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AUDIO NOISE

AND AC SYSTEMS

THE TWO-PHASE SOLUTION TO A DIFFICULT PROBLEM

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By Martin Glasband

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OVER THE PAST 30 YEARS, the art of sound recording and production has advanced perhaps as much as any of the modern technologies. However, these advances have outdistanced electrical standards to which we have grown accustomed in the electrical power industry.

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Although the consoles, transports and digital electronics are more refined, where AC power is concerned, we are still relying largely on the same technology used in the vacuum tube years. It seems that no one has given enough thought to tailoring AC power to meet the needs of sophisticated audio equipment. Some would argue that isolation transformers with Faraday shields address this need. But in fact, the root of the problem goes much deeper. This may sound stunning, but power distribution systems in widespread use today are not fully compatible with audio electronics. There are serious shortcomings in conventional electrical engineering methods that apply to audio systems. Use of any of today's standard 120-volt single-phase AC systems mean potential problems for audio equipment.

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Since the development of the JBL "T" circuit in the mid-1950s, balanced audio signal circuits have become the industry standard. Audio power supplies are not truly designed to operate in an unbalanced manner; commonly they are "force-fed" (unbalanced) single-phase AC anyway. Why? Because that's just the way it's done; because that's the electrical industry standard. Amazingly, most audio engineers and manufacturers, studio designers and electricians, share the same blind spot. These standards have remained unchanged (and unchallenged) for a very long time.

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Ideally, the correct type of AC power for audio gear in the United States is 120-volt, two-phase (balanced) power. Commercial use of two-phase (equi-potential) AC isn't new. In fact, it is probably the first type of AC power ever put into widespread use in this country (Chicago in the early 1900s). Nevertheless, its commercial use is all but dead these days. Properly grounded 120-volt two-phase wiring systems aren't mentioned at all in the National Electrical Code (NEC). However, this unfortunate fact didn't deter a few innovative engineers and studio execs from employing a 120-volt equi-potential AC system at the Zoo Studios, Studio City, CA.

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Soon after its opening, a studio musician plugged in his Marshall amp and declared it was broken because there was no hum. Much to his surprise, his amp worked just fine. He mentioned to the studio engineer that this was the first studio he'd ever worked in where his amp simply made no noise at all. It should also be noted that it was not necessary to drive a single ground rod. That's pretty clean AC.

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EXPLAINING THE BASICS

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The root cause of most studio noise problems has little to do with grounding techniques. Accordingly, the balance of this text addresses the source of studio noise, unbalanced AC, and of course the remedy. First let's examine some typical AC systems:

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Figure 1 is the most common type of 120-volt AC system. The transformer outputs furnish both 120 Volts and 240 Volts to the rest of the electrical system. Figure 2 and Figure 3 are 120/240 delta and 120/208 wye 3-phase systems. In both of these 3-phase systems, single-phase 120-volt power can be applied to branch circuit wiring. Figure 4 is an example of a system more commonly found in recording studios. It furnishes 120-volts to all branch circuits.

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The common thread in all of these systems is a grounded neutral conductor. Here lies the sole cause of almost all AC-induced noise in audio installations.

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Figure 5 is a low-pass filter (EMI/RFI filter) that can be found in nearly every power supply in audio equipment. Note that current flow exists between the 120-volt side of the AC line and the chassis ground. Simply put, here is the source of most AC induced audio noise. As more pieces of audio equipment are turned on, noise in the grounding system increases. This appears as hum in most high-gain and high-impedance equipment. There have been numerous remedies used to eliminate this noise. The common belief is that an improved earth ground will lessen the problem. This is true to a slight extent, but simple logistics make desired results all but impossible. The theory assumes that a good earth ground will somehow "vacuum" the system clean of AC interference. However, the transmission of electrical interference within a studio grounding system, being a measurable and even predictable phenomenon is of course subject to Ohm's Law.

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Commonly, the chassis-to-chassis impedance in an electrical grounding system is fractional. (For a more accurate estimation of ground circuit conductivity and impedance, refer to Table 8 in Chapter 9 in the NEC, "Conductor Properties.") Even under the most optimum conditions, substantially greater conductivity to true earth ground is virtually impossible to achieve. Even if it were possible, the low conductivity to earth ground would at best result in only a very slight improvement in system noise levels. A good example of this is a real case where a studio owner decided to have a well drilling company sink a 60 ft. x 3 in. copper shaft into the earth for his studio's grounding electrode. Unfortunately, the midi-racks and effects returns still had a lot of noise.

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A somewhat more effective approach is to break the equipment grounding connections. Lifting the capacitors from the chassis or using AC ground lifters are ways this is done, however electrical safety is severely compromised. Star grounding is at best a crude approach to the problem. The system's impedance to earth is still far greater than chassis-to-chassis impedance. Noise continues to be a problem to a greater than desirable extent.

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The only cure to this dilemma is to adapt the power system to the gear being utilized, and eliminate the noise at its source. It seems like a simple enough task. Unfortunately though, the electrical code lacks direction in this area. Before we tackle that one, let's look at why a balanced (two-phase) 120-volt system works. If one is going to attempt to amend the NEC, it's a good idea to have a clear reason for doing so.

