"Half a watt goes in here . . ."
". . . must come out there!"
"I’m talking about power, Chucko."
"I know you are, or you wouldn’t be here."
(Mechanical noises)
"Look! It heard the word power and responded, just like we do!"

~Firesign Theatre

Those of you who are old enough might remember this routine from Firesign Theatre’s classic comedy album I Think We’re All Bozos on This Bus. Those merry jokesters were talking about a mechanical model of the government, but I’m talking about sonic power, Chucko–raw, un-adulterated audio wattage.

In any sound system, this undeniable force is supplied by a power amplifier. The technology that underlies this indispensable device has remained essentially unchanged over the past 40 years, ever since transistors replaced vacuum tubes. But now, at the dawn of a new millennium, we also face a new era in power-amp technology. The industry is making quantum leaps as it pursues its goal of designing power amps that are smaller, lighter, less expensive, and more powerful than previous generations.

EM author Rudy Trubitt examined the design philosophies behind the power amps of several major manufacturers in his article "The Power and the Glory" (see the August 1993 issue). One might think that little has changed over the past half-dozen years; after all, power amps seem like the type of product that evolves at a glacial pace. Indeed, some designs have changed very little, but several companies have developed new or significantly revised designs since Trubitt visited the subject. Clearly, it’s time to take a fresh look.

To this end, I interviewed representatives from five power-amp manufacturers: Mackie Designs, Hafler, QSC, Crown, and Velodyne Silicon Systems. Some of these companies have begun to manufacture power amps relatively recently, while others are well-established names in the field. Of course, their spokespeople agree on some points, but it’s what they disagree about that can be especially enlightening.

Learn Your ABCs

Before we delve into the fine points of power-amp design, let’s review a few basics. As its name implies, a power amplifier boosts the power of a signal. Typically, a power amp accepts a line-level signal and increases its voltage and/or current without changing the shape of the input waveform. The amplified signal is sent to a speaker, which converts the signal into acoustic sound waves. Power amps are used in three primary applications: studio monitoring, live sound reinforcement, and instrument amplification.

Unlike most studio gear, a power amp draws some serious current from the AC outlet. This current is converted to DC by the amp’s power supply. In a traditional power supply, a power transformer decreases the incoming AC voltage, which is then converted to a DC voltage by a set of diodes and several large capacitors. One common type of power transformer is called toroidal because it looks like a doughnut (a shape known as a toroid in mathematical terms). This shape plays an important part in how the transformer functions because the toroid’s magnetic field is confined more to its core, reducing leakage into the audio circuitry. Toroidal transformers are made of iron for its electromagnetic properties.

The DC voltage from the power supply is symmetrically arranged around the ground point (0V). For example, the output from the power supply might be ±50 VDC. These positive and negative voltages, which are called the power-supply rails, operate the amp’s internal circuitry.

In particular, these voltages provide power to a set of output transistors, which perform the actual amplification. The output transistors amplify the input signal by drawing power from a set of capacitors in direct proportion to the input signal’s voltage as it varies over time. As the capacitors discharge in this process, they are replenished by the power supply.

The power-supply rails determine the maximum amplitude that the amp can produce. For example, if the rails are at +50V and -50V, the amp can produce signals of nearly 100 volts peak to peak. If the amp produces a signal that exceeds this limit, the tops and bottoms of the waveform are cut off; this is called clipping.

One of the most important characteristics of any power amp is the efficiency with which it uses AC power to amplify the input signal. Unfortunately, most conventional designs are very inefficient, using less than 50 percent of the AC power they draw from the wall. The remainder of that power dissipates as heat within the amp. Most power amps therefore require large heat sinks, and many use fans to cool their components. In addition, many include thermal-protection circuitry, which shuts down the amp if things get too hot.

That Amp’s Got Class

Power amps fall into several classes, the simplest being A, B, and AB. Class A power amps have output transistors that handle both the positive and negative swings of the waveform. Class B designs have one set of transistors that handles the positive swings, and another that handles the negative swings. Class AB amps also have separate sets of transistors to handle the positive and negative swings, but there is some overlap as the signal changes polarity; when the signal is near 0V, both sets of transistors are conducting.

Most of Mackie’s current power amps, including the ones within the company’s powered speakers, use a conventional Class AB design with conventional power supplies. According to Cal Perkins, Mackie’s director of new technology (and self-described corporate cynic), the company’s amps have the most efficient Class AB design in the marketplace because they use lightweight toroids, which reduce the transformer’s weight by about 50 percent.

Hafler uses a variation of Class AB called trans-nova, designed by Jim Strickland in 1980. Jerry Cave, Hafler’s managing director, explains: "It’s a different way of using transistors in the circuit, requiring fewer gain stages and a much simpler signal path. This lets us get voltage and current gain out of both transistors instead of one or the other. Trans-nova combines the linearity of Class A with the efficiency of Class AB. Class A is typically only 25 percent efficient; trans-nova is typically 65 percent efficient."

