Bruno Putzeys Συνέντευξη στο Sound and Vision

[Spyros Koutsogiannis]

https://www.soundandvision.com/content/bruno-putzeys-head-class-d

image: https://www.soundandvision.com/images/styles/600_wide/public/051617_bruno_putzeys.jpg?itok=7xec4LhD

You may not have heard of Bruno Putzeys but if you’re an audiophile and have purchased a high-performance power amplifier in the recent past, you might know his work. Putzey’s ground-breaking NCore Class D amplifier module, created under the aegis of Netherlands-based Hypex Electronics, is used in amplifiers from ATI, Marantz, Jeff Rowland, and Bel Canto, to name a few. That Morris Kessler, founder of ATI and long-time champion of Class AB amplification, chose NCore for his new AT527NC and AT524NC amplifiers, is telling. Both models received Sound & Vision’s Top Pick designation, earning five stars in the Performance category, suggesting designs that are a far cry from Class D devices of just a few years ago. We tracked down Putzeys, now CTO at Kii Audio, to learn more about the new Class D and the apparent revolution he has started.

S&V: Your diyAudio profile lists your interests as audio and philosophy. An interesting combination. Why audio?
Bruno Putzeys: It’s congenital I’m afraid. My maternal grandfather and great-grandfather ran a shop manufacturing radios in the 1930s. My grandfather reportedly claimed in the 1950s that the day would come when a powerful amplifier would fit in a matchbox. He died early and I never met him but I was infected by this prophecy well before I seriously considered making a career in electronics. My dad started building amps and speakers at age 12 but never thought you could make a living doing so. As a teen I was, like any geek of my generation, heavily into computers but after a particularly complex software project I decided that I didn’t want to spend my life tied to a keyboard and screen. So I wormed my way out of the attic and went to see what my father was up to, which was a shootout between a solid-state amp and a 10-watt valve [tube] affair. The valves won, and the sheer unruliness of that outcome is what got me interested.

S&V: Walk us through your professional audio history. Which accomplishments are you most proud of?
BP: My first job was in the Philips lab in Leuven (Belgium) where I’d done my thesis work on Class D. Far from being allowed to continue along those lines I was told to tweak the umpteenth round of cost optimization for one of those ugly black micro stereos everyone wanted to make in the ’90s and that all used Sanyo Class B hybrids. It was soul-crushing work and I took every opportunity to talk to management about Class D until finally someone gave me the chance to do a Class D design for TV’s. Apparently the Philips TV department had gotten nowhere with a Class D chip the research lab in Eindhoven had been working on for years. So I got a month to do a discrete circuit that could do what that chip couldn’t. Thirty days later I presented them with a small 25-watt self-oscillating amp that could be made for $5 or so. I’m still wondering if the intention hadn’t been to hand that young whippersnapper a loaded gun to shoot himself with because, looking back, one month was perhaps a bit short for a project of this nature. But it bought me a bit more time to do a 100-watt version and so on—each time a few months more to try out something new.



image: https://www.soundandvision.com/images/051617_marantz_PM10.jpg

The Marantz PM-10 integrated amplifier is based on Putzey’s Ncore module: “I had been trying to get my amps into a Marantz product ever since my Philips days so it was quite a moment when they decided to go with Ncore.”
Grudging admiration from the management slowly turned to enthusiasm and I got a right-hand guy for engineering work and another to try and turn into money whatever it was that the two of us came up with. By 2000 I enjoyed an enviable degree of protection within the lab, both concerning projects I wanted to do and against the consequences of my rather forthright attitude regarding, well, pretty much anything really.

Inventing that ”UcD” circuit was, of course, one of my highlights at Philips, but two others are worth mentioning. You have to remember that around the turn of the millennium there was still confusion about the fourth letter of the alphabet, to wit “D,” and whether that was supposed to mean “digital.” Of course, Class D was only called that because the amplifier class invented before it was called Class C, but confusing semantics and engineering is another way of spelling “hi-fi.”

