what size studio you have, power-related noise can cause serious problems. The typical scenario in the personal studio goes something like this: the console is on one side of the room, and the keyboard and rack modules are on the other side. The system, however, always has power-related noise, sometimes even with the mixer faders down.
Power-related noises with analog gear are obvious and fairly easy to troubleshoot, because you can hear changes instantly as you experiment. Hums and buzzes in the analog domain may be the only clue to mysterious problems in your digital devices; power-related noises may not be audible but they can affect equipment performance.
When I used to make house calls for a living, I was usually able to pinpoint and resolve such issues. Even if I offered advice over the phone, it was never quite enough and my presence was requested anyway. Perhaps my explanation was too matter-of-fact, or the client never believed that they had the power to fix many of the problems. But you do have that power. In this article, I will address a number of power-related topics, from chasing down and minimizing a variety of noise problems to determining your power capacity and optimizing distribution.
If you choose to take the DIY approach, have respect for electricity: wear socks and shoes, don't stand in a puddle of water while working, and always keep one hand in a pocket while probing (with a meter, for example). Basically, you don't want the jolt to hit you across the chest, from hand to hand or from hand to foot. If the electrical investigation requires you to go beyond your comfort level, find a knowledgeable electrician who is sensitive to the needs of a multimedia system.
Tree, Trunk, and Limbs
Randomly plugging your gear into the most convenient outlets around a room can be the beginning of trouble. Power distribution in a home or office is not configured to audio standards, so there is an increased potential for noise when using more than one outlet.
FIG. 1: With the Tree, Trunk, and Limbs method, imagine that the outlet is the tree trunk and the outlet strips that are plugged into it are the branches.
The first questions that I ask a customer is how many outlets are in the room, and how many are being used? I suggest that everything be plugged into one outlet using the Tree, Trunk, and Limbs (TTL) approach to power distribution (see Fig. 1). Think of the outlet as the bottom of a tree trunk. Plug an outlet strip into the trunk and connect the branches (more outlet strips). Then plug your gear into the branches.
Typically, when I visit a customer's studio, I notice that at least two outlets are being used. When asked why, the customer usually explains that he or she didn't think one extra outlet would make a difference and that it's inconvenient to use only one outlet.
Although it may not be an ideal solution — it does have limitations — the Tree, Trunk, and Limbs method distributes the same power and ground to all your outlet strips and gear, which is the ultimate goal no matter what size system you have. If you have noise problems, that approach will most likely reduce or eliminate most of them.
Noise Noise Everywhere
Every electronic product you own dumps noise back into the power line. Power-related noises can be distributed by the power wiring or through the air by induction. The best receiver is the electric guitar, which acts as a divining rod for radiated electrical noises. That's because a single-coil guitar pickup and a power transformer — such as those found in amps, power supplies, and wall warts — have something in common: they are coils of wire wrapped around a hunk of iron. A transformer radiates an electrical field, and a single-coil guitar pickup does just as its name suggests.
A humbucking pickup has two coils, one of which is wired out of phase with the other. Any noise that is common to the coils and in phase is rejected. That type of phase cancellation technique is a common strategy for dealing with noisy lines.
Computer and video monitors also have coils, which coerce electrons into creating recognizable images on screen. Flat-panel displays have coils to generate voltage for the light source behind the panel. Both technologies radiate noise, so you should gather your audio harnesses with cable ties and dress them as far away from power sources as possible.
Light dimmers and fluorescent lights also generate electromagnetic interference (EMI). Bill Whitlock, president of Jensen Transformers, recommends using an AM radio to track down noise sources, such as defective transformers in fluorescent light fixtures. To do this, tune the radio to an unused frequency and walk close to any potential noise sources: the radio will pick up and amplify the interference.
Speaking of noisy lights, small, affordable dimmers do not belong in a recording environment. Instead of varying the voltage to change bulb brightness, those super-efficient dimmers chop up the 60 Hz wave into small pieces for dim and passes the full wave for bright. If your light needs include atmospheric control, chose a Variac-based dimmer. Although these are transformers and should be positioned away from sensitive gear, they don't generate high-frequency noise, which tends to travel better through the air, just like Radio Frequency Interference (RFI).
Power transformers radiate energy into the air and into nearby metal objects, such as the chassis that houses your gear. Connect any two devices, and noise current from one chassis will flow through the signal cabling to the other (and vice versa), specifically through the shield that is supposed to protect the signal wires from radiated noise. All mic- and line-level audio cables have a shield, no matter what type of connector is used on the end.
