Power Lines: AC Regulation And Conditioning
Power factors that can seriously compromise your system

By Garth Powell

Figure 1: Note how average AC line noise levels have steadily and dramatically increased in noise levels. (120 Hz to 1 GHz)

Do I really need an AC (alternating current) power conditioner or regulator? Isn’t my surge protector enough? Two of the most commonly asked questions regarding power, for sure. Answers, in part, come from an understanding of modern-day realities. AC power is far noisier and contaminated than at any time since its inception. This is due not only to an increased population taxing utility lines, but also because of the proliferation of computers and microprocessors. These devices typically run on switching power supplies, which can have a devastating effect on noise levels as well as on the bandwidth and character of the noise.

The past 20 years or so have seen a 100-fold increase in AC noise (Figure 1), while at the same time, performance expectations from audio (and video) devices/systems has risen dramatically. AC power is the foundation of every critical component used in a system, so as AC power quality deteriorates, so too does audio and video performance (and reliability).

A bit more specifically, line noise can smear, mask, and distort high-resolution signal, seriously compromising performance. These AC anomalies arrive from your breaker panel or service, and ultimately flow into every sensitive circuit.

BETTER YIELDS LESS

Experiencing the benefit of system components like superior audio processors, microphone pre-amps and even line array loudspeakers requires RESOLUTION. To achieve this, low-level audio information must be rendered without distortion, un-masked, pristine.

Thus the paradox: as the AC noise level continues to rise, advanced audio technologies yield less and less of their potential, because low-level information is either masked or distorted. Additionally, trouble-free performance with unwavering microprocessor presets calls for highly stable AC power.

Figure 2: Common mode noise attaches itself to AC lines in even proportion (both line and neutral wiring.)

Years ago, we all drank water from the household tap. How many of us continue to do so? Under current conditions, a rack of components that’s been without problems in 10 prior projects may hum, buzz, and worse come project number 11. Why? Because the circumstances that produce ground loops can be quite complicated and are affected by the internal grounding scheme of the components.

In spite of efforts to create dedicated AC lines and “star-configured” AC cable grounds, in the end, few systems render an ideal “zero ohms” between ground connections. Lighting equipment, large appliances or machinery that share the same power grid can easily cause ground noise.

Quite often discrete power from generators is not possible, or may not be desired, given current limitations, ambient noise, and voltage stability concerns. Unfortunately, AC ground contamination, loops, and distortion may result.

Figure 3: To help eliminate common mode noise, balanced power is achieved by running incoming power into a 1:1 ratio isolation transformer.

PURSUIT OF SPECIFICS Given the “Murphy’s Law” situation of our profession, many times these problems happen to occur just before the concert or prior to your fixed system’s first “big show”. Thus we arrive at a useful tool in helping to keep Mr. Murphy at bay, the AC isolation transformer.

As the name indicates, these devices are intended to isolate AC power (particularly incoming AC ground), thus eliminating the source of many AC problems. In the pursuit of specifics that can lead to further understanding, let’s have a look at the primary types of AC noise as well as solutions to help with the problem.

Common mode noise attaches itself to AC lines in even proportion (both line and neutral wiring referenced to ground). It comes primarily from AC fields and all 60-cycle harmonics, as well as a sizable portion of RF (radio frequencies) inducing noise into the AC line. (Figure 2)

Figure 4: Transverse (differential) mode noise does not attach itself to the lines in even proportion.

Though some products are designed with some form of common mode rejection from input to output (such as most audio power amplifiers), the majority of products are still susceptible to performance corruption from induction of this noise into their circuitry. The most frequently seen common mode noise rejection built into audio components offer “choke and capacitor filter” designs, and most certainly, many of these do aid in reducing the problem, particularly in terms of RF noise.

However, the addition of a symmetrically balanced transformer goes many steps further in reducing and often completely eliminating common mode noise across the audio and video bandwidth. Symmetrically balanced power is achieved by running incoming AC power into a 1:1 ratio isolation transformer, with a precisely placed center tap on the secondary.

This takes the inc-oming voltage (120 volts on the line terminal, and 0 volts on neutral and ground) and splits it in perfect halves on the output secondary of the transformer. Thus the output line terminal now has 60 volts AC while the neutral terminal also has 60 volts when referenced to its center tap ground, which remains at 0 volts AC. (Figure 3)

What’s significant about this is that the two 60-volt terminals are now in opposite polarity. So, like opposing magnets, the fields cancel. This canceling of common mode noise is extraordinarily efficient and linear across a huge bandwidth. (Re-cording and broadcast microphones have utilized this same noise reduction principal for over 60 years.)

And in the modern era, live sound professionals never consider using a microphone (or other components, for that matter) that is not balanced. Further, a host of ground loop problems can be eliminated in this manner, due to the transformer’s isolated signal ground.

WILL BALANCED SUFFICE?