Some amp designs use multiple power-supply rails. When the peaks of the output signal are small to moderate, the low-voltage rails are used; most musical material falls within this range most of the time. When the output peaks are large (as in momentary, loud transients), the high-voltage rails are used. If the same set of output transistors is used with both sets of rails, this is called a Class H design; if different transistors are used with the different rails, it is called Class G.

The multirail approach is more efficient than single-rail designs. According to John Subbiondo, marketing manager for QSC, "Transistors are most efficient when they are all the way on; if they’re only halfway on, half the power is lost to heat. With a multirail design, the transistors are closer to being fully on more of the time." Consequently, multirail designs can lower the AC current draw and cooling requirements by as much as to 40 percent. QSC’s most powerful amps use four rails on each side of ground.

The New Switcheroo

Another way to improve the efficiency of a power amp is to use a switching power supply (also known as a switch-mode, active, or electronic power supply). Long used in computers and other devices, a switching power supply converts the incoming 60 Hz AC power to a much higher frequency, often in the 200 to 500 kHz range. This improves the performance of the transformer (the behavior of which is frequency dependent), allowing smaller, lighter transformers to be used.

QSC uses switching power supplies in its PowerLight series of amps. Subbiondo cites several additional advantages to amps that use this approach: "You can make them very quiet; you don’t have a big hum field from a large transformer, and the hum field you do have is outside the audio range. In addition, you can have a purer path from the audio circuitry to the speakers. And because the transformer is smaller, it tends to have lower impedance and fewer losses, which translates into less voltage sag and better performance under high demand."

However, Perkins points out that switching power supplies have their own set of problems. "True, a switching power supply eliminates most of the low-frequency magnetic fields, but it produces a lot of high-frequency noise. This noise can be eliminated, but in many cases it isn’t."

Many people believe that using a switching power supply results in a more open sound with a clearer high end, but that it also inherently compromises bass response. Others say that this problem is related to poor design implementation: correct design provides better voltage regulation, which results in improved low-frequency performance over conventional power supplies.

Mick Whelan, vice president of new product development at Crown, sees this debate as similar to the one about analog versus digital audio. "Some people love it and others hate it," he says. "A lot of people believe that you don’t get good bottom end without a good chunk of iron."

Perkins believes that this debate is more smoke and mirrors than anything else. "A power supply is nothing more than an energy-conversion and energy-storage mechanism," he says. "It sucks AC from the wall, converts it to DC, then reconverts it to a modulated signal, which is the audio. This process is a function of the total enegy-storage capacitance and/or inductance of the system. If you have the same energy-storage capability, it doesn’t matter if it’s in a capacitor or an inductor; the number of joules is the same."

Perkins gives an example from his past experience working at JBL: "I designed an amp for JBL with a switching power supply that had more energy storage than the biggest iron supplies they had at the time. A competing product on the market at the time had roughly the same average power level but had one-twentieth the energy storage. When we connected the two products to a pair of speakers and cranked up the sound using material with a lot of bass, it was like listening to entirely different speakers. There was no bass in the competing product."

According to Subbiondo, switching power supplies offer another opportunity, called power-factor correction. "This is a way of drawing current from the wall more efficiently," he says. "Supplies that aren’t power-factor corrected do something called peak-voltage rectification, in which the filter capacitors charge up only when the incoming AC voltage equals or exceeds their reservoir voltage. It draws current only at that time, so you get big current spikes. Power-factor correction lets you continuously draw current over the entire AC waveform. This lowers peak AC requirements by 40 percent, which might not mean much in a 200-watt amplifier, but it makes a big difference in a 9,000-watt amp."

Switcheroo, Part Deux

One of the most interesting recent developments in power-amp technology is the switching power amp, also known as Class D. In this design, the input signal is converted into a pulse-width modulated (PWM) square wave by alternately turning two output transistors on and off in the 100 kHz range. (Class D amps are therefore sometimes called PWM amps.) This square wave is then processed through a lowpass filter, which yields an amplified version of the input waveform.

Theoretically, Class D topology offers some significant advantages over more conventional designs, the most important of which is much higher efficiency. This means that you need less heat management–which translates into lighter weight, smaller size, and lower cost. Class D amps have been used in powered subwoofers for some time, partly because their distortion is less audible in the low range. One of the premier manufacturers of powered subs is Velodyne Acoustics, which recently started a new division, Velodyne Silicon Systems (VSS), to develop Class D amps for other manufacturers to use in their products.

However, this type of amp presents a number of significant obstacles to high-fidelity operation. According to Bill Ciullo, vice president of engineering at VSS, "Class D is very difficult to do well. In the real world, it tends to sound terrible unless you do some fancy design tricks. For example, if there is any overlap or gap between the transistors turning on or off, you get major distortion." This can even cause the amp to literally explode!

In addition, Cave says, "Class D amps have output inductors that act as filters. When you change the impedance of the speaker load, the frequency response of the amp changes. If you know the impedance of the driver you’re using, it works fine."

Ciullo says that Tripath was the first company to make Class D amps sound good (although, in typical marketing fashion, Tripath calls its design "Class T"). "They solve the overlap/gap problem at the front end by adding broadband noise to the input signal," Ciullo says. "This tends to mask the switching errors, which are averaged in with the noise. This approach is somewhat similar to digital dithering. However, it requires DSP at the front end, which is expensive, and the transistors are switched in the megahertz range, which presents its own difficulties.

