Currently on the market are 120 Volt balanced AC power conditioners which are large AC transformers whose secondaries are "center tapped" much like the power supply transformer secondary in a standard bipolar DC audio power supply. These transformers then become the source for the distributed AC power for all or critical portions of a sound system or in an installation like a recording studio to power the studio electronics. Improvements in signal to noise ratios for systems have been advertised as dramatic and grounding problems greatly reduced. I believe in this simple but very effective technology. Minimum recommended areas would be to balance the power for the Front of House and Monitor electronics locally using balanced AC transformers capable of supplying enough current for each area.
The theory behind the operation of these devices lies at the very heart of one of the biggest causes of "ground loops" as described by Martin Glasband in his articles posted in his Equi=Tech web site. Also, there are few people whose recommendation can be more highly regarded by fellow audio people than Roger Nichols, the recording engineer for "Steely Dan". He also has an article posted there called "Power Management in the Studio" which first appeared in the July '96 issue of EQ Magazine.
Equi=Tech is a manufacturer of balanced AC transformers. Equi=Tech also manufactures a balanced three phase isolation transformer known as the "Neptune Series", capable of up to 500 kVA 3ø. A typical transportable "arena rock" sound system is usually quite happy to be powered with 75 to 150kVA 3ø (about 200 to 400 amps per leg three phase power).
Similar in nature to the standard bipolar DC power supply, the center tap of the secondary of the conditioning AC transformer is bonded to the incoming local earth ground. This bond point is now used and distributed as the AC earth ground reference for the equipment receiving balanced AC power. This forces the transformer secondary into a balanced condition - producing half of the full secondary voltage (about 60 AC volts) from each leg to the earth ground referenced center tap. This balancing act also takes full advantage of a transformer's inherent isolation and common mode (noise rejecting) capabilities and greatly reduces AC line noise.
Under normal circumstances, our unbalanced distributed AC source pollutes the audio reference by reflecting reactive components onto the ground bus. The is because the source AC transformer is typically unbalanced by the bonding to earth ground of one leg of the AC transformer secondary selected as neutral in accordance with the National Electrical Code. In three phase systems, the common neutral gets bonded to earth ground (see Section 1 about ground referencing). The bonding of the neutral to earth ground unbalances the AC line the same way shorting XLR pin 2 (or 3) to ground unbalances an audio transformer. There is no "neutral" as we know it in balanced AC power but two "hot" legs. The best way to think about balanced AC power is to view it the same way as you view the difference between balanced and unbalanced audio transmission. The advantages are the same. Balanced AC power reduces noise the same way a balanced audio line does. A fully balanced AC transformer feeding another fully balanced AC transformer as in audio, is a quiet interface due to the high common mode rejection of transformers in general.
Another benefit of balanced AC power is in the area of AC noise bypassing of DC power supplies and audio circuits. Noise components from the (+) and (-) DC rails normally arrive roughly in phase and accumulate on the ground bus. Balanced AC power renders these noise signal "out of phase" with each other as they arrive at the ground bus.. This also reduces the cumulative ground current in large sound systems.
Equi=Tech recommends that you run around and reconnect all the dropped or "lifted" audio shields in your sound system. They say you no longer need to telescope them and that a system will get progressively quieter as you do this. In an installation such as a studio, I would be the first to implement such a plan in conjunction with careful study of signal routing permutations and equipment internal grounding design status to insure performance in all circumstances. However, for live transportable audio systems I would NOT recommend reconnecting lifted shields for several reasons:
- There is a preponderance of audio equipment with signal grounded shields that would have even less noise problems with a shield lift. Even with balanced power these devices will still exhibit poor noise characteristics due to that condition. This cannot be ignored when considering a system wide ground scheme. If every piece of manufactured pro audio equipment had an internal star ground design I would be the first one with a soldering iron in my hand reconnecting shields. Having painted themselves into a bit of a corner with their "reconnect all your shields" statement, Equi=Tech qualifies their position via a technical paper on their site called "The 'Dirty' Chassis Condition", which grants that there is equipment out there with inherently poor internal ground designs that balanced power can't help and needs to be addressed, but doesn't acknowledge its overabundance. The "Dirty Chassis Condition" basically describes design examples 5, 8, 11, and 12 and cautions that some hums and buzzes are the result of EMI proximity effect induction.
- If you are implementing the use of balanced AC power conditioners to solve an existing buzz problem in a transportable sound system, chances are your ground scheme is incorrect to begin with! A properly instituted ground scheme even using conventional wisdom shield telescoping will give you a quiet sound system. The addition of balanced AC power conditioning will only make it better! It should be added only to further enhance signal to noise performance and make the system less prone to differences in ground potential - making interconnections more forgiving when not adhering to accepted or conventional grounding practices. That by itself is a good enough reason to incorporate this technology.
- With portable sound systems, especially rental systems, you never know what you might have to interface with or connect to, which could negate, or render moot, the benefits of these devices. To abandon a functioning ground scheme that works with or with out balanced AC power conditioners just seems foolish. If you are in the design phase of an sound system and its associated AC distribution, then carefully consider all the equipment and interface permutations your system may encounter.
- If you are the owner of a sound equipment rental company, you should only consider wrecking your carefully laid out company-wide ground scheme (by reattaching all your shields which in itself is an additional cost!) if you plan to buy enough of these devices to insure that there will be enough to provide balanced AC power for every critical area of every system you own in every conceivable circumstance of interface. Otherwise, you are inviting potentially confusing interface problems that would not otherwise exist for the people who field your systems. I know of at least one national sound company that has built (or purchased) a custom portable 3ø seven wire balanced AC distribution system delivering balanced AC power to every area and component in the system. Click on the link to see a comparison of a balanced 7 wire 3ø AC transformer and a standard unbalanced 5 wire 3ø AC transformer. I have used that system and can attest to it's effectiveness in situations that would have normally been an absolute grounding nightmare. However, I don't think that system was as quiet as it could have been if it also employed shield telescoping to address the issue of the equipment with signal grounded shields.
I know it would preferable for shields to be connected at both ends. I will address the concept of shield reconnection by bridging open shield connections with a capacitor or capacitor/resistor networks or by deploying special rack signal panels that redirect incoming and outgoing shields to minimize the RF contamination and circumventing any audio grounded equipment shields. See Bridging lifted shields with capacitor and or resistor networks and Custom Rack Panels Interfaces (the next two pages) for more detail. This practice is especially effective on long signal lines.
A word of caution about balanced AC power transmission. If you are interested in applying balanced AC to your system, please consult a professional as there are safety and electrical code issues to be addressed. Equi=Tech's web site contains very useful and educational reference material when considering this as a option. It is their opinion that:
"An unsettling practice continues by a few well-meaning audio technicians who attempt the electrical installation themselves. Often, this turns out to be an overzealous and uninformed approach to hazardous electrical safety matters. Beware! Electrical power is a technology requiring years of education and training. Numerous safety factors, and in many cases manufacturing specifications, must be considered in any electrical installation. Misapplication can result in damage to property, liability problems and serious injury or worse."
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