EIGENTAKT: IS IT THE ULTIMATE CLASS-D AMPLIFIER TECHNOLOGY?
IS IT THE ULTIMATE CLASS-D AMPLIFIER TOPOLOGY?
Is Eigentakt, developed by the famous Bruno Putzeys at Purifi, a version of which is used by NAD in its brilliant M33 amplifier, the ultimate Class-D amplifier topology?
There’s a new amplification technology in town, which purports to take Class-D concepts to a new level, with negligible distortion, extraordinarily low noise, load-invariant response, exceptionally clean clipping, low losses and high efficiency, while delivering 400-watts of power without even getting hot. It comes from Purifi, in Denmark, which has named the technology ‘Eigentakt’, which is German for ‘self-clocking’.
We’d exercise our usual caution with such remarkable claims, but what makes us more inclined towards credence in this case is NAD’s rush to adopt it, and also the team that’s behind it. The three listed co-founders of Purifi Audio are Bruno Putzeys, Lars Risbo and Peter
Lyngdorf — a pretty illustrious trio.
MEET THE TEAM
Bruno Putzeys is a Belgian engineer who spent a decade at Philips working on switching amplifier technologies before striking out on his own with Hypex, also as a collaborator in Grimm and later as CTO with Kii Audio. NAD’s adoption of Hypex’s Universal Class-D amplification and Ncore revolutionised its amplifier range a decade ago, and for the last few years Ncore has been the go-to audiophile Class-D topology for many audiophile brands. Putzeys seems an entertaining individual beyond his audio prowess: his personal website is well worth a visit, where he describes his professional activities as “including most things audio, analogue hardware in particular.”
“My first hobby is trying to be relaxed about being not at all good at my second hobby, which is trying to be zen about failing miserably at the first.” Other posts include his formulation of a ‘Quantum Theory of Female Vestimentary Preparedness’, which states that “Teenage women preparing to go out will remain in an indeterminate state of readiness until observed, at which point the wave function instantly collapses into an immaculately dressed and quite stunning apparition.”
Lars Risbo, meanwhile, first established his musical credentials as principal cellist of the Copenhagen Youth Symphonic Orchestra, going on to develop ‘equibit’ technology in his Danish company Toccata Technology, as used in the highly regarded TacT Millennium amp way back in 1999, this early ‘digital amp’ circuit gaining rare audiophile praise. He was also early in more carefully defining the technology to overcome the general disdain for ‘digital’ amplification, really being a powered DAC in which the signal remains digital all the way through to the point it exits to the speaker terminals, achieved by switching from a PCM bitstream to pulsewidth modulation (PWM), although he notes that “the PCM-to-PWM process is the easy part — the power stage was the hard part”, where the PWM is integrated at the output by switching a steady voltage.
Toccata was subsequently purchased by Texas Instruments in 2000, where Risbo started a project called ‘SmartAmp’ and went on to be elected a TI Fellow in 2012 and Audio CTO in 2013, now holding more than 30 patent ‘families’. This connection has seen several other TI personnel come to work with Purifi. One of these is the company director Claus Neesgaard, who developed several core innovations in Class-D amplification for Toccata and went on to head TI’s Audio
DSP product line, leading the transition into streaming-based system solutions.
Also involved in the TacT amplifier was hi-fi luminary Peter Lyngdorf — also now a co-founder of Purifi. Lyngdorf brings his connections to the likes of DALI, Steinway Lyngdorf and the Hi Fi Klubben retail network, as well as a close relationship with NAD dating back almost to its earliest days.
Put these Purifi co-founders together and you have a team able to develop switching amplification at its highest level, to bring it to market, and to enjoy themselves in the process. The resulting Eigentakt module is small given its stated output of 400-watts, although it is notable that NAD, which adopted a version of the technology for its Masters series, the first model of which to use it was the M33, rates that model with a power output of 200-watts continuous, perhaps because the distortion characteristics of the module rise dramatically above 150-watts into eight ohms, reaching 1% THD+N, whereas below that the figures are astonishingly low: 0.00017% at 100-watts into eight ohms across the full audio spectrum, and output noise of just ~11.5 V A-weighted. Putzeys says the module has the frequency and phase response of a second-order Butterworth filter cornering at 60kHz, so very nearly ‘linear phase’ in the audio band. The quoted dynamic range for the module is 131dB, and its efficiency greater than 94%. Chunky heatsinks are not be required.
