Everywhere you look today, digital technology is taking over. In photography, digital cameras are the hot sellers. Digital controls and displays are the norm for everything from kitchen scales and microwave ovens to thermometers and wristwatches. And in audio, CDs and DVDs long ago displaced vinyl records.
Analog technology still dominates in audio power amplifiers--but not for long. Today, amps based on digital principles are already having a profound effect on equipment efficiency and size. They are also beginning to set the standard for sound quality.
Known as Class D amplifiers, these devices have the potential to be extraordinarily efficient--to waste very little power. That means they can be made quite small and light because they have no need for large metal heat sinks to get rid of their waste heat. The result: gear like a 500-watt amplifier you can hold in the palm of your hand and lower-power portable equipment that runs much longer on a set of batteries.
By the way, the "D" in Class D does not stand for digital, but was simply the next available letter for classifying amplifiers. What distinguishes Class D amplifiers from all others is that their power transistors are always operated either fully on or fully off. This is the only and complete definition of Class D and is the reason they are so efficient--typically around 90 percent at rated power.
The article by Bruno Putzeys of Philips Digital Systems Laboratories, in the March issue of IEEE Spectrum, describes Class D amps and explains in detail how they differ from conventional amps. It also discusses technical problems they have faced and shows how these problems were solved in the past and the different ways they are likely to be solved in the future. Accompanying the article is a sidebar by audio expert Dan Sweeney, who describes a listening test and compares the sound of a Class D amp with those of comparably priced tube- and transistor-based analog amplifiers.
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