Here are some industry heavyweights talking about the technologies.
PS Audio’s Paul McGowan wrote: “If you look at class-D, there are limitations you will always have to deal with. One of those, of course, is the analogue low-pass filter at its output, used to remove the switching pulses between transitions. While much wonderful work has been performed on this limitation, especially by class-D’s resident genius, Bruno Putzeys, the fact remains it’s there in the signal path. Another limitation is the dynamic range. In a typical PWM-modulated signal running at 100kHz or so, you’re basically limited to about 16-bits of resolution, about the same as a CD, which ain’t bad, but still.
“On the positive side,” he added. “The linearity of a class-D amplifier will almost always exceed that of an analogue-based power amp. They are different, and they sound different. In our products that use class-D output stages, we work with them, as we do all our circuit topologies, in the same way a fine wine vintner [works his wines]. We blend this and that to come up with an award-winning output. For example, in our class-D amplifier Stellar line, we use an analogue input stage to feed the PWM modulator. In the lower wattage models, that input stage is sweetened [using] low feedback FET designs, while in our highest wattage model, the M1200 monoblock, we add a tube input stage to perform a similar function.”
“Analogue power amps, too, have their limitations or quirks,” Paul said, “which we also solved in the design by blending the proper amount of technology and topology to come up with winning design choices, all in service of the music.”
He concluded: “I think using class-D technology for the power supply, as opposed to the huge analogue transformers, etc., and using analogue output and input stages for the audio signal is likely the best topology currently available today. Time will tell if that reverses or changes.”
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Belgian engineer and Kii Audio co-founder Bruno Putzeys
About that resident genius Paul mentioned—Belgian engineer and Kii Audio co-founder Bruno Putzeys. I think it’s fair to say that class-D amplification would not have the status it has today if not for the fact Bruno spearheaded the class-D revolution with his Hypex UcD and Ncore class-D designs used by most high-end audio manufacturers today. I also doubt that, if not for Putzeys seminal role in making class-D sound as good as it does, that the tube designer I spoke of at the start of this piece would’ve told me what he did that blew my mind.
When I asked Bruno if he’d be willing to contribute a paragraph to this discussion about class-D’s future, he sent me his response with an intriguing foreword: “I do hope you can afford me a few more words than a paragraph, particularly because I’m fairly certain that it’ll run directly counter to at least some of the other replies you’ll get.
“To be completely blunt,” his response began. “State-of-the-art class-D amplifiers are good, not because they’re class-D, but in spite of it. I chose a career in class-D because I liked the [the technology’s] efficiency and compactness, and I was hoping to combine that with high fidelity.
“My first attempt immediately sounded appealing and engaging. The idea that class-D sounded ‘harsh’ was never true and was only perpetuated by people who’d never heard one. As proof of that, rudimentary zero-feedback designs still regularly hit the shelves and garner praise on account of their striking sonic character. But Hi-Fi, they were not. I wanted an amplifier you couldn’t hear, one that anyone could use and be happy with in any stratum of the market. And that turned out to be inordinately difficult [to design].
“The secret lay not in the power stage but in the control circuit, i.e., the modulator and the error correction. The mathematics required to fully understand a class-D amplifier is similar to what’s used in sigma-delta AD/DA chips, only more complicated. It’s not taught at any school. But the change you can make by only tweaking the power stage (faster FETs, etc.) is tiny compared to the effect of better error control. So, mathematics it was.
“Designers of traditional amplifiers are not known to geek out on mathematics,” said Bruno. “They wouldn’t have to anyway; it’s not that difficult to build a respectable class-A amplifier using nothing but a few well-worn rules of thumb. The result was that while class-D crept steadily forward, class-A pretty much stagnated.
“This has caused a curious paradigm reversal,” he continued. “The question is no longer whether class-D is approaching the quality of class-A, but how many class-A amplifiers can really claim to be up there with the best of class-D? Make no mistake, the fundamental fact still holds: any given level of performance is much easier to achieve in class-A than in class-D. But designers of class-A amplifiers have, by and large, sat on their laurels. I could easily design a better class-A amplifier, but I see no one waiting for a better petrol engine.
“The level of difficulty explains why virtually all demonstrably good class-D-based products use prebuilt modules,” he wrote. “The effort of working this stuff out only ever pays off if you can re-use the design in hundreds of products. That’s a mixed blessing. On the one hand, class-D modules have turned high-end amplification into a commodity. On the other, it’s still de rigueur in the audiophile market to have distinguishing (i.e., home-grown) technology. Mine and my competitors’ work of decades raising class-D to adulthood now lends a halo effect to any class-D amplifier. Today, almost any class-D amplifier gets a rave review, no matter how crude the design. This could well prove perilous in the longer run to class-D’s hard-won reputation.”
He added: “[One] way to escape from this is for the audiophile market to [focus less on] separate components. If you want to know what’s scaring younger customers away, it’s the idea that they should suddenly school themselves in amplifiers, DACs, cables, and whatnot before they can buy something that plays quality sound. Active speakers are a way out of this. If high-end audio has a future, it’s in system [integration], where the amplifier is simply a necessary functional block but where the real cleverness lies in the concept of the system as a whole and how it functions, sonically and practically.
“There is a whole future in front of us with radically improved sound systems,” said Bruno. “If only the market were ready to accept that the amplifier part is basically a solved problem.”
