

Technology has democratized the music industry in many obvious ways – notice how every band, regardless of stature, has a MySpace page? But one of technology's more subtle effects has been to make our sound systems smaller, lighter and greener.

The special sauce: Class D amplifiers, the first real advance in speaker design in decades. These amps were once thought to be appropriate only for low-end audio devices like cellphones. But with continuing refinements they have started showing up in everything from flat panel televisions and computers to high-end sound systems.

"Imagine a light bulb in your house," said NHT audio engineer Gordon Chang. "If you turn it on all the way, it's too bright, and if you turn it off, it's too dim. Now if you turn it on and off really quickly, you can get the light to look like it's halfway between on and off."

Traditional amplifiers achieve the same effect using electrical resistance, similar to a dimmer switch that controls the light. Those resistors turn valuable electricity into unwanted heat, which necessitates bulkier design. Ever wonder why your ultra-thin panel television sounds so great despite having so little real estate dedicated to audio? The Class D amplifier, which is having a massive, albeit largely unacknowledged effect on consumer audio devices, is responsible.

"The big advantage with Class D amplifiers is that they're much more efficient from an energy usage point of view than conventional Class AB amplifiers, which are what you've seen in most audio equipment up until now," said John Widder, market development manager at STMIcroelectronics, a semiconductor manufacturer.

He said Class AB amps typically run at between 15 and 40 percent efficiency when playing back music, while Class D amps run at between 40 and 85 percent. And that's only part of the picture.

"The advantages of that efficiency then ripple through the entire system … power supplies can be smaller, or if it's battery operated equipment, the battery life is longer," said Widder.

"But then, in addition, because the amplifier's more efficient it has less heat to get rid of, so heat sinks can be smaller, or maybe (the devices) don't even have a heat sink at all, which makes the equipment then smaller and lighter. You can have things like five or seven channel home theater systems in very small boxes with very high output power."

These efficient designs lower shipping costs and ultimately contribute less mass to our planet's landfills. The effect may be negligible on a per-unit basis, but when you consider that Class D could result in the vast majority of the world's sound-producing electronics consuming less than half as much energy as they used to and they are being manufactured at much smaller sizes, it's clear that the economically-driven trend towards Class D amplifiers will have a significant positive effect on the environment.



Class D amplifiers are superior to their predecessors in many ways,

but none of them matter to music fans unless they sound good, too.

"Initially," explained Widder, "Class D was only used inapplications where audio was a secondary consideration" such as incellphones. No longer. Most high-end plasma and LCD televisions now useClass D amplifiers, as do several home theater systems.

I too can bearwitness to how excellent Class D can sound, having reviewed the Kuller Aux Out400, a $1,000 audiophile-quality bookshelf system.

Longconsidered unworthy of high-end music applications, Class D's more recent specs are praiseworthy:

sub-tenth-of-a-percent total harmonic distortion and frequency responseequivalent to that of Class AB (see diagram). Class D even has somesonic advantages over its predecessor, says Widder: "low – or even zero –

crossover distortion" (which occurs in Class AB amps when one outputstage takes over for another) and "very high dynamic range andsignal-to-noise ratios" (meaning that it has many levels between softand loud, with very little noise interfering with the sound of themusic).

So who's making this stuff? ABI Research analyst Steve Wilson told us that twocompanies are responsible for a good amount of the high-end Class D

electronics in home theater setups: IcePower (a division ofBang & Olufson) and D2Audio. Meanwhile, a company called TriPath made the chips used by the Aux Out 400 speaker set I liked so much – a version of Class D it calls "Class T" (for TriPath).

Atthis point, anyone with even a mild geeky streak will want to know howClass D's sound improved enough to spread from cellphones to hometheater systems, where sound quality is a big priority. As it turns out,

the change has been largely a matter of filtering and design.

Sincea Class D amplifier has an output that switches between on and off, thenoise created by that switching must be filtered out. "You just have tobe careful with the filter design and circuit board layout," saidWidder. "It takes a little more work on the part of the designer." Asword spreads among the audio community that Class D amps are ready forprime time, we can only expect these design improvements to multiply.

Class D amplifiers maynot have much name recognition, but they're already changing the ways welisten to music, with the welcome side effect of shrinking and greeningthe world's audio hardware. A little respect is long overdue.

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Images courtesy of STmicroelectronics