Another reason for the power discrepancy might well be that NAD says that it and Purify co-operated to adapt the Eigentakt circuitry it uses in NAD products to meet NAD’s own signature design requirements, so although Eigentakt circuitry is at the core of the design, the circuitry in NAD amplifiers is different to modules used by other manufacturers using Eigentakt circuitry, as well as to the modules Purify is selling as kits (about which more later).
THE SECRET OF EIGENTAKT
Eigentakt’s secret, says Purifi, is the application of nonlinear control theory, with a “mathematically exact” optimisation of the feedback circuit that improves performance by at least an order of magnitude over existing implementations. The result is a large-signal self-oscillating amplifier which is extremely stable, has an output impedance below 65 at 1kHz(!), and makes the impedance curve of the speaker irrelevant (though it is, of course, subject, to a minimum impedance requirement). There’s also an immunity to noise from even simple
Eigentakt has raised the bar both technically and subjectively over Bruno Putzeys’ previous Hypex Ncore Class-D designs. If there were any slight deficiencies in the sound of older Hypex Ncore designs they are now banished
switched-mode power supplies, as well as a comprehensive protection system which makes it robust in operation and particularly easy to integrate into complete amplifier designs.
There’s one particular development highlighted by Putzeys and Risbo (who admit to their ‘bro-mance’ being the spark which brought the new company into being), and that is best explained in a Q&A with the pair published by audiophilestyle.com, where Putzeys explains: “The only real surprise we had recently was to do with the output choke. Magnetic materials have something called hysteresis, but there is precious little information about what this really does. If you test a magnetic core with a sine wave, the distortion looks a little like soft clipping, perfectly benign. But what came out of tests on iron parts in loudspeakers was that hysteresis has a long-term memory, so you can get intermodulation between things that happen now and things that happened 10 minutes ago. With music, this distortion sounds like half correlated noise.”
“Crackling,” interjects Risbo. “You hear when each magnetic domain flips.”
“When you put the coil inside the amplifier’s feedback loop, that distortion gets reduced along with the distortion of the power stage and everything else,” continues Putzeys. “We have a strong suspicion here that the most audible distortion in typical Class-D amplifiers may very well be that.”
In the Eigentakt circuit, he says, the extreme amount of loop gain (about 75dB all the way to 20kHz, 20dB better than Putzeys’ previous designs) reduces the sonic footprint of the output choke.
While NAD claims a first in the use of the Eigentakt module, it has appeared elsewhere, initially in a prototype Lyngdorf 8-channel amplifier, and more recently in amplifiers by Nord Acoustics as well as amplifiers sold by Australia’s own March Audio, such as its P451 monobloc amplifier (which retails for $1,295). March Audio also uses Hypex Ncore modules in its P701 amplifier, but founder Alan March is clear about his preference.
“The 1ET400A [Eigentakt] modules from Purifi have raised the bar both technically and subjectively over Bruno Putzeys’ previous Hypex Ncore designs”, he says. “If there were any slight deficiencies in the sound of older Hypex Ncore designs I think they are now banished. They have a sweet extended high frequency range with great power and definition and tightness in the bass. They are just neutral — but don’t think for one minute that means clinical or unengaging. They just let the music through. Tremendous power output with low power consumption and cool running temperatures in a very compact format. What more do you need?”
EIGENTAKT DIY?
Purifi also sells its Eigentakt modules (as well as other of its Class-D modules) to the public, via its website and other suppliers, for those indivduals who prefer to build their own amplifiers (DIY). Its EVAL1 kit combines two 1ET400A amplifier modules and a stereo front-end board which are said to be “highly suited for DIY projects”, for 4,700 Danish krone, which at current exchange rates, is around A$1,000. Because such modules are available direct to the general public, we asked March whether there were any disadvantages to building one of these kits.
“Well it’s having the engineering knowledge behind it,” he says immediately.
“Some of the OEM manufacturers do see it as a problem, but do-it-yourselfers are a different market. DIYers are going to DIY — I used to DIY. They’re going to do what they’re going to do, so they’re not a customer you’re going to lose. “And if you look at the total cost required to build an amplifier yourself, it’s not economically worth it. We’re charging not much above DIY prices, and for that you get a nice enclosure milled from a solid block of anodised aluminium that looks great, is professionally put together, and is fully tested and guaranteed for three years.
Eigentakt has a sweet extended high-frequency range with great power and definition and tightness in the bass