I was just as confused and as a result concocted a power DAC that directly converted DSD to analog in the power domain. The thought process that made me embark on this folly is a prime example of how not to start an engineering task but the result is still the most efficient (97 percent) and lowest distortion (0.007 percent) zero-feedback power amp ever made. Sonically, it was immediately killed by the first UcD prototype, which got me thinking. I decided that an aesthetic precept like “digital all the way” or “no feedback” can’t hold a candle to the simple question, “What problem are we trying to solve here?”

Thus chastised, I quickly grew exasperated at the continuing cottage industry, both in business and in academia, of spending years looking for the ideal algorithm to turn a pulse-code modulation (PCM) signal into pulse-width modulation (PWM) while the power stage was left as an afterthought. Ostensibly they all believed that that was the trivial part while the reverse was blindingly obvious to anyone who’d actually tried building one. One day I thought it might be amusing to throw a stick into the hen coop as we say here and see if I could come up with a decent digital PWM algo of my own and publish it. In evidence that creativity is at its highest when there’s no pressure, two hours was all it took to find one that was simple to implement and had no distortion at all, for every reasonable definition of “none at all.” It was intended as a stunt but the core idea turned out to have very serious applications wherever PWM was used. It is still generating new insights into the fundamental nature of PWM. That was completely unexpected. I wouldn’t be surprised if in the very long run that little invention gets to be my real contribution to the art.

Still, if I had to choose a personal highlight, it must be my “Krikkit One” moment—that little 25-watt amp that got my Class D career moving.



image: https://www.soundandvision.com/images/051617_ncore_formula2.jpg

 

[Spyros Koutsogiannis]

In 2004 I moved to Hypex, which was mostly a period of consolidation, refinement, and branching out. I left Philips out of ire with their inability to turn great ideas into a commercial success. By and large Hypex did realize that success and gave me the gratification of seeing a good idea recognized by a wide audience. Describing Ncore as a “refinement” might sound irreverent but the crucial insight was to find a concise mathematical expression for the stable operating point (the “limit cycle”) of a self-oscillating amplifier. Ncore merely followed when I started applying this as a tool to optimize a UcD style circuit and increase loop gain whilst keeping the thing stable. The improvement in sound quality was impressive. Sometimes an evolution can be so great that it’s worth being called a revolution.

I also got to branch out into DSP for loudspeakers and to start a new high-end brand called Mola-Mola where I could design electronics for end users and do it exactly the way I thought was right. In particular the DAC I did for this project was quite something. It ties together a lot of loose ends from my past, ranging from sample rate conversion to discrete op amps and the PWM algorithm I mentioned earlier.

S&V: Generally speaking, what are the key benefits of Class D versus the traditional Class AB and Class A designs that have long been favored by audiophiles?
BP: Efficiency and therefore the ability to construct amps that are powerful for their size. Only that. Modern Class D amps, in particular mine—ahem—sound good not because they’re Class D, but in spite of it. I can’t repeat that often enough. Left to its own devices, a switching power stage tries to do just about anything except amplify audio. You choose Class D to save energy but it’s all elbow grease after that. People don’t realize how much more challenging Class D is compared to Class AB. It’s truly an order of magnitude.

S&V: Class D amplifiers have been around for years but are most common in powered subwoofers and non-critical listening applications. What is it about older designs that made them unsuitable, or less suitable, for use in full-range audio applications? Put another way, what prevented Class D from performing at the sonic level of a high-quality Class AB design?
BP: They weren’t good enough. Distortion was high and output impedance was all over the map, especially near the end of the audio band. At the core of the problem is fear of math. Even serious papers about power electronics for things like electric cars tend to resort to well established rules of thumb like “phase margin” and “gain bandwidth” rather than taking things from the ground up—even when the subject matter clearly requires it. So that meant little help was coming from the industrial power electronics community where you’d expect it to be most readily available.
Class D is also thoroughly unforgiving. Every detail has to be right and there simply isn’t room to indulge scientifically unproven flights of fancy. Established audiophile habits become a blocking item. You can always construct a passable Class A amplifier that tiptoes around any conceivable set of personal taboos. Not so in Class D. If you try to replace the tiny output capacitor with a reassuringly big paper-in-oil equivalent, you end up with a nonworking amplifier. If you believe in star grounding, fireworks ensue. A high-performance Class D amplifier contradicts every single item of audiophile superstition. Designing one is the ultimate test to see if you’ve got your head screwed on right.

