Solid State Power Amp Reviews

Buckeye Purifi Eigentakt 1ET9040BA monoblock power amplifier

Kalman Rubinson  |  Dec 19, 2024

Back in 2016, I documented the rise of class-D amps using the early TriPath technology. Used in the Bel Canto eVo 200.2, TriPath cracked open the door to the High End but was never admitted due to a dim and opaque treble. The second wave was based on B&O's ICEpower technology, again via a Bel Canto amplifier, the Ref1000M monoblock. ICEpower had more credibility and was accepted by many but, often, only with, ahem, due consideration for size and efficiency. There was an explosion of new class-D amps in 2016 when Stereophile featured glowing reviews of Bel Canto's e.One Ref600M monoblock, Theta's Prometheus monoblock, and NAD's Masters Series M22 stereo amp, all based on Bruno Putzeys's Hypex NCore technology modules. Finally, it seemed that class-D was "in the room," though, even to this day, there remain critics and quibblers who continue to deny them as true high-fidelity products.

Well, time does not stand still and neither did Bruno Putzeys. He, along with Lars Risbo and Peter Lyngdorf, had founded Purifi in 2015, and in 2019 he unveiled the Purifi Eigentakt power amplifier modules. Eigentakt (meaning "self-clocking") is the result of years of research by Purifi into the self-oscillation behavior in class-D amplifiers with algorithms that, when applied in control loops, "improve existing designs by an order of magnitude or more. ... The Eigentakt circuit is expected to outperform any audio amplifier known, regardless of technology or class, with THD and IMD quoted as below 0.00017% at all frequencies and power levels, and the frequency response in the audioband remains within ±0.01 dB under all load conditions."

The 1ET9040BA is currently the most powerful device that Purifi has in its range of Eigentakt modules tailored to different applications. That's great but that's not really why I was attracted to this amplifier. The clue is the word module. I was a committed DIY-er for most of my life, working from scratch or, if I was lucky, working with prebuilt modules where the critical elements were assembled with carefully selected wiring, connectors, and enclosures. All the class-D amplifiers mentioned above are based on factory-built, factory-tested modules supplied as OEM parts to the audio companies (or, in some cases, built by the companies under strict licensing agreements).

This creates opportunities for smaller companies to build amplifiers based on these modules as long as the minimum OEM purchase is reasonable. In this case, the OEM purchase minimum is only 25 pieces, although I am certain better pricing accompanies larger quantities (footnote 1). Of course, there's more to a functioning audio amplifier than the module. First, since the Eigentalkt modules have a relatively low gain (14.4dB for the 1ET9040BA), an input stage is needed to raise it to a more useful level and, in this case, to offer gain options. Second, one must add a power supply to power the amp module and the input stage. Third, of course, is everything else: a chassis; input connectors; output connectors; power switch and indicator; options for signal-sensing; 12V trigger; gain selection; possible heatsinking; internal wiring; and, literally, nuts and bolts. There is nothing daunting to those with relevant expertise and, over the past decades, this has resulted in a new cottage industry that builds amps from these modules or supplies kits for home assembly.

Buckeye Amps is a relative newcomer. After more than a decade of buying and selling to upgrade his system, founder Dylan Launder built a set of Hypex NCore MP amps and never looked back. He was so impressed with the sound quality that he became a North American OEM Hypex builder and sold his first Buckeye Amp in October of 2020. Customer interest, based solely on word of mouth and positive technical reports, has grown his business, and he has gained over 4000 customers in the past two years. His line now includes amplifiers featuring the newest Hypex NCx and Purifi Eigentakt modules, of which the subject of this review is, to date, the most powerful.

Where's the beef?
The 1ET9040BA monoblocks cost $2300/pair, including free shipping in the US. The two amplifiers arrived in a single cardboard carton that weighed less than 15lb! For a pair of power amplifiers, this was surprising, even puzzling. Inside were two smaller plain boxes, each containing an amp and a power cord, and a single-page Purifi Quick Start Guide that also serves for other Buckeye amps. Each amplifier was in a nicely finished black chassis, on the front of which was a single LED to indicate On/Active (blue), clipping (orange), or fault (red), and a painted Buckeye logo. Around back left is sturdy pair of gold-plated, multiway speaker terminals, a balanced (XLR) analog input, and a gain selection switch (25.5dB, 20.5dB, 15.5dB). On the right, there's a 15A IEC AC socket and a power rocker switch. Between these elements, there is a 12V trigger signal socket and a three-position slide switch for power modes (AutoSense On, AutoSense Off, 12V trigger).