"Velodyne’s founder, David Hall, solved the problem differently, on the back end of the process," Ciullo continues. "He put an inductor between the two transistors. When they are ready to switch, the energy is temporarily stored in the inductor until the switch is complete, at which time the energy is released to the transistor that’s on. This process is controlled by two more transistors. There is no voltage drop across the transistors when they switch, so they are not stressed at all."

Interestingly, Velodyne’s design uses no power transformer. Rather, it rectifies the wall voltage and produces ±82 VDC rails. (Standard wall voltage is 120 VAC RMS; peak-to-peak is 164V.) Classes A and AB can’t use rails at these extremes because they’d have too much heat to dissipate; the transformer steps the voltage down to reduce heat. According to Ciullo, Velodyne’s design is more than 97 percent efficient; its 250-watt and 600-watt Class D amps need no heat sink or fan, and they are quite small compared with more conventional designs.

Crown’s K series of full-range Class D amplifiers uses a strategy similar to Velodyne’s (that is, an inductor between the output transistors); however, the two companies have separate patents on their designs. Crown’s approach is called Balanced Current Amplifier (BCA). "This is an enhanced Class D topology," Whelan says, "that lets us make a 2,500-watt amp [1,250 watts per channel into 2 ohms] in a two-unit package with convection cooling. No other topology can do that and maintain a frequency response of 20 Hz to 20 kHz with low distortion.

"BCA efficiency is around 90 percent," Whelan continues. "However, measuring efficiency with sine waves is a waste of time. The energy density of a sine wave is very different from that of actual music; a sine wave has much more energy than most music signals. For example, an amp that’s 90 percent efficient with sine waves might be only 40 percent efficient with music." Perkins agrees with this assessment, adding that the efficiency of an amp changes dramatically when you measure it at the typical average level of most music, which is only 20 to 30 percent of full output.

Class D amps have a potentially bright future as "digital" amplifiers. According to Ciullo, "Essentially, the output of the transistors is ‘digital’ in that it oscillates between the two rails. In the near future, we’ll see Class D amps that accept a digital bitstream, decode it, and use it to determine when the transistors should be turning on and off. This would keep the signal in the digital domain all the way to the lowpass filter and would be a very noise-free design. You would have no chance to pick up analog noise anywhere along the way."

Power to the Speakers

Another hot topic in the world of power amps is the concept of powered speakers, in which a power amp is mounted within a speaker cabinet. This subject sparks lively debate in the audio community.

Cave points out some of the advantages to this approach. "The speaker’s drivers and enclosure act as a system with impedances, pressures, and so on. If you drive it with an external amp, you don’t know what kind of performance you’re going to get if the amp is not matched to that speaker. With a powered speaker, you can match the amp to the speaker in terms of power, impedance, and crossover points, which optimizes everything."

Perkins agrees. "The main advantage of powered speakers is that you can optimize their performance with the internal electronics," he says. "You can dramatically change the sound of any speaker. For example, the Mackie HR824 uses basically the same midrange and tweeter as another popular powered speaker. The bass driver is similar, too, and the enclosure is the same size. But they sound totally different from each other because of the electronics package."

On the other hand, some people point out the disadvantages of powered speakers. Subbiondo offers an example: "Instead of running one cable to your speakers, you have to run a signal cable and a power cable, which can introduce EMI into the signal cable. Also, if you’re flying the speaker in a live venue and it fails, you’re stuck. Powered speakers are well suited for monitoring, which is why they’ve seen so much success there. But when you’re powering many speakers with the same signal, it can be more cost-efficient to use a single, large amplifier."

Whelan sees an emotional factor, as well. "One of the beauties of having a separate amp is that people have their favorite speakers and amps. Separating them gives you the flexibility to assemble your own system and get the sound you’re looking for. When you put an amp in a speaker, you remove some of that flexibility."

Thermal Shutdown

Power amps are essential to any sound system, and their basic function is unlikely to change in the future. However, the relative merits of the different ways they perform their function will be debated as long as engineers devise new methods of amplifying an audio signal.

With 30 years of experience in this field, Perkins recognizes two schools of belief when it comes to power amps: "One school says they’re all the same; there are no differences. The other school says that each design sounds different–and manufacturers use this to tout their own products as better than the next company’s."

To which school does Perkins subscribe? "The reactive load of a speaker is totally different from the resistive load on a test bench," he says. "Because of that, you do have sonic signatures and differences between products and manufacturers. Some of these differences are subtle, and others can easily be discerned even by someone who doesn’t care a whole lot."

Of course, the readers of EM do care. So if you’re in the market for a power amp, listen critically to several models in your price range and take along recordings that you know well. Audition amplifiers that use different topologies so that you can become familiar with their sonic signatures. In addition, keep your purpose firmly in mind. As Cave says, "There are a lot of different designs, and you have to choose the one that is best suited to your application."

Scott Wilkinson, a contributing editor to EM, knows that what goes in must come out.