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HOW IT'S DONE

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Figure 6 is a 120-volt, two-phase grounded system applied to a typical EMI/RFI filter. The phase to ground voltage on each side of the AC line is about 60 Volts. Each line of the iso-transformer output is 180 degrees out of phase to the other, therefor a 120-volt potential between the lines is present. When this voltage is applied to an EMI/RFI filter, the current passed through the capacitors is nulled at the common chassis ground by its inversely phased counterpart. Obviously, this noise cancellation effect on the ground is beneficial for audio circuits.

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Single-phase systems cause noise flare-ups in other related applications. P.A. systems are particularly vulnerable because of the high-impedance and high-gain equipment used. Mobile recording units can also be adversely affected. Radio production and broadcasting facilities, and even video editing and telecine installations, are subject to unbalanced AC induced interference. Here, too, an equi-potential AC system would greatly help matters.

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At the Zoo Studios, there was some initial apprehension when the 120-volt, two-phase system was first fired up. Just how would the audio gear respond to this type of power? .Optimistic predictions proved to be correct. There were no examples of any equipment running afoul on two-phase power. Equipment power supplies operate normally with equi-potential power. Line-to-line voltage is critical, but line-to-ground voltage is irrelevant as far as power supply operation is concerned.

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The only real problem occurred when a new electrical inspector showed up and began quizzing the contractors about the unorthodox wiring system. Fortunately his concerns about electrical safety had been anticipated and appropriate safety measures had been taken. After some discussion with the inspector (and his boss), it was agreed that the system was safe. Though not covered in any section of the NEC, the "spirit" of the code had been maintained. In other words, it was deemed to be safe and passed inspection.

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Short of changing an entire electrical system, there are some procedures that can be followed in the interim to improve noise problems in many studios. Referring once again to Figure 4 (the more common studio system), if the grounded side of the output on the transformer is lifted, a more balanced AC system will result. This is called an ungrounded 120-volt split-phase system. It has some drawbacks, but its use is recognized in the NEC for application in hospital anesthetizing rooms. System monitoring equipment is required as well as other conditional installation methods. You will also need special permission from the local code authority to use it in a studio.

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One drawback is the lack of system grounding protection. Should there be a short-to-ground, the entire system would "tip over" (with one side grounded) and revert to a common single-phase-type system with a neutral wire. Furthermore, every chassis power supply in the studio would then have a neutral wire in its power supply and have a chassis that is referenced to ground through a shorted out piece of gear -- not a very nice picture. Worse perhaps would be an ungrounded chassis that became shorted; this could create a serious shock hazard.

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The simple solution to potential safety questions is the use of two-pole GFCI circuit breakers, or at least a two-pole GFCI main breaker on the whole system. Additionally, when there isn't a grounded center tap on the transformer, a typically uneven magnetic flux dispersion across the output windings will result in unequal phase-to-ground voltages at the outputs. This difference may vary as more gear is turned on. A difference of 10-15 Volts from line-to-ground between the outputs is not uncommon. Nevertheless, noise levels will be reduced by l0dB to 12dB.

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When a grounded center tap system is used, equal output windings on both sides of the center-tap ground reference balances the output voltage. The need for costly GFCI circuit breakers is also eliminated. This is because of a much higher ground fault current potential at the transformer line outputs. Additionally, AC induced noise is rendered inaudible.

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At the time of this writing, a course of action is being undertaken to include in the NEC provisions for 120-volt two-phase electrical systems for special use applications (audio/video installations). In the mean time, local authorities will need special attention until new standards are adopted. Each project needs to be handled on a case by case basis. More than likely, local electrical inspectors will want to know why a two-phase system is needed. They will also require a number of safety measures to be employed in the design and wiring methods used. Some of the basic safety requirements are as follows:

 

The isolation transformer used must have a center-tap terminal on the output windings (refer to Figure 6) that is appropriately grounded. This will ensure sufficient fault current to trip a circuit breaker in the event of a short circuit somewhere in the system.

Double-pole circuit breakers are to be used for all AC branch circuit wiring. Both line conductors (unlike a normal circuit with a neutral wire) are ungrounded. Therefore, both require over-current protection. The breakers selected should include a thermal overload mechanism.

Always use orange iso-ground receptacles to identify balanced AC system outlets. Though they are designated as neutral-wired devices, there is no NEMA receptacle configuration that can be used as a practical alternative. This is part of the code change process currently underway. Nevertheless, it is important to identify (using orange receptacles) these special system outlets. Additional labeling may also be required. Other use outside of studio equipment should be discouraged.

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Inspectors may have other considerations, but it is doubtful that anyone will want to stand in the way of progress. The concepts discussed here are new to many and as stated, there is little if any knowledge of these engineering methods to be found throughout the electrical industry. Local code variances and national code changes will take time. Electrical equipment needs to be standardized and textbooks need to be rewritten. To quote one NEMA code representative, "This is a multi-headed beast." Perhaps a few letters to the NFPA (National Fire Protection Association) in Quincy, MA, will expedite these changes, Though it may be more than a simple job getting a 120-volt, two-phase system legally installed, the rewards are well worth the effort. The savings in downtime and freedom from noise problems will most assuredly justify the installation and the investment.

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Martin Glasband is an electrical engineering consultant and contractor in Selma, OR. He has designed and built electrical systems for KCET-TV, Music Animals (now The Post Complex), Baby'O Recorders, New World Pictures and the ABC Radio Network.

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