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Class Divisions

An Overview of New and Old Approaches to Power Amplifier Design

Its really in fits and starts that amplifier technology has advanced over time. So said Bob Carver, whose design credits span several decades in the audio business, with such companies as Phase Linear, the Carver brand, and his current work with Sunfire. Manufacturers still havent yet decided on the final topology that will take us into the future, he explained.

Today, the most-talked about trend in power amplifier technology is the Class D switching amp, despite the fact that its been around for a long time and has been unable to sweep established legacy technologies from the field.

Theres enormous engineering work being done right now on switching amplifiersin Japan, in China, and the United States, Carver observed. Theyre getting better, and better, and better. Theyre becoming more sophisticated. I always think that next year theyre going to be perfected, and then Im surprised that they arent.

Jeff Francisco, VP of engineering at SpeakerCraft, opines that the biggest story in digital amplification is really what hasnt happened. If we were to have this conversation five years ago, everyone would have expected all amplification in our industry to have become digital by now, he explained.

Audio Design Associates (ADA) is one company thats been therebuilding Class D power ampsbut then returned to the tried-and-true Class AB topology. We believe in performance as our guideline for how we do things, said ADA VP and COO Richard Stoerger. Technology is neat, but how do we apply it, and how do we go ahead and deliver value from that technology?

Stoerger said that ADA sees the power amplifier as, the essential component for delivering proper sound. The companys focus is on what it calls high-current amplificationamplifiers that arent current-limited in their operation. But, Stoerger added, Todays breakthroughs in amplification technology are actually in the opposite realmClass D amplifiers, and I dont necessarily believe that thats going to get you the best performance.

The knocks against Class D amplifiers range from poor sonic performance to unreliability to an inability to handle high power levels. You can design a [Class D] module thats small, Stoerger said. Its not going to throw off a lot of heat. But its simply not going to have the current output that will make the woofer respond more accurately and with more warmth, more bass. The Class D amplifiers that we were looking at werent stable below four ohms. And we worked hard at making sure that our Class D amplifier design was stable down to two ohms.

ADA utilized that Class D amplifier in the companys Suite 8100, an eight-source, eight-zone receiver. Stoerger explained that that particular product is no longer in ADAs product lineup, having been replaced with the Suite 8200. There, the upgrade was our moving away from Class D back to Class AB current amplification, he explained.

In Defense of Class D
David Dever, VP of technical development for NaimUSA, argues that although theres a tendency for some to look at digital amplifiers with a bit of scorn, it is possible to get reasonably decent musical results out of them, as long as you pay attention to good engineering. Some designers are better at doing that than others, he said. And, I think that really shows in the way that the units sound.

Naim will be utilizing Class D amplification in a new product range that the company calls NaimNet. It uses technology that we licensed from NetStreams, Dever said. Their technology is called StreamNet, and that is largely the mechanism by which the audio is transmitted over the [ethernet] network. StreamNet ensures tight synchronization in the delivery of the audio to various zones, which is very critical in party mode, where the same program material may be playing in many zones.

Dever says that in contrast to a traditional situation where there is one big closet loaded with power amps, NaimNet will utilize distributed room amps. Rather than existing in a massive stack of equipment that shoots high current through long runs of speaker wire, Dever explained, we see the power amplifier [under NaimNet] as really being part of one bigger scenarioin a per-room environment. Our approach is closer to what we would call connected HiFi.

One issue with distributed amplification is how to power those far-flung amps, but the NaimNet amplifiers will look much like stereo components; theyll be placed in individual zones that they need to power, and will plug into an AC wall outlet. DC versions that can be remotely powered will also be available.

Breaking the Rules
As a general rule, Class D amps seem to be finding their biggest application in areas such as multi-room audio, where per-channel power levels are frequently 35 watts or less. But one company that seems to be breaking all the rules is the Danish manufacturer Lyngdorf, which is debuting its new TDA-2300, a two-channel digital amplifier producing 300 watts per channel (into 4 ohms; 150 watts into 8 ohms) thats packaged in a slim 1U-high rack-mountable chassis.

The TDA-2300 is designed to team up with Lyngdorfs new D-1, an eight-channel home theater preamplifier/processor that features the companys trademarked RoomPerfect room correction technology. TDA-2300s are designed to directly connect via a proprietary digital Lyngdorf Link. (Theres also a S/P-DIF coaxial input, for use with other preamps having digital outputs.) When utilizing the Lyngdorf Link, multiple TDA-2300s can be daisy-chained, with up to 24 of the amplifiers controlled by a single D-1.