I found this feature set entirely logical and suitable, though I did think that the input XLR and the speaker terminals were too close to each other. I was also surprised that Buckeye used a latchless XLR jack for the input over the security of a latched connection but, considering how close the XLR is to the speaker terminals, there really isn't enough room for a latch. I therefore recommend plugging in the input cable before the speaker cable.

Inside the chassis all is neat and clean. Towards the front is a Hypex SMPS1200A180 1200VA, high-efficiency, switch-mode power supply which provides 2×46VDC (footnote 2) for the 1ET9040BA module and also powers the input board. The latter is a proprietary Buckeye design mounted to the rear panel. Wiring among the elements is by plug-in cables, making for easy replacement:

• AC input to power supply.

• Power supply to input board and 1ET9040BA module.

• Input board to 1ET9040BA module.

• 1ET9040BA module to speaker outputs.

Installation was as easy as can be imagined (since you can hold the amp in one hand) as I just transferred the XLR input cables and speaker cables from my bridged Benchmark AHB2 amps to the 1ET9040BA monoblocks. The Buckeyes were set to medium gain (20.5dB) and, after I flipped the power switches, internal relays clicked once and the LEDs on the front panels glowed blue. In operation, there was no mechanical noise nor any perceptible noise whatsoever, even when I applied my ear to each of the drivers in each of the speakers. I do not recall what track I played first because my intent was not to listen but simply to find out if everything was connected, operating, and quiet. It was.

Footnote 1: The price of a sample 1ET9040BA module from Purifi is $660 or, with a very simple mono front-end board, about $760 plus relevant taxes, fees, and shipping.

Footnote 2: This power supply limits the ultimate output of the amp to 1200W into 2 ohms. Buckeye offers an optional Micro Audio SMPS1K-SN power supply that offers Active Power Factor Correction, higher power output (a 0.67dB increase from 1200W to 1400W into 2 ohms), and standby control, which allows for under 1W power draw when the amp is not in use.

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COMPANY INFO
Buckeye Amps
4280 Weckerly Rd.
Monclova
OH 43542
(419) 351-5704
buckeyeamp.com
ARTICLE CONTENTS

COMMENTS
David Harper's picture
Submitted by David Harper on December 19, 2024 - 8:29am

"Still, it would have been easy to excuse the Buckeyes for any flaws in performance because of the low price, but I do not have to. I do admit that it will not offer the purchaser such ancillary rewards as exclusivity, a prestige name, impressive size and weight, an elegant sculpted chassis, or even a booklet extolling its goodness. In the absence of those features, its appealing price and outstanding sound do not characterize the Buckeye as "low price" but as "high value."

It sounds very much like you're saying that this low priced and lightweight amp is equal in performance to the multimegabuck audiophile high end gear that is typically reviewed in these pages. Call me crazy.

supamark's picture
Submitted by supamark on December 19, 2024 - 10:52am

You literally asked for it :D

supamark's picture
Submitted by supamark on December 19, 2024 - 10:50am

"The result? A full 75dB of feedback..." is why Eigentakt amp modules will never regarded as hi-fi. 75 freakin' dB of feedback. It also sounds terrible, very grey and lifeless due I suspect to the low pass filter (greyness) combined with that absolutely bonkers amount of feedback (lifelessness).

I keep hearing he's a great EE, so why can't he make a stable amp without gobsmacking amounts of feedback? He also needs to move the switching frequency into the GHz range to get the low pass filter a lot higher in order to address the phase issues of his amps.

Purifi make some really interesting woofers though. Very spendy, with a funky fresh surround.

Anton's picture
Submitted by Anton on December 19, 2024 - 9:48pm

Is Kal wrong in his listening experience?

Kal must have greyness and lifeless discernment disorder.

supamark's picture
Submitted by supamark on December 20, 2024 - 1:23pm

Kal does not have synesthesia (I do, along with misophonia), and does have different taste than I do. The way he hears, and what he values in his sound, is objectively different than how I hear and what I perfer.

That's the nice thing about Stereophile, there's no editorially imposed "correct sound." Herb and Kal have *very* different tastes for example. HR, KM, and MT are the tubes 'n transformers 'n turntables subjectivists, KR is the objectivist, the two JA's are kinda "standard" middle of the road audiophiles with STEM backgrounds, JVS is the ultra high end and esoteric tweeks person, and the other reviewers mostly cover more (relatively speaking) affordable gear and tend to be less "pigeon holed" into one catagory.

