This is part of IEEE Spectrum's special report: Top 11 Technologies of the Decade
Even in the go-go world of high tech, it's pretty rare that a technological leap delivers both markedly superior performance and stunningly greater efficiency. That neat trick happened with class-D audio amplifiers, which now dominate the market for applications in car stereos, home-theater-in-a-box systems, television sets, and personal computers.
Their success has been a long time coming. The first commercially available class-D amps came in the 1960s from the British company Sinclair Radionics (now Thurlby Thandar Instruments), but they didn't work well. Somewhat better ones came along in the 1970s and 1980s: John Ulrick designed a couple of class-D amps for Infinity Systems in the early and mid-1970s, and a decade later Brian Attwood did a series of "digital energy conversion amplifiers" for Peavey Electronics Corp. But in those days class-D theory was ahead of implementation, because the available components simply weren't good enough to produce high-quality sound.
The problem was speed: Class-D amps sample the input audio waveform hundreds of thousands of times a second before amplifying it, and it wasn't until the 1990s that cheap and reliable MOSFETs became available that were fast enough to sample a waveform at such high frequencies, says Bruno Putzeys, the chief engineer at Hypex Electronics. Hypex, in Groningen, Netherlands, is a leading maker of audiophile class-D amplifier modules, which are incorporated into products sold by other firms.
"What it took was for a couple of companies to take the plunge," Putzeys adds. Those pioneers were Tact Audio, ICEpower, and Tripath Technology, all of which released their first class-D offerings in the late 1990s. Tact, now called Lyngdorf, rocked the audio community in 1998 with a US $9800 class-D amplifier called the TacT Millennium, which was designed by the Danish engineer Lars Risbo. It dazzled as much for its industrial design as for its engineering: The amp had a large volume knob with a digital display in its center.