How the mating connector is mounted to the chassis determines a device's noise immunity. What you want is an electrical firewall, a safe place for the shield to dump its noise so that it doesn't get into the box. (Once it is inside, it's harder to remove.) Renowned Toronto-based engineer Neil Muncy named that the pin 1 issue, but it's not exclusive to XLR connectors. RCA, ¼-inch, and even FireWire and USB cables are all vulnerable if the shield/pin 1 connection does not go directly to the metal chassis. This is the case for some connectors that are isolated from the chassis for ease of manufacturing. If the path from pin 1 to the chassis is through a printed circuit board trace, then all the noise infiltrates the ground scheme, where it can be amplified.
An unbalanced audio cable has two conductors: a wire for signal and a protective shield in the form of a multistranded wire-wrap, a braided screen, or an aluminum-foil wrap. A balanced audio cable has three conductors: the shield and a twisted pair of wires for the signal. In either case, the shield alone is not enough to stop a transformer's strong electrical field, whether it's the pin 1 issue or the system's inherent ability to reject noise.
For example, if an unbalanced cable is too close to a wall wart, the electrical field radiating from the transformer will pass through the shield and into the signal wire, producing a pronounced hum. Even if pin 1 is correctly implemented, you will get noise through induction. But if the source and the destination are balanced, the noise will be minimized as much as the circuit topology allows.
FIG. 2: The audio signals—two out-of-phase sine waves—are on the red and black wires and attached to pin 2 and pin 3, respectively. Common-mode noise, which remains in phase, is represented by the red noise spikes.
Balanced input and output circuits come in two primary forms: as active electronics or as a passive device known as a signal transformer. Transformers are more tolerant than poorly implemented transformerless designs. Both amplify a differential signal pair — that is, two signals of opposite polarity — but reject noise that is common to both signal wires. That relationship is known as the Common Mode Rejection Ratio, or CMRR. Look for it when checking out mic preamp specs. It also applies to line inputs, where it is usually taken for granted.
In Fig. 2, the audio signal is on the red and black wires, connected to pin 2 and pin 3, respectively. Notice that the two sine waves are out of phase with each other, but the red noise spikes (common-mode noise) are in phase. (Another common mode signal, 48V phantom power, travels over the yellow wire feeding the two resistors. It's called phantom power because no additional wires are required to power the microphone.)
There are two types of active balanced configurations: full balanced and impedance balanced. Full balanced has two identical signals traveling down a shielded twisted-pair cable; one signal, however, is 180 degrees out-of-phase with the other. The amplifier driving each wire has a defined output or source impedance, just as some vacuum-tube guitar amps have a back panel selector switch to match the impedance of the amp with that of the speaker cabinet.
The math of amplification and rejection in an ideal balanced system is simple. For example, I'll call the positive (pin 2) signal +1 and the negative (pin 3) signal -1. If each of these signals were on a mixer fader, the result of the addition would be zero, or cancellation. But a balanced input amplifier, also known as a differential amplifier, looks for differences in a signal. A differential amplifier subtracts what is common to both wires — in this case, the noise. But when it attempts to subtract two mirrored signals ([+1] - [-1]), the double-negative becomes addition, yielding two instead of zero.
Because an impedance-balanced configuration has only one modulated signal wire, it is crucial that the unmodulated signal wire be sourced by the same impedance (it's simply a resistor to ground). Matching those two impedances allows the differential amplifier at the other end of the cable to do its job. Maximum noise rejection occurs when both wires are equally susceptible to noise. For really troublesome noises, a transformer at the input (destination) is more tolerant of circuit idiosyncrasies and can deliver 90 dB of CMRR.
In the Unbalanced World
If you are using unbalanced gear, the devices must be physically close together (preferably in the same rack), so that the interconnecting cables can be as short as possible. Long cables act as antennas for noise. In addition, use high-quality cables, with robust shields. If you have only one rack of gear, the noise situation is relatively manageable.
While most sound modules and keyboards are unbalanced, it doesn't hurt to route them to a balanced mixer using balanced audio cable. While it's far from an optimum solution, there will be some noise canceling from doing so. In hostile environments, installing unbalanced-to-balanced converters at those sources will give you a fighting chance against noise. Use digital outputs wherever possible (it's worth the extra money to buy gear that has that option), and make sure you have separate physical paths for power and audio. It's always a good idea to keep wall warts and power cables as far apart from audio cables as possible.