In other words, Stereophile tries to cover all the bases. JA1 (Mr. Austin) is probably closest to me in sonic taste, and coincidentally also has synesthesia. JA would be a close second. Musically, it's JA (for the 80's 4AD label stuff that he likes).

Contrast to SoundStage! where I used to work, they're pretty much objectivist in viewpoint (the owner/publisher Doug sets the tone) though they follow the subjective review + measurements template like Stereophile. I was sort of the pet subjectivist (relative to them) lol.

Oh, and one more thing - amplifier feedback and its' sonic desirability have been tested many times, and low/no feedback is always preferred to a bunch of loop negative feedback by a statistically significant margin. Some people are more sensitive to it than others, and there are of course some who prefer the feedback. The THX-AAA circuit uses both lots of feedback along with a feedforward distortion cancellation system and I want to kill it with fire.

mocenigo's picture
Submitted by mocenigo on December 21, 2024 - 8:36am

Feedback is per se not dangerous, nor detrimental to the sound.

It is not the amount of the feedback that can affect the latter negatively, rather a wrong use.

If feedback is used only to reduce overall distortion and provide stability, then it is only a partial, and limited application of the concept. Bruno Putzeys has shown that the so-called "moderate" amounts of feedback actually worsen the distortion profile. If the feedback loop is carefully designed (and, as in recent design, combined with feedforward/current dumping) it improves linearity, the SNR and kills distortion effectively, also at high powers, without reducing dynamics or "slowing" the sound (whatever this means), but giving you essentially a wire with gain.

If you find the Purifi "terrible, very grey and lifeless" then you are used to amplifiers that add something to the sound that was not in the original signal, additional distortion, a loudness effect (typical of low powered tube amps with very soft clipping), maybe low damping ratio (because you like the bass roilling in the room), and some equalisation depending on the loudspeaker's impedance.

Which is fine, but that does not mean that Putzey's amp is "grey" and "lifeless". You are like those that purchase TVs because they are hypersaturated colors in the shopping mall, thinking that this is quality and how stuff should look like. EXACTLTY like that.

You are also repeating some uninformed wrong criticisms of class D like: the switching frequency must be higher to get the low pass higher in order to address the phase issues of his amps. Which phase issues? Have you seen the actual values? And have you seen how linear is this in the frequency? Then there is no signal alteration whatsoever, and it is orders of magnitude lower than ANY loudspeaker.

DaveinSM's picture
Submitted by DaveinSM on December 22, 2024 - 9:17am

I’m sure Class D has come a long way, but I’m sure I’m not the only one who found them to sound harsh.
Whether that’s from the inherent switching, or feedback, or whatever. Or whether those artifacts were in the audible bandwidth or not, it often resulted in a sound profile that wasn’t pleasant.

It should be interesting to see if Class D can continue to improve to the point where the best designs are truly competitive with uncompromising Class A designs. I’ve been satisfied with my Class AB amp, and doubt I’d be able to reliably ABX it with a Class A amp, especially at lower volumes.

But I think that given the long history of power amp designs, people are rightfully skeptical that the highest high fidelity sound quality (whatever that means) already comes in Class D form.

I’m not one of the tube guys, but I have a feeling that you will never convince them that Class D sounds better than tubes.

Kal Rubinson's picture
Submitted by Kal Rubinson on December 22, 2024 - 10:06am
Quote:

But I think that given the long history of power amp designs, people are rightfully skeptical that the highest high fidelity sound quality (whatever that means) already comes in Class D form.

Skepticism is appropriate but it is not determinative. Testing is.

Quote:

I’m not one of the tube guys, but I have a feeling that you will never convince them that Class D sounds better than tubes.

I would not think of trying to convince them. They have to test their convictions and reach their own conclusions.

mocenigo's picture
Submitted by mocenigo on December 23, 2024 - 4:38am

@DaveinSM, class D can achieve the absolute top, just as any other class. If it is harsh to your earsm iut can depend on many factors, but it is not an assessment of quality, merely of subjective preferences.

The fact that you like "uncompromised class A" — which is often low powered — tells me you have an unconscious soft spot for soft clipping and "loudness" compression of transients, and indeed you write "I’ve been satisfied with my Class AB amp, and doubt I’d be able to reliably ABX it with a Class A amp, especially at lower volumes."

Nothing wrong with this. As long as you mention this is a personal preference.

But such amps are not "better". In fact, you are considering flawed technologies, that only work linearly if you have extremely sensitive speakers, as preferable. But they compress the transients and have a roll off at the treble, They both result in a "harsher" sound impression when yoiu are listening to something more transparent.

