April 03, 2023 / By mobanmarket
I’m writing this column on the long flight back to New York City following High End Munich, the big hi-fi show that in regular times takes place each May. Because these are not regular times, this was the first Munich show since 2019. This show was smaller than other recent Munich shows: COVID in the Far East limited involvement by people and companies from East Asia, and German government–mandated attendance caps limited the number of people who could enter at any one time. Even so, it was a big show, with some interesting product introductions and prototypes.
A big hi-fi show is a great place to see and audition new hi-fi equipment. Of at least equal importance to me, it’s also a good opportunity to talk about hi-fi with leading people in the field.
Early Thursday afternoon, the first day of the show, I visited an off-site “service apartment” temporarily occupied by folks from Purifi Audio, the company best known for its advanced, low-distortion “Eigentakt” class-D amplification modules. Purifi is also developing loudspeaker drivers, which they were demonstrating in Munich.
Amplifier modules and loudspeaker drivers may seem an odd combination, but both are basic building blocks of a hi-fi system and both are significant sources of distortion. As such, both are obvious foci for people aiming to make hi-fi better.
But haven’t people been working to improve speaker drivers and amplifiers for more than 100 years? Surely all the low-hanging fruit has already been harvested—hasn’t it? I guess that depends on how tall you are, metaphorically speaking.
Bruno Putzeys, who co-founded Purifi with Peter Lyngdorf and Lars Risbo (the latter also present, as was co-owner/director Claus Neesgaard), told me that when they started Purifi, the main idea was to correct hi-fi problems with DSP—digital signal processing. They soon changed course and started to use science and engineering to attack some of those problems at the source. A result is the Eigentakt module used in some of the best current class-D amplifiers. Another is their loudspeaker drivers, seven of which are already marketed, with others in development. Purifi’s drivers are documented by extensive performance data—far more than is provided by most other driver manufacturers.
Many advancements in loudspeaker drivers involve fancy and expensive new materials—anything that makes the cone stiffer and lighter. Purifi’s new drivers use old-school cone materials: aluminum and paper. In place of real-world trial and error, Purifi solves many problems in silico before they build the first prototype. Then starts a cycle of measuring, listening, and refining—but less iteration is needed now because their simulation work sets them closer to the optimal solution. Such an approach led to modifications in the shape of the cones used in Purifi’s loudspeaker drivers that look subtle but have important sonic consequences.
Less attention has been paid to surrounds, those “floppy pieces of rubber” as one of the Purifi folks—I think it was Putzeys—called them during our meeting. Because they’re at the outside edge of the driver’s radiating surface, and because area is proportional to the square of the radius, the surround can be close to 20% of a speaker driver’s moving surface. You can’t ignore that if you want low distortion.
There’s nothing special about the material in Purifi’s surrounds, either; it’s just rubber—but because of the way they’re molded, Purifi’s surrounds are less floppy than most. I lack the space to detail the technology (footnote 1), and I’d likely get it wrongnif I tried—so I’ll merely report that while Purifi’s cones look ordinary, their computer-optimized surrounds look kind of, well, funny. Not that you’d care a whit after you heard them.
Purifi’s work on drivers isn’t limited to the parts you see. Innovations in the motor system reduce hysteresis distortion and solve several other important problems.
But it’s not their specific innovations I want to draw attention to. It is, rather, some big ideas that the Purifi approach illustrates:
• Straightforward, clever thinking combined with simulation can still, in 2022, realize major gains in performance even in hi-fi’s most basic technologies.
• Easily audible improvement can result from lowering distortion that’s already at levels far lower than you (or anyway I) would imagine audible, often lying far below the signal level and deep in the noise. In certain circumstances, lowering distortion from –80dB to –100dB (relative to the signal level) can deliver audible gains. And this ain’t some artsy-poetic intuitive tube-amp manufacturer; it’s Purifi, home of serious engineering and class-D modules that solve that technology’s problems by flooding them with feedback.
• Speaker drivers that sound (and measure) fine with single tones—like, say, a frequency sweep—can run into serious trouble with more complex signals. Like, say, music. Intermodulation, via various mechanisms, is where things get interesting (footnote 2). What happens in the bass can have profound effects on the midrange. But these problems can be solved, and some of them already have been.
Purifi’s people are serious engineers, but don’t make the mistake of thinking they mindlessly ally themselves with the simple-minded objectivists who populate certain online discussion forums. They understand that science and engineering must be carefully deployed. Members of the Purifi team seemed to me as impatient with measurements-happy reductionists as with the radical antimeasurements crowd. Sure, it’s depressing when a forum post asks, “Is it possible for a component that measures well to sound good?” But owning an Audio Precision analyzer doesn’t make you an expert, and the unguided application of analytical tools can lead you far astray.
Knowing what to measure is critical, and getting to that point is hard work. As is written in a Purifi blog post—it’s unsigned, but the voice and some biographical details sound like Putzeys’s—”We don’t so much hear distortion levels as distortion mechanisms. You need to understand the mechanism before you can design a test that will quantify it sensibly.”
Footnote 1: If you’re interested, check out patents.google.com/patent/WO2020208070A1.
Footnote 2: See, for example, this Purifi blog entry. If you’re a tech-minded audiophile, I encourage you to monitor the Purifi blog, at purifi-audio.com/blog/. New posts aren’t frequent, but they’re meaty.
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