This morass of folk lore has slowed down real progress in high-end audio for over a generation now. When it comes to Class D, it bars access completely. You have to love great sound and great music but you have to be equally fanatic about facts. Reality makes logical sense. Anything that doesn’t make logical sense isn’t real.

S&V: Thanks to your work, Class D is coming into its own as an audiophile-caliber amplifier option. What changed?
BP: Mostly the modulator and feedback loop design strategy—the math bit. I could build an Ncore amp nearly as good as today’s using only parts that were available in the 80’s. But it would be completely impossible with only the knowledge available then. What was missing until recently was a solid understanding of how to make high gain control loops in a limited bandwidth. It’s not too difficult to build a linear amplifier with a bandwidth of 10 MHz. A classical first-order design around one of those would give you 50-something dB in the audio range. It’s no surprise then that designers of linear amplifiers like a wide bandwidth.

A Class D amplifier, by contrast, maxes out at a few hundred kilohertz. Traditionally trained amplifier designers will say that even 20 dB would be hard to get under those circumstances. Until the mid-1990s Class D designers said exactly the same thing. A few knew about second-order control loops but that’s where it mostly ended. They certainly saw the output filter as an insuperable barrier (John Ulrich being the notable exception). It limits the bandwidth down to a few tens of kilohertz so folks felt that taking feedback after the output filter was out of the question.

 

[Spyros Koutsogiannis]

A high-performance Class D amplifier contradicts every single item of audiophile superstition. Designing one is the ultimate test to see if you’ve got your head screwed on right.
We now know that you can build so-called high-order loops to get a lot more loop gain within a given bandwidth. That was the idea that made sigma-delta AD/DA converters possible. That only really got underway in the 1990s but before that could be adapted to Class D we had to wait for the realization that the output filter could actually be used to leverage this. If you put a third-order feedback loop around an amplifier that includes a second-order output filter, you can make use of that to get a fifth-order loop. All you need to find out is how to keep it stable. That’s been the major contribution of UcD and of NCore: the output filter actually produces about 20 dB of loop gain for free! That goes completely against intuitively held traditions. It often helps not to watch the competition too closely or you end up repeating their mistakes.
S&V: Conventional wisdom says a great amplifier has no sonic character of its own and, beyond boosting the signal level, is transparent in the audio chain. But does the nature of high-quality Class D amplification—perhaps through the absence of distortions found in other circuit topologies—produce a particular sonic signature or specific attributes you could describe?

BP: Well, if the amplifier is truly great that’s absolutely right. Sonic signatures are what you get when you approach the same ideal from different angles. There are a few distortion mechanisms conspicuously missing in Class D, mostly those related to the input stage of a Class A(B) solid-state amplifier and nonlinear capacitances. Those are also missing in valve [tube] amplifiers so it’s quite common for people to notice that a Class D amplifier is somehow reminiscent of valve amplification in terms of “sweetness” for want of a better word.

I’ve heard several reports of valve aficionados ditching their glassware and switching to Ncore. All I can conclude from that is that those people clearly weren’t actively seeking the distortion of valves as many believe, but instead had a legitimate beef with certain sonic aspects common to most solid-state designs. That’s one thing I have to explain again and again to my fellow doubters: when audiophiles report a particular listening experience, that experience is real. Trust that. Just don’t trust the explanation they proffer.



image: https://www.soundandvision.com/images/317atiamps.2.jpg

ATI’s AT527NC amplifier is based on Putzey’s Ncore module.
S&V: The new generation of Class D amps are obviously good enough to herald audiophile kudos, as we saw most recently with David Vaughn’s review of ATI’s AT527NC and AT524NC amplifiers, but what lingering issues might still need to be resolved to move performance to an even higher level?
BP: I’d say that basically the cat has been skinned. Further refinements will surely happen but the same can be said of the other amplifier classes. In terms of things affecting sound, I don’t see any fundamental outstanding issues that merit much attention—it’s mostly down to lesser implementation details, ordinary technological progress, and perhaps some more adjustments in the math department. I would like to see magnetic materials with less hysteresis though.

S&V: If we assume Class D has achieved full audiophile performance, what opportunities does this open up for future products in categories apart from home audio? Say, portable audio, for instance?

