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Listening to Class D

Listening to Class D

You needn’t look further than Class D audio amplifiers to find a technology whose proponents tend toward exuberant advocacy at best and bald-faced hype at worst. Although these ICs are not yet the be all and end all of audio amplification that their makers would have you believe, they are remarkably good-certainly more listenable than the clamor about them.
If you’ve ever perceived the slightest gap between the marketing and the metal in an electronic product, you ain’t seen nothin’ yet, as the song goes, until you read the promotional materials that accompany audio products. You’d think that the engineering community would have dispensed with the hype long ago. Electronic audio amplifiers, starting with Lee De Forest’s triode vacuum tube, are nearly a century old. Even the relatively newfangled Class D is no longer a novelty. From an instrumentation perspective, audio bandwidths aren’t challenging, the amplitudes are generous, and the impedances look kindly upon probing. With other products, you would just compare data sheets and be done with it.

But the data sheets and supporting documentation available for audio amplifiers often say precious little that forecasts how the products will perform in their primary role: driving speakers in various environments for humans to enjoy. Lest this sound like an appeal to subjectivism, let me assure you that it is not (see sidebar “The trouble with subjectivists”). Unfortunately, data sheets typically don’t reveal the kind of measurable detail that allows for meaningful comparisons. This state of affairs is not entirely the fault of amplifier manufacturers. They are not, after all, trying to drape their wares behind a veil of secrecy or false promises. However, no industrywide consensus exists on what comprises a measurement set that correlates to the listening experience: a surprising and disappointing fact considering the conceptually simple task that audio amplifiers have to perform and how long they have been doing it. The problem is not that we can hear things that are immeasurable—the fatally flawed claim of the subjectivists—but rather that we haven’t a defined set of measurements that adequately predict what we hear. Though the fi is as hi as an elephant’s eye, there’s no “fidelimeter” with which to measure it.

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