The second type of noise found with AC power is called transverse mode, and it’s essentially the opposite of common mode in that the noise does not attach itself to the lines in even proportion. (Figure 4)

Also, transverse mode is even more devastating than common mode, and any common mode filter design (including symmetrically balanced) is generally useless in defeating it. This is important to understand, because there are those in our industry who would have us believe that “balanced power” alone will suffice in the quest to reduce AC noise.

Transverse mode noise is typically produced by motors, appliances, switching power supplies and digital processing circuits. Significantly reducing this noise requires a low-pass filter of considerable range. Here’s why.

Figure 5: Typical audio frequency versus amplitude. Note how low in level the upper harmonics are when compared to the lower frequencies and sustained signals.

Recall the previously mentioned dramatic increase in AC line noise seen in the last 20 years. What’s just as critical is that the character and bandwidth of the noise has changed. As late as 1980, most electronic circuits used large transformers and linear power supplies. But with the massive rise in personal computers in both business and residential applications, the character of AC noise changed forever. Two decades ago, AC noise was mostly limited to 60 Hz harmonics up to almost 400 Hz, followed by virtually un-measurable noise levels up to approximately.100 kHz. Most of this noise appeared in the AM radio band (and beyond), in the 100 kHz to 1 GHz range.

This is significant, because it still serves as the basis of the design (or architecture) of nearly every AC conditioning filter produced today. Most transverse mode filter designs are merely refinements of filters that were designed by Bell Laboratories nearly 100 years ago. These were conceived by tremendous engineers, but the designs were created to meet standards of the 1920’s.

This was a time when the AM radio was a pinnacle of technology, all electronic circuits were vacuum tube and switching power supply and microprocessors were barely, if at all, conceived. Now we’re faced with a constant stream of new technologies and the challenges they present ­ for example, today it’s possible to measure more noise at 2 kHz, or 10 kHz, than octaves above in the RF band.

HOW WE HEAR IT

Though it’s as important as ever to filter the RF bandwidth, it’s just as important to reduce noise in the audio band, particularly at 2 kHz to 20 kHz, where all the low-level harmonics occur. For premium audio, any noise in the upper octaves is devastating because of the way we hear music, along with the way that musical instruments work. (Figure 5)

For example, if we’re listening to music at our mixing console at an average level of 100 dB, most of the frequency content (at that amplitude) will be lower midrange and bass frequencies, and this will be primarily sustained energy. However, the harmonics, upper partials of music, reverb-ambient information, percussive transient attacks and high-pitched instruments will be riding along at a FAR LOWER decibel level.

In fact, a great deal of information in these upper frequencies will be 20 dB, 40 dB, even 60 dB below the upper decibel level. This is how we hear music. The reason a musician will pay a quarter of a million dollars for a Stradivarius violin, or the reason we can hear a significant difference between a Hamburg Steinway and a lesser grand piano is essentially the quantity and quality of harmonics that are typically very low in level compared with the fundamental tone.

We are capable of deciphering layers of information simultaneously, and often it’s these low-level sounds or signals that are most prized. This is one of the reasons it’s important to have the headroom to raise volume levels well above the din of an audience. Comprehensive AC noise filtering can increase both dynamic range and audio quality.

Figure 6: With AC regulation, widely variable voltage is converted into stable 120 volts.

Another valuable tool in the management of AC power is regulation, which essentially takes incoming voltages that are either too low or dangerously high and converts them into a constant, stable 120 volts. This is particularly important in systems where the incoming voltage is either continuously or intermittently above 125 volts or below 115 volts. Low voltage can be particularly troublesome for power amplifiers because their rated power specifications are based on a constant 120 volts.

If incoming voltage ranges as low as 100 volts, a power amplifier will only produce a fraction of its rated power.

Further, high voltage or poor power regulation can play havoc with video projectors. If this equipment is fed from a generator, regulation can make the difference between a successful show and a rack full of broken equipment.

TRANSIENT IN NATURE

AC power regulators are designed to insure delivery of a constant 120 volts output (typically within +/- 4 volts). Because the load demands of live audio equipment are transient in nature, AC voltage feeds may be taxed.

Further, portable AC generators are designed to produce a constant voltage only when connected to a constant load. Thus your amplifier rack could be receiving 116 volts during an opening set only to surge up to receipt of a dangerous 135 volts during an acoustic set because the amps are not in heavy use.

Particularly under these variable live conditions, it’s clear that AC regulation is essential. Surge protection and under/over voltage shutdown circuits are certainly as important as they’ve ever been. But further steps are required if we wish to reap the full potential of today’s advanced technologies. Without comprehensive AC power management, both sound quality and reliability can be seriously compromised.

After all, the show must go on!

Garth Powell works in engineering and technical support for Furman Sound, and can be reached at Garth@furmansound.com

November 2003 Live Sound International