BP: Live sound and studio monitors are fast switching to Class D out of necessity so the easy availability of audiophile-grade prefabbed modules is certainly going to make the world a nicer sounding place. But don’t get your hopes up too high for portable audio. The biggest limiting factor there is distortion in those small loudspeaker drive units. I mean, driver distortion is already a bigger bottleneck than amplifiers in high-end audio (we’re just so used to these types of distortion that we fail to notice them). In portable systems it’s so bad that nothing else matters. In a little Bluetooth speaker, all effort not spent improving driver distortion is wasted outright. By that I mean either using active correction or by making the drivers better in the first place.

S&V: We can’t leave without talking about your work in loudspeakers. Your bio on the Kii Audio site says you have turned your “full attention to loudspeakers with the intention to revolutionize them in the same way as [you] did amplifiers.” Tell us about your work in this area?

BP: Well I guess I’ve already thrown down the gauntlet in my previous answer. Our industry’s fixation on separate components is holding us up. The ultimate goal is sound, acoustical output. If you split the signal chain into separates and you require those to be exchangeable, you’re adding technical requirements to the interfaces between those boxes, just to standardize them. Case in point, why are we making amps with super high damping factors? Because a separate power amp means a passive crossover filter and if you don’t define the source impedance you get unpredictable results. But if you take the system level view and ask, “what problem are we trying to solve,” you can improve the distortion performance of a speaker driver enormously by tailoring the output impedance of the amp that’s driving it. That option isn’t available in a separates world. A system level approach allows you to get clearly better performance at a lower price, or something that is completely unattainable using classical means if you spend a bit more.



image: https://www.soundandvision.com/images/051617_kiiaudio_three.jpg

Kii Audio’s Three speaker is an active design.
We suffer from what I call “loss of perspective.” Compared to amplifiers, speakers distort tremendously. But if you keep the speaker and the room the same you can still do an electronics shootout and clearly hear a difference. So you may be tempted to think: Oh, I really need a new preamp. We fool ourselves into thinking that our sonic horizon is expanded immeasurably because we can just change the preamp for a better one. It isn’t. Separates allow us to tinker at the boundaries of that horizon but only after shrinking it to something the size of our backyard.

Anyhow, that’s why I decided to leave amplifiers for now and do speakers instead. The first aim of Kii is to reduce the impact of the room acoustics. The correct list of items in the playback chain in the order of importance is: Room acoustics, speaker acoustics, drive units, then a long void followed by electronics as a remote third. The Kii design tries to make the speaker and the room switch position by reducing the amount of sound that excites room modes without first going to the listener. To do that at bass frequencies is quite a novelty and that’s where rooms need most help. You can’t build a passive speaker that does that so it’s only natural to make it active. The other benefits of combining electronics and speaker in one product follow naturally.

You didn’t ask why I like philosophy but I guess I made that point.


Read more at https://www.soundandvision.com/content/bruno-putzeys-head-class-d-page-2#1RptW6W25pARpbeV.99

 

[Spyros Koutsogiannis]

Μία σύνοψη των πλέον ενδιαφερόντων σημείων καθ' εμέ:
1) The correct list of items in the playback chain in the order of importance is: Room acoustics, speaker acoustics, drive units, then a long void followed by electronics as a remote third.


2) Our industry’s fixation on separate components is holding us up. The ultimate goal is sound, acoustical output. If you split the signal chain into separates and you require those to be exchangeable, you’re adding technical requirements to the interfaces between those boxes, just to standardize them. Case in point, why are we making amps with super high damping factors? Because a separate power amp means a passive crossover filter and if you don’t define the source impedance you get unpredictable results. But if you take the system level view and ask, “what problem are we trying to solve,” you can improve the distortion performance of a speaker driver enormously by tailoring the output impedance of the amp that’s driving it. That option isn’t available in a separates world. A system level approach allows you to get clearly better performance at a lower price, or something that is completely unattainable using classical means if you spend a bit more.

3) I mean, driver distortion is already a bigger bottleneck than amplifiers in high-end audio (we’re just so used to these types of distortion that we fail to notice them).