Interestingly, when using the Lyngdorf Link, the D-1 sends volume control information across the link to the TDA-2300, where output level is adjusted by varying the power supply voltages for the output stage. The company also has a proprietary digital process (trademarked Equibit) for converting the pulse-code modulated (PCM)-encoded digital signal information directly into the pulse-width modulation (PWM) needed to drive the output deviceswithout converting digital signals to analog and back again. This means that all signal processing is accomplished in the digital domain, and the company claims this is the worlds first fully digital home theater preamplifier/processor/power amplifier solution.

More Efficient Linear Designs
For every amplifier manufacturer that has jumped into the Class D world, there are likely just as manyif not morewho have opted to stick with, or return to, tried-and-true linear designs. Rather than changing the output stage to a switching type (as in a Class D amp), those working on linear amps have chosen to keep the output section more or less intact, but look for efficiency gains in how the output stage gets powered.

In linear amps, much of the electrical power coming from the power supply gets dissipated in output transistors. The crux of the matter regarding amplifier efficiency is that for a high-power amplifier, you need high-voltage rails. (Rails are the electrical buses that distribute various voltages created in the power supply to the various circuits throughout the amplifier.) Those high voltages are needed because an amplifiers peak output voltage can never exceed (or even quite match) the rail voltage.

But those high rail voltagesneeded to produce peak output voltagesare actually an efficiency detriment most of the time. When playing most real-world music and program material, loudspeakers only need those peak power levels to deliver brief transients, at musical crescendos. Most of the time, even big, hulking amplifiers are just loafing along at a few wattsor tens of wattsmuch like a high-powered sports car cruising around town.

How does the amplifier produce the lower voltages needed most of the time? The output transistors interpose themselves between the high-voltage rails and the load being driven. Essentially, they throttle the current so that most of the voltage drop is across the output transistor (or transistors, if multiples are used), while very little voltage is seen by the load.

The way to boost the efficiency of conventional linear amps is, therefore, to avoid having large voltage drops across the output devices. Class G and Class H amplifiersoften used in professional audio applicationsutilize either multiple voltage rails or step the rail voltage up and down according to the power demands of the input waveform at any given moment.

This is a strategy that AudioControl has taken. Tom Walker, the companys president, says that his firms power amps are Class H designs. The output stages can use the lower-voltage rails when only small voltages need to be output, and step up to the high-voltage rail when there are peak demands. Walker says its like having an amplifier that can shift gears as needed. With a multiple-rail, Class H amplifier, Walker explained, it can operate as a 15-watt amplifier under most conditions, and only switch to being a 150-watt amp when it needs to.

NAD uses a somewhat similar strategy, called PowerDrive, a technology that allows the amplifier to alternate between a rail optimized for voltage, and another optimized for high current.

Another Carver Innovation
Bob Carver has a patented approach that he calls a Tracking Downconverter. Here, instead of stepwise changes in supply rail voltages, the Tracking Downconverter provides a continuously varying voltage thats always six volts above whats needed at the output, at any given moment. This provides the voltage needed at the output, while the output devices never see a voltage drop in excess of six volts.

With such a small voltage drop across the output devices, Carver said, that really strips the amplifier of a need for heat sinks. Instead of dissipating hundreds of watts, it dissipates tens of wattssay, 30 watts, instead of 200 or 300 watts. Carver does allow that, theres a couple [of heat sinks] in my big amps, but all in all, much less heat sinking is required.

The Tracking Downconverter is one of the signature technologies in Sunfires power amplifiers. Among recent developments at Sunfire, the company has been planning an update to its Theater Grand line of multichannel home theater amplifiers that was scheduled to be unveiled at this months Consumer Electronics Show.

Theres clearly a diversity of philosophies among manufacturers and designers, and that extends to more than just what class of amplification is used. For example, Naims Dever observed that theres a real sense that people like to sell a five-channel power amplifier for home theater, but whether such a thing is really sensible, practical, or necessary is a whole other issue. Noting that different channels have different powering needs, Dever doesnt see the sense in bundling multiple channels into a single chassis. In [Naim Audios] opinion, the center channel, being the dialog channel, is really the most important section of the home theater, so it makes sense to focus a lot of the amplification there. Followed, of course, by the obvious left and rights, he said. In most cases, the surrounds need the least amount of power.

By contrast, ADAs Stoerger noted that Albert Langella, ADAs president, CEO, and chief design engineer, saw the advent of multi-channel sound as a game-changer for power amps. Whereas big monobloc amps are regarded as the Holy Grail for two-channel stereo, multi-channel soundtracks can levy power demands that move from one channel to another. Thus, ADAs power amps feature one big power supply thats shared by all the channels. That way, regardless of where that explosion may be, the power supply can deliver the current to the output stage that needs it.

A former loudspeaker designer, Alan R. Frank ([email protected]) is a networking consultant and freelance writer.