There has always been considerable fuss about the dreaded and misnamed ground loop which, in reality, is the unavoidable ground current. As soon as a piece of gear is rackmounted, plugged in, and connected to another piece of gear, there will be ground currents in all cables. It would seem as if we are doomed at the start and that meeting any electrical code and achieving a quiet system are disparate goals, but that is not the case.
FIG. 3: The purpose of the ground adapter is to temporarily modify an old-fashioned two-prong outlet and provide the third-prong ground through the outlet''s plate screw.
The most common bad-practice electrical activity is the misuse of the ground adapter as a ground lifter (see Fig. 3). Use it only as a testing tool, never as a fix. Nothing is more permanent than a temporary solution.
There are many so-called fixes applied to how the ends of interconnecting cable wiring are terminated. Tricks such as flying shields, where the shield at one end of the cable is not connected, complicate the wiring scheme. Keep things simple by connecting the shield at both ends to let the noise-prone gear reveal itself. Rather than compromise or complicate the system, the funky gear should be fixed, modified, or thrown away. There are several products that can interface unbalanced gear with balanced gear. Transformers may cost more, but they are also more effective.
Using the TTL method to solve power-related noises might also push the limits of the single circuit breaker now powering the entire system. For example, you might not want to turn everything on at once because the power-up current draw is much more than current draw once everything is on. The total power-draw for any breaker should be 75 percent of its rated value (such as 15 amperes [amps] for a 20-amp breaker).
If you are concerned about overloading the electrical system or popping breakers or fuses, there are two ways to determine your total power consumption. The easy way is with a clip-on ammeter (see Fig. 4). It eliminates all of the guesswork, but requires a professional electrician's respect for electricity.
FIG. 4: A clip-on ammeter can be used to measure current consumption at the breaker box. The orange jaws open so that the power wire can be inserted.
Start by determining which circuit breaker feeds each outlet. Current is measured with a clip-on ammeter while your gear is on and doing work. The red-orange jaws of the ammeter open so that a single wire from a breaker can be inserted. The jaws are then closed and the current is measured without making a physical or electrical connection, proving that wires radiate an electromagnetic field.
After measuring the circuit breakers to which all the audio equipment is connected, add up each reading. If the total is within 75 percent of 20 amps (15 amps or less), you are safe to plug all gear into one outlet using the TTL method.
Another way to determine your power consumption is to calculate it using the specs from each device you use. Circuit breakers are rated in amps (A), but appliance power consumption may be rated in amperes, watts (W), or volt-amperes (VA). Remember that power (in watts) equals volts multiplied by amps. The difference between watts and VA is the power factor. Unless the power factor is known, use the following formulas:
(W to VA) W ÷ 0.85 = VA
(VA to W) VA × 0.85 = W
Depending on your enthusiasm for calculating the total power consumption of your system, this might be the time to look in the Yellow Pages for a local electrician.
If the total draw of your system exceeds 15 amps, power down any items that do not seem relevant to the problem but leave everything plugged in to maintain the conditions that may be causing the hum or buzz. We don't want to pop the breaker from excess current and, as mentioned, the goal is to localize and weed out the noise-prone gear. If your system requires more than 15 amps and you have the luxury of rewiring, put each outlet on its own breaker with its own ground wire back to the box using the isolated ground outlet detailed in the following Faulty Outlet section.
FIG. 5: Use an outlet tester to confirm that your outlets are wired correctly.
Another warning sign that you are pushing the power strips and extension cables to their limit is when power plugs are warm to the touch. However, plugs can become warm if the wires in the plug or socket are not secure. Molded power plugs should have spot-welded connections. Unfortunately, cheaper products only crimp the wires to the prongs. Over time, and through repeated cycles of heating and cooling, oxidation builds up resistance at the junction of wire and prong, causing the plug to warm. The same is true for outlets that have lost their grip.
Replacing power strips is easy, but you should also have your electrician inspect all power outlets, tightening or replacing them as necessary. Use either heavy duty outlets or the orange hospital-grade power outlets.
If plugging everything into one outlet lowered the noise, then your problem is related to the wiring in one of the outlets. Use an outlet tester, available at most hardware stores, to confirm that your outlets are correctly wired (see Fig. 5). An outlet tester can detect gross wiring errors, although problems are typically more subtle than that.
Fig. 6 shows how an outlet should be wired: black for hot, white for neutral, and green for ground. Neutral is also referred to as the return wire that completes the circuit.
