November 2006
By Robert Harley

Class D Amplifiers — Promise Or Peril?

 

  The controversy over whether Class D power amplifiers are an advance in sound quality or merely a concession to convenience (Sec our special report on page 72) parallels previous technology introductions in high-end audio. The most obvious example is the transistor itself Small, lightweight, and powerful, the transistor seemed to be a significant advance compared with the ancient technology of glowing filaments in a glass envelope.

Yet, the first transistor amplifiers were hideous in sound quality Despite competing with mature and great-sounding high-end tube designs such as the McIntosh 275, transistor technology still came to dominate audio until the tube renaissance was ushered in by William Zane Johnson in 1970 with his founding of the Audio Research Corporation.

Today, of course, transistor designs have reached a high level of refinement and musicality, and are no longer the clearly inferior technology (although some would argue with that assessment).

The second technology introduction that parallels today's state of Class D power amplifiers is the Compact Disc. Again, a new technology came along that offered advances in convenience, form-factor, size, weight, and cost. The first examples of the medium were crude and amusical, but that didn't stop CD from quickly replacing the LP. Nonetheless, CD sound went on to achieve a musicality (in both software and hardware) that would have been unthinkable in the early 1980s.

And now we have Class D power amplifiers. Their light weight, small form-factor, high output power, and low heat dissipation are certainly compelling, just as the transistor was compelling to the vacuum-tube age and CD to the vinyl era. But high-end audio is not about form-factor and convenience; it's about the pursuit of musical realism in our homes. That's why the cognoscenti continued listening to vinyl records through vacuum tubes long after those technologies vanished from the mass market. But how will history judge Class D amplification? As a fundamentally inferior technology that has no place in the high end? As a potentially superior technology that just needs dine to evolve so that it can compete sonically with the best of today's conventional amps? Or as a clearly superior technology that will render obsolete the massively inefficient linear amplifier?

Although it's very early in the learning curve (both for designers and critical listeners), I think we can reach a few reliable observations about Class D. First, the technology is in its infancy, suggesting that sound quality improvements are inevitable. Look how long it took solid-state amplifiers and CD to sound musical. From an historical perspective, Class D is now in the transistor amplifier's 1960s era, and in the CD'S late-1980s era.

Second, certain sonic qualities — dynamics, bottom-end weight and control, transparency — seem to come much more easily to Class D than to linear designs. This is somewhat true in an absolute sense, but startlingly so when Class D amplifiers are compared to similarly priced linear models. At a given price level, Class D outperforms conventional designs in some performance parameters.

Third, our observations about the strengths and shortcomings of Class D amplifiers in this issue's reviews and Editors Roundtable are made in comparison to megabuck reference-quality linear amplifiers. Even the most expensive Class-D amplifier in our survey — the $6800-per-pair Kharma MP ISO — is a fraction of the price of the linear amplifiers to which it was compared. The NuForce Reference 9 monoblocks, at $2500 per pair, are roughly one-tenth the price of my reference Balanced Audio Technologies VK-600SEs. The linear amplifiers have the advantages of being a fully matured, significantly more expensive technology, and of being hand-picked by the individual reviewers as reference-quality The Class D amplifiers were also judged purely on the basis of their sound quality; with no concession to their advantages in price, size, weight, and lower heat dissipation. As you'll see in the Editor's Roundtable this issue, those of us whose primary daily experience is with affordable amplifiers tend to view Class D more favorably than those of us who listen on a regular basis to cost-no-object reference units.

Fourth, it appears that Class D amplifiers are extremely sensitive to the loudspeaker load and cabling. They will sound different in different systems to a much greater degree than linear amplifiers. This factor helps explain the polarized reaction to some Class D amplifiers, and also suggests that the audition of a Class D amplifier with the loudspeaker and cables with which it maybe used is prudent. Class D amplification is a potentially revolutionary technology; holding Out the promise of high output power, relatively low cost, efficient operation, small size, and yes, great sound quality; It's far too soon to make definitive judgments, but based on what's already been accomplished by Class D in just a few short years, the future will be fascinating.