WHY POWER RELATED NOISES VARY

Copyright 1994 by Eddie Ciletti
 
  Figure One:  What "the juice" should look like when it comes from the outlet.
 

Power at the outlet is supposed to be pure hum, or more precisely, a sixty-hertz sine wave. Buzzes are the harmonic contribution of power-hungry appliances such as air conditioners, electric heaters, computers and light dimmers. Any device that can make your lights dim when powered also creates spikes and mangles the waveform. These are distortions that can be reflected back into the power line. Susceptible gear will receive these "transmissions" either directly through the power cable or radiated into the air. Electric guitar pickups, for example, are susceptible to all sorts of radiated noises, a fact not lost on guitarists who intuitively position their axes for minimum noise. (Hopefully they can still face the audience!) By wiring two standard pickups out-of-phase, most of the noise will cancel. This is the principle behind the humbucking pickup; two coils wired back-to-back into one assembly. Only one set of coils faces the strings, so the music comes through relatively unscathed. Some dynamic mics also include a humbucking coil. It’s not so easy with audio gear. As your system grows, so too does its susceptibility to power related noises. Specifically, gear can be grounded by an audio cable, the third prong of the power plug, contact with the rack rail and via computer interfaces such as the SCSI port. Each additional connection causes a ground loop that can bring out the demons in certain pieces of gear.

Power ( in the US ), as distributed to a standard wall outlet as 120-volts, is "un-balanced."  Hopefully the illustrtation above will illuminate and not confuse.  As shown, the smaller flat socket on the "top" of the standard power outlet is, in electrician’s jargon, "hot."  The electrical code designates that this wire will be BLACK, though for the purpose of creating a mental picture the connection is RED (as in "hot). The larger, lower flat socket  is connected to a white wire called "neutral."  (Depicted as blue as in,  "cool.")  This is the return-to-earth connection, but (as shown in the tug-of-war illustration below) high current demand can raise it from zero to a few volts above ground.  In doing so, it becomes contaminated with the harmonic backwash from every electrical device on the line. This one of the reasons un-balanced power has no inherent noise imunity, ESPECIALLY if the equipment design has been compromised during production by cost- and labor-saving steps. Though it can be effective at reducing hum and buzz, but not noise such as "hiss," many view Balanced Power as a Magic Cure-All, which it is not. Balanced power will reduce the hum IF you plug all your gear into it, so, the transformer has got to be of significant power handling capability. Balanced power will not regulate your juice, for that, you want a UPS ( which also has to be of significant power handling capability).  A UPS is better at both regulation AND eliminating power spikes than any other product. Balanced power will not fix what's bad about gear that is sensitive to hums and buzzes (and other stuff you don't realize).  Gear (and wiring) that take advantage of balanced audio will inherently have better imaging and transient response. There is no magic about having gear that can pass audio and ignore noise.

 