4)
S&V: Conventional wisdom says a great amplifier has no sonic character of its own and, beyond boosting the signal level, is transparent in the audio chain. But does the nature of high-quality Class D amplification—perhaps through the absence of distortions found in other circuit topologies—produce a particular sonic signature or specific attributes you could describe?

BP: Well, if the amplifier is truly great that’s absolutely right. Sonic signatures are what you get when you approach the same ideal from different angles. There are a few distortion mechanisms conspicuously missing in Class D, mostly those related to the input stage of a Class A(B) solid-state amplifier and nonlinear capacitances. Those are also missing in valve [tube] amplifiers so it’s quite common for people to notice that a Class D amplifier is somehow reminiscent of valve amplification in terms of “sweetness” for want of a better word.

I’ve heard several reports of valve aficionados ditching their glassware and switching to Ncore. All I can conclude from that is that those people clearly weren’t actively seeking the distortion of valves as many believe, but instead had a legitimate beef with certain sonic aspects common to most solid-state designs. That’s one thing I have to explain again and again to my fellow doubters: when audiophiles report a particular listening experience, that experience is real. Trust that. Just don’t trust the explanation they proffer.

 

[kostas Papas]

Το ποσο παλια είναι η φωτογραφια του, δεν λεγεται !

 

[minas]

Μία σύνοψη των πλέον ενδιαφερόντων σημείων καθ' εμέ:
1) The correct list of items in the playback chain in the order of importance is: Room acoustics, speaker acoustics, drive units, then a long void followed by electronics as a remote third.


2) Our industry’s fixation on separate components is holding us up. The ultimate goal is sound, acoustical output. If you split the signal chain into separates and you require those to be exchangeable, you’re adding technical requirements to the interfaces between those boxes, just to standardize them. Case in point, why are we making amps with super high damping factors? Because a separate power amp means a passive crossover filter and if you don’t define the source impedance you get unpredictable results. But if you take the system level view and ask, “what problem are we trying to solve,” you can improve the distortion performance of a speaker driver enormously by tailoring the output impedance of the amp that’s driving it. That option isn’t available in a separates world. A system level approach allows you to get clearly better performance at a lower price, or something that is completely unattainable using classical means if you spend a bit more.

3) I mean, driver distortion is already a bigger bottleneck than amplifiers in high-end audio (we’re just so used to these types of distortion that we fail to notice them).

4)
S&V: Conventional wisdom says a great amplifier has no sonic character of its own and, beyond boosting the signal level, is transparent in the audio chain. But does the nature of high-quality Class D amplification—perhaps through the absence of distortions found in other circuit topologies—produce a particular sonic signature or specific attributes you could describe?

BP: Well, if the amplifier is truly great that’s absolutely right. Sonic signatures are what you get when you approach the same ideal from different angles. There are a few distortion mechanisms conspicuously missing in Class D, mostly those related to the input stage of a Class A(B) solid-state amplifier and nonlinear capacitances. Those are also missing in valve [tube] amplifiers so it’s quite common for people to notice that a Class D amplifier is somehow reminiscent of valve amplification in terms of “sweetness” for want of a better word.

I’ve heard several reports of valve aficionados ditching their glassware and switching to Ncore. All I can conclude from that is that those people clearly weren’t actively seeking the distortion of valves as many believe, but instead had a legitimate beef with certain sonic aspects common to most solid-state designs. That’s one thing I have to explain again and again to my fellow doubters: when audiophiles report a particular listening experience, that experience is real. Trust that. Just don’t trust the explanation they proffer.

Τα ίδια ετοιμαζόμουν να παραθέσω (με λίγο διαφορετική σειρά)

 

[costas EAR]

Εγώ αντιθέτως, μόνο αυτό θα ήθελα να παραθέσω:

That’s one thing I have to explain again and again to my fellow doubters: when audiophiles report a particular listening experience, that experience is real. Trust that. Just don’t trust the explanation they proffer.

 

[costas EAR]

Άντε, κι αυτά τα δύο:

confusing semantics and engineering is another way of spelling “hi-fi.”

You have to love great sound and great music but you have to be equally fanatic about facts. Reality makes logical sense. Anything that doesn’t make logical sense isn’t real.

Κοινή λογική. Σπάνιο είδος.

 

[vasilisnt]