FIG. 6: In a properly wired outlet, the black wire is hot, the white wire is neutral, and the green wire is ground.
In the wiring of a standard household electrical socket, ground travels through the round prong of a power plug. It's important to note that ground is there for safety purposes, not for providing low-noise audio. Standard outlets receive ground when the mounting screw touches the outlet box. Hospital-grade orange power outlets have a ground screw that is isolated from the mounting flanges. That gives you an isolated ground wire all the way back to the breaker box, avoiding potential interruptions such as loose screws at each conduit extension or interference from the spiral metal jacket of BX cable.
The route from orange outlet to a dedicated ground wire terminating into a 6-foot spike set into damp earth requires extra time and money, but it's not cost prohibitive. That said, it still doesn't ensure a quiet ground connection. The isolated ground wire is easily contaminated by noise currents in the black and white power wires because of their close proximity within the conduit. To prove that, I experimented with running ground wires outside the conduit with greatly improved results, but I have never investigated the electrical code to see if it's a legal and safe implementation. Consult your local electrician.
In a typical home or office building, outlets are daisy-chained, and every loose connection between them — whether it is on the hot, neutral, or ground line — is a potential noise source. With the breaker off, each outlet should be pulled, inspected, and tightened by a licensed electrician if necessary.
FIG. 7: Tightening the screws on the buss bars in your breaker box can help alleviate noise problems.
In the breaker box, there should be two buss bars (hunks of aluminum, copper, or copper-alloy) tapped with screws for multiple wires (one for neutral and one for ground). Because every connection is a potential noise source, all of the screws should be checked and tightened if necessary (see Fig. 7).
All of your gear reflects noise back into the power wiring, but if a more serious variety of noise is coming from outside your facility, the best way to tame it is with an uninterruptible power supply (UPS). A UPS converts AC power to DC power by charging up batteries instead of capacitors, and then converting the power back to AC. A UPS effectively cleans up noise and provides a safeguard against spikes and power failures. Remember that isolation and balanced power transformers do not filter noise, and to be truly effective, they must be properly installed where power enters the building.
I'm not a big fan of rackmount voltage regulators and surge protectors. I would rather have a UPS take care of all that. It also gives me a backup in case of power failure. Not every model of UPS offers voltage regulation; the American Power Conversion (APC) Back-UPS Pro line (www.apcc.com), however, does provide voltage regulation. Unless you live in an area that has frequent voltage swings, most of your gear has sufficient internal regulation. Most computers, for example, will run on 90V to 200V by design.
Heavy Metal Noise
Balanced power enters the home as 220V to 240V. Neutral comes from a center tap on the utility pole transformer, hence the 110V to 120V standard outlet power (see Fig. 8). Neutral is tied to ground as it enters the home, but it tends to get dirty from all the appliances that are constantly switching on and off (such as a microwave oven, a refrigerator, a washer, or a dryer). It's even worse in a commercial building, which may have elevators and HVAC.
FIG. 8: Electrical power enters the 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.
A 1:1 isolation transformer can help avoid the neutral noise by taking the 220V on its primary side. A new, clean neutral is then established on the secondary side for your audio power network. It's not a big deal to add one, but it's also not a DIY project. I have mixed feelings about balanced power. It is somewhat effective, but not a panacea. Like the impedance-balanced method of interconnecting audio gear, balanced power requires a similar form of impedance equity in the power-distribution system to be effective. Not all gear treats incoming power in the same way. A customer once bought a balanced-power transformer because he had read in a magazine that it was a miracle box. When it showed up, it was too heavy to move and there was nowhere to put it. The real problem, however, was summer brownouts, something that a balanced-power transformer doesn't fix.
All transformers that are big enough to run an entire recording area — including lights — are big, heavy, and potentially noisy. They make physical noise as well as radiated noise, so choosing a proper location is vital (preferably one where the power lines enter the facility and is as far away from guitars and analog tape machines as possible). Although there are project-studio-size transformers available, consult an electrician about the best solution for your needs and the optimum place to install it.
RFI and Television Interference — also known as RFI/TVI — are a lot more squirrelly than your garden variety hums and buzzes, but the root of their evil is the same. RFI has an obvious sound. TVI, on the other hand, sounds similar to a power-buzz, except that it tends to vary in character, with a phase-sweeping quality. That is because TVI is video picture noise with a 59.95 Hz fundamental — 0.05 cycles per second less than 60 Hz AC power. Hearing them together causes the slow-phasing artifacts.