A BRIEF DIVERSION... In the typical home, you’ll find a single phase of 220 volts in the breaker box. This power is reserved for large air conditioning systems or electric ranges. From each phase-to-neutral is 120 volts, the sauce that feeds standard appliance outlets. The electrician’s goal is to balance the loads on each phase. The analogy is tug-of-war, where you get to be the rope, A.K.A. Neutral.  Both the "A" and "B" phases are pulling, but so long as the load of each phase is equal, you (Neutral) stay put. When the load becomes unbalanced your system is likely to pick up bizarre intermittent noises. This is especially true when more than one outlet source is used. When possible, avoid potential problems by plugging everything into one dedicated outlet. Before doing so, determine the total power consumption in either of two ways: One: read the specs either in the documentation or on the rear panel of the unit. Unfortunately, adding up power specs may be difficult because power consumption can be specified as Watta, Amps and VA (Volt-Amperes). Two: use an "Amprobe" clip-on ammeter to measure the current being drawn. Be sure to add a 30% to 50% safety margin. For example, if the total load is 10 amps, the typical 15 amp breaker will be happy. (Running too close to maximum will make the breaker run hot or it may blow if everything is powered up at once.) FORMULA RELATIONSHIPS Fuses and circuit breakers are always specified in amperes (AKA "amps"). The information on the back of the unit or in the operator’s manual will be specified in watts (W) or volt-amperes (VA). The Power Formula is: Power = Volts times Amps ( P = V I ). Since "I" is the current in Amperes, "VA" is volt-amperes, which should be the same as Watts except that the "power vector" must be considered (this detail is not yet fully developed on this page). Since the current is "alternating" and not "direct," we can not simply multiply volts and amps to get watts. Expect to make a few phone calls to the manufacturer. Meanwhile, here are a few tips to keep the beasts at bay. Connect all of the gear into power strips, but Don’t daisy chain any power strips, yet. Count all of the strips, then Determine how many strips it will take to accommodate them. Now, think like a tree... Each piece of gear is a leaf... Each branch is power strip... All strips feed the trunk... Which is like the main outlet, OK? Some trees have two trunks... It could happen! If so, make sure both breakers are on the same power phase. ISOLATED GROUND and BALANCED POWER Is it worth the added expense? Consider the following An Isolated Ground (IG) can be worth the added expense, but ONLY when approached in a very specific and consistent way.  Otherwise the benefits will be nominal at best.  Almost all new systems are sonically clean, so don't kid yourself into thinking you got what you paid for (IG) or got away with not paying for (IG). Assuming a New Installation If everything is connected to the same noise source there will be no noise.  The problem with most systems is that, over time, connections become loose.  Inconsistent noises are the result of changes in temperature, moisture and current demand over TIME. FOR EXAMPLE It is also important to consider the habit of electricians.  Most do house and commercial wiring, few have the sensitivity to do audio installtions.  It is quite common, for example, for outlet boxes to be run in series RATHER THAN running a fresh wire back to the breaker box. Consider strings of "holiday" lights where, if one bulb is pulled, the others go out. In power distribution, daisy-chaining outlet boxes puts a potential noise-maker at every juntion: hot, neutral and ground. Again, the tightness of each connection is important, because the lack of same generates heat, causing expansion (and contraction), which, over time, generates intermittent noises into the system wiring. Power hungry appliances and audio gear could potentially heat up every weak junction. An Isolated Ground system requires the following: 1.) three wires: hot, neutral and insulated ground to the IG outlet. PLUS 2.) To be legal, a metal jacketed cable securely connected to a metal outlet box (if used).  When metal studs and metal boxes are used, there will be multiple ground connections.  The isolated and INSULATED ground wire can not be used for, nor can it touch, the metal box or metal stud. HOWEVER Using the spike-in-the-ground approach, there is still the problem tht results when all of the wires are in the same jacket.  Consider that even audio gear generates noise.  The proximity of the ground wire -- isolated or not -- to the hot and neutral means that gear noise will be induced from the power lines into the ground wire. To legally take full advantage of an true IG system, balanced power distribution (BPD) is required. Yet another expense, BPD requires a power transformer with 120 volt windings, precision center-tapped at the mid-point to yield 60 volts (60-0-60) to both the former NEUTRAL and the somewhat less HOT.  With balanced power, minimal, if any. noise will be generated into the ground wire, isolated or not. IG sans BPD To keep the ground wire clean, it should be run away from -- and not parallel to -- the power lines.  This is not "to code," but for the moment, consider this: In additon to the legal requirement of the standard hot-neutral-ground cable to the outlet box, run a separate ground wire -- that is not paralllel to any of the power lines -- to the isolated-ground outlets.  ALL of the ground wires -- legal and clean -- will go back to the same ground bar in the breaker box.  Assuming the same clean, tight connection for every wire, the system should remain clean. NOISE will occur if you join a clean system to a noisy system as might occur when connecting to a guitar or bas amp via direct box.  Of course, ALL potential audio outlets should have their own ground wire and a straight return to the breaker box. Glossary of terms Hertz (abbreviated Hz) is the frequency, in cycles per second, of an alternating current (AC). When specifying voltage, the RMS (root mean square) value is used so that the effective power is equivalent to that of direct current (DC). For example, the voltage at the power outlet is 120 volts RMS which is actually about 340 volts peak-to-peak.

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