|by Martin Glasband
Not long ago at a Fog Hat concert in Portland, Oregon, a engineer from a well known console manufacturer commented to the engineer in a remote recording truck that this was absolutely the quietest installation of this console he had ever seen in all of his years with the company. But an even greater surprise was in store for him when he was shown where the truck’s AC mains were tapped into the Portland Rose Quarter’s power system — on the three-phase stage lighting panel with “dimmers ablazing.” A true story.
Now in contrast, it takes only a little stretch of the imagination to assume that somewhere another sound engineer was at that very moment in time working in a night club, scurrying frantically about chasing down ground loops after finding out that the lighting check ruined his quietly arranged sound system just minutes before showtime. (And he was also doing a live mix for an album.)
These are examples of two possible experiences one might have working in live sound. So this may come as some good news. The simple difference between these two stories was the presence of balanced power in the remote truck.
What is Balanced Power?
Balanced AC is a revolutionary new technology that has been quietly growing in popularity for about the past 5 years, but for the sake of those who aren’t familiar with its application, let’s briefly cover some basics.
Balanced AC is simply 120 Volts that has been split evenly across two AC mains. One phase is +60V while the other is -60V. The mains are always 180 degrees out of phase across the load and therefore sum to 120 Volts, the same voltage and frequency for which equipment power supplies were designed. In this case however, the reference potential (ground) has been located at the midpoint between the two mains so there is no “neutral” wire.
|Here’s an example of a balanced AC circuit.Fig. 1
|As a reminder, here is what standard (or unbalanced) AC looks like:
|This is the common AC power circuit. There is a “hot” wire, a neutral wire and a ground wire connected to the neutral. Perhaps, the truth is that the most taken for granted of all electrical circuits (the unbalanced AC power circuit) is responsible for more problems with sensitive electronics than any other known cause. How is this possible|
|Electrical noise is a natural phenomenon. Here in the next illustration, along with the fundamental 120V 60Hz power, reactive currents are also indicated on the mains.
Reactive currents represent “unused” energy that has been discharged onto the power lines from the impedance load as the source voltage modulates. Each piece of connected equipment adds to the total reactive current on the neutral wire which in turn is connected to the electrical system’s ground. The level of these reactive neutral/grounding currents can be considerable. In many cases in the past, there have been high enough levels of reactive currents present in the power to crash data processing systems. Historical evidence attesting to this fact is overwhelming. Data processing facilities have long had problems with AC power. It’s no wonder that the audio industry is finding out that digital sound reproduction is no panacea. In other related areas such as digital broadcasting, malfunctions from poor power often prove to be the “Achilles Heel” of the system, particularly where extremely high bit rates and wide bandwidths are used. Compressed noise can cause real problems.
Electronic power supplies have long been known to create these reactive currents. In the electrical power industry, an entire segment of that field has grown up around the study of “power quality.” Power quality (PQ) has much to do with the purity of AC power as it affects the operation of sensitive electronics.
Aside from reactive currents, there are other PQ issues involved that can affect equipment performance. Among them is Power Factor. Whenever the true power used by a load equals something less than the apparent power being supplied, reactive currents make up the difference. This ratio between true power and apparent power is called “Power Factor.” Ideally the ratio would be 1:1. However, the presence of reactive current means that some portion of the power that is present has shifted phase. This is due to the effect of reactive current from a typical non-linear (impedance) load. Some of the voltage is lagging 90 degrees from the fundamental voltage phase. This shift in voltage away from the active current phase means that to that extent, simply, there is no power present. Voltage and current must be in phase for power to exist — for electricity to actively create an effect. Low power factor means that the efficiency of equipment power supplies will be affected. Often studio owners will comment that their equipment sounds better at night, a symptom of heavy commercial use on the power system during daylight hours and an indication of poor power factor.
|Down to Busines|
|So what does all of this power business have to do with noisy or poor sounding audio equipment? First, let’s take a look at what is termed “The Noise Circuit.”Fig. 4
Note in the (typical amplifier) illustration above how these reactive currents in the ground find their way into audio signal circuits. The amount of this reactive current potential is also proportional to the impedance of the source power system. So in reality, an alternate current path is created through the electronics between the opposite reactive potentials across a power supply via the AC ground. These reactive currents complete the circuit back through sensitive signal-circuit electronics, looping in a matter of speaking through whatever grounding or signal path that is available. This is noise, commonly called hum. But as one might imagine, the harmonic structure can be infinite so all sorts of “sonic qualities” in the noise are possible. At the very least, noise at low levels creates IM distortion in program material which can cause loss of imaging and compromise signal accuracy across the entire audio bandwidth.
The interesting thing about this phenomenon is that it has been going on for a very long time without much being done to directly address the cause of the problem. Yes, there have been band-aids of all sorts; ground lifts, .01mfd capacitors in grounding circuits, special power supply transformers and many other similar sorts of remedies have been tried — but nothing that deals with the source of the problem, not directly that is, not until the advent of balanced power. But even then, balanced power is often mistaken more of as a remedy than as what it actually is, a properly designed source of EMF for the electronics. Balanced power is better understood as a foundation, not as a remedy.
|How it Works|
|Let’s take a look at how balanced AC better fits the power requirements of sensitive equipment and answers noise problems so elegantly and effectively.
|Note in the illustration that on opposite mains, reactive currents are out of phase to each other as is the voltage phase across the mains from the driving power source. Traveling back through the mains, eventually this reactive current flows into ground at the center tap of the transformer. Here exists the equal presence of inversely phased reactive currents flowing from the mains into ground; everything collapses at the signal reference. This is phase cancellation or common mode rejection.