FIG. 9a and b: The ferrite clam, which attaches to the end of a cable, can help eliminate RFI and TVI problems.
One of the DIY solutions for eliminating RFI/TVI is the ferrite clam, which is easy to apply to cable ends (see Fig. 9a and 9b). You have probably seen these before on computer video-monitor cables, hidden under shrink tubing. In essence, the ferrite acts as a near-short circuit at frequencies from 1 MHz to 100 MHz — exactly where you need them to work. The clam-shell filter comes in various sizes to accommodate wire thickness. Guitar cables are also good candidates for ferrite clam filters.
Another RFI/TVI fix requires the most basic soldering and mechanical skills and it applies to the male end of an XLR cable. A standard 3-conductor XLR configuration has shield/ground on pin 1, signal high/hot on pin 2, and signal low/cold on pin 3. There is also a fourth pin available that connects directly to the chassis by means of the connector shell. XLR pin 1 is supposed to go directly to chassis, but when it takes a more circuitous route, high-gain mic preamplifiers see hum, buzz, and RFI/TVI. If adding a jumper wire from pin 1 to the chassis-lug fixes your problem, you may want a technician to go inside the box to make it permanent.
Video and computer monitors have their own way of displaying hum, as a rolling horizontal bar. You will typically see and hear hum when integrating a cable TV box into an audio system because the cable is grounded when it enters the building, and that will not be at the same potential as the audio system ground. As a result, current will flow. The fix for that noise is to insert an isolation transformer between the cable wire and the cable box or video recorder (see Fig. 10).
Cable quality can also affect noise reception in hostile environments. Typically, multichannel snakes have a foil shield with a drain wire. Those may be acceptable for line-level applications, but for mic and unbalanced lines, a heavy copper braid or a wrap is better because it reduces noise by 15 dB. Canare Star Quad cable, which has a braided shield, is best, because it has a 40 dB reduction.
FIG. 10: Inserting an isolation transformer between the cable wire and your cable box or video recorder will alleviate hum that appears on video and computer monitors.
Now it's time to address the wiring in the rack. Here it would be helpful if all manufacturers adhered to a standard location for the power connector. When applicable, I place a vertical power strip on the cabinet side that has most of the outlets, using short, 18-inch IEC power cords. Avoid coiling any power or audio cables so that you minimize noise generation and pickup. Don't forget to use cable ties to bundle audio cables on one side of your rack and power cables on the other side.
The Knowledge of Power
Understanding potential noise sources and using the TTL method of power distribution should allow you to identify and in most cases reduce the major noise problems in your system. There are situations, however, that require the combined support of an experienced technician and an understanding and sympathetic electrician — for example, one that may also be a musician. Although it may be more expensive than the DIY approach, a system that is consistently safe and quiet will be worth the investment. (For more information about cleaning up your studio's AC power situation, see Scott Wilkinson's article “Power to the People!” in the October 1997 issue of EM at emusician.com.)
Eddie Ciletti winds up by repairing and modifying gear (www.tangible-technology.com) and unwinds by editing a new instructional music video. He also teaches recording and maintenance one day a week atwww.iprschool. com.
While a new audio system may start out quiet enough, a gradually elevating noise floor is not uncommon. Cable connectors are one of the first and easiest places to look. For example, silver-plated XLR connectors turn black when oxidized. It never hurts to disconnect everything, clean the contacts with 99 percent isopropyl alcohol, and then apply a contact cleaner or preservative using a cloth or a cotton swab. Avoid spraying the cleaner directly on anything because it will attract more funk than it washes away.
Note that the propellant that forces the cleaner/lubricant out of the can is cold, and water vapor may condense on cold connectors, causing additional corrosion. Other contaminants can accelerate the corrosion process, causing sulfur-based films to coat the contacts, which are harder to see than copper or silver oxide.
A CHEAP BUZZ
Noise-prone equipment is not necessarily due to poor design. It is more likely due to shortcuts taken to aid mass production. Eighties-era gear often used plastic-insulated jacks mounted directly to a printed circuit board (PCB). As a result, the noise was dumped directly to the PCB ground, to which many high-gain amplifiers are referenced, instead of being stopped at the firewall (the metal chassis that houses the circuitry).
For an example of good design, check out any Mackie mixer. Notice that all of the ¼-inch jacks have metal threads and a metal nut securing them directly to a metal chassis. Minimizing noise susceptibility is almost that simple.