This principle is identical in theory to a balanced audio circuit. Both examples of balanced circuitry reject noise very effectively. Why balanced audio has grown in practice but not balanced AC is somewhat of a mystery. The phenomenon of phase cancellation or common mode rejection is at the heart of how balanced power eliminates interference. Remarkably, this simple method of dealing with AC harmonics has been entirely overlooked in engineering texts and code books until very recently. However now, balanced AC has been recognized as an approved wiring method in the 1996 National Electrical Code, Article 530 Part “G”.
|A Closer Look at Analog Audio|
|The weakest link where noise is concerned in an analog audio chain is in the high-gain circuitry. Here’s where low level signal in the presence (or not) of low level interference is amplified. The ratio between signal and noise levels here is most critical. Where there may be only milivolts of signal present, any electrical interference at all could pose a problem. In line level equipment the presence of low noise levels is obviously less of a factor.
It is known that every power supply creates some reactive current potential. The more equipment involved, the more of these reactive currents are present. And so it follows, the more channels that are mixed, the more background noise will be present. This occurs in the summing amps in a console. Some equipment is more sensitive to noise than others so problems are often hard to predict. As a rule though, high gain and high impedance equipment is where most noise is introduced because here is the most gain.
At this point some may cleverly ask the question: “Does all of the equipment need to be running on balanced AC to get good results?”
The answer is no. The standard practice of lifting the shields at the inputs of power amps suits this application very well. Power amplifiers may run on unbalanced AC with little effect on hum. Balanced power can still be used effectively to rid a system of hums, buzzes and other more subtle artifacts in the most sensitive equipment. One can expect a very clean performance when (e.g.) front-of-house and backline electronics are running on balanced AC with the line level amps running on unbalanced AC, shields lifted.
The only drawback to this approach are the limits imposed by noise on the line from the huge amplifier load. Accompanying the lower power factor, a general “thinning” of the sound quality (in comparison using balanced AC) is inevitable. Power supplies simply don’t deliver power the same way. But, assuming that the amps are in good shape, there won’t be any audible hums or buzzes. So in numerical terms, due to the attenuation of noise, an average 12dB increase in dynamic range can be expected from running high-gain equipment balanced and another 3dB can be gained when the amps are included.
|The Digital Domain|
|In audio as with other digital data processing applications, AC power harmonics cause data corruption. Short of complete system failure, their is a “gray scale” effect of signal degradation that differs somewhat from analog applications. The main difference are the frequencies at which problems appear. For example in 16 bit audio, 16 bit chunks of data are processed at the rate of 44.1kHz. The bit stream rate in 16 bit audio therefore is about 700,000 bits per second (16 x 44,100.) Various other digital clocking functions may run at much higher frequencies, but they too are subject to high frequency AC noise.
How this sounds to the ear is a matter for subjective evaluation, but it is also measurable. Recent tests done on a well known manufacturer’s DAT machine revealed some interesting results. First, under standard power, peak jitter was measured under test in a live performance situation. The results yielded a peak jitter of 18ns. At the same time, the average jitter measured was 6ns. Then, the test was repeated using balanced power. The results were surprising even though they were expected. Average jitter was cut by 1/2 to only 3ns and peak jitter was cut by 2/3, down to only 6ns.
When high frequency interference is present, proportionally there will be timing errors. Digital jitter appears in a manner not unlike intermodulation distortion in analog circuits. Jitter is compounded as more equipment is added to the digital signal chain the same way low frequency noise compounds in analog systems. Digital jitter is essentially “digital hum!”
|The Future of Balanced Power|
|These test results may be viewed in retrospect one day as a harbinger of things to come. In the PQ industry, it is widely known that AC harmonics and grounding currents have invaded nearly every branch of modern technology. The impact that balanced power will have in sophisticated applications across the board has yet to be fully realized.
So in the broadest sense, is it accurate to say that balanced power is really just a patch? For some it can be viewed that way, and it certainly can provide an effective solution. But in fairness, balanced AC is far more than that. It is a simple, elegant and clean way of applying power to electronics. Electrically, it is a solid foundation for sensitive equipment.
For some, accepting conventional AC just because that’s how things have been done for almost 100 years has been a very limiting belief. Sometimes, using unbalanced AC is just a bad habit, low in priority on the list of changes to be made. But mostly, it remains an environmental problem that’s already there. But is settling for unbalanced AC truly worth the consequences that limit the performance capabilities of both equipment and personnel? Things can be different. The performance from everyday equipment that is unleashed by using balanced power can be quite astonishing. Doing one’s job with complete freedom from noise problems is a luxury few have experienced. Balanced power does make a great patch, but understanding it as a foundation on which to build high performance audio systems makes a lot more sense.