Also "given the long history of power amp designs, people are rightfully skeptical that the highest high fidelity sound quality (whatever that means) already comes in Class D form". This sounds like "given the long history of computing machines (no need to invoke Antikhitera, just consider at the Z1), people are rightfully skeptical that the highest performance alreadyt comes in semiconductor form" — and you would clearly realize the logical mistake here. There are metrics of fidelity, and both the best Class AB an the best Class D have reached the level that they would pass zero tests at -120Db. This means absolute fidelity, something NO class A design can do, unless you measure the very first milliwats — and this is the only area affordable Class A can achieve the top sound quality: You need to buy a 100W Class A amp that will constantly consume 600W to provide you with just the first few watts of absolute fidelity. But it can. In a horrible, environment-fiendly way.

Of course I (or the person you were replying to) would never be abkle to persuade those folks. They reject objectivity and progress, in a very narrow field (I am sure they would agree theis smartphone computes "better" than the Z1). But this does not make them right — same as a few billion of people believing in some god does not make that god real.

Fact is, you can always add distortion or some roll-off in the digital domain (or with an analog circuit) to a very faithful circuit, and you have only degraded the signal once, and get the sound signature you prefer. If you want to "fix" the sound of a distorting and equalising amplifier, in the other hands, you end up with two degradations of the signal. Same if you want to change or tweak the sound signature to another one of your liking. This proves that very linear circuits (well designed AB, such as Benchmark, or Class D like the recent designs out of Icepower, Hypex, and Purifi — DACs like the Topping D90 series, and so on) are objectively better. And can assist in a subjective tweaking with more precision and lesser degradation of other parameters.

Everything else is snake oil.

DaveinSM's picture
Submitted by DaveinSM on December 23, 2024 - 7:28am

I disagree with all this.

kai's picture
Submitted by kai on December 19, 2024 - 11:31pm

If you look at the figures at page 5 and 6 of Purifi‘s 1ET9040BA datasheet, it becomes clear that Buckeye‘s own input board spoils the distortion figures of the module.

https://purifi-audio.com/document/share/59/a82eb980-951a-4387-85ce-43caaa1e5c2e

The believable Purifi 1ET9040BA published charts don’t show such a drastic increase in harmonic and IM distortions as the Buckeye amp.

In fact the charts show a remarkable clean behavior.

Personally I think the board is completely obsolete,
14.4 dB of voltage gain is enough to have the sensitivity just right to match with most contemporary preamps and domestic situations.
The typical 30+ dB gain of power amps always had lead to the use of passive attenuation to get to a suitable control range at my preamp’s volume dials.

Only the low input impedance of 1.5 KOhm of a direct coupled module might tax some preamps, specifically tube driven ones, too much and justify an active input impedance converter.

yys310's picture
Submitted by yys310 on December 19, 2024 - 11:38pm

The IMD performance is underwhelming.
Also, what’s the problem with the input impedance? Why does it deviate so much?

georgehifi's picture
Submitted by georgehifi on December 20, 2024 - 12:02pm

If your source has volume control ability, low output impedance <100ohms, and higher than 2v output, many are these days, your far better off sonically, direct in with no buffer at all, as I do with my NC500 modules. The difference is not subtle.

Cheers George

pma's picture
Submitted by pma on December 20, 2024 - 2:47am

My take is that the high level of CCIF distortion measured by John is most probably the result of iron steel amplifier case, thus the filter coils in the module get core non-linearity. And this always reflects in the odd distortion twin-tone HF components.
My measurement of 1ET400A placed in Al box, at 40W/8ohm (true rms, not peak power) is of 35dB better than in the test here.

https://pmacura.cz/1ET400A_CIFF40W_8R.png

mocenigo's picture
Submitted by mocenigo on December 21, 2024 - 8:38am

I also suspect that something else may be at play, for instance, some defect in the buffer board, or in the batch of amps. The data sheet of the 1ET9040BA shows 40DB better IMD values, and Purifi is known for not cooking their datasheets.

pma's picture
Submitted by pma on December 21, 2024 - 9:58am

Purifi Audio have already acknowledged a problem with HF distortion in that specific module that was assembled by Buckeye and tested here in Stereophile. You may find their acknowledgement on Facebook, posted by Purifi Audio. They say they corrected the QC process and will replace the units that have had the described issues.

mocenigo's picture
Submitted by mocenigo on December 21, 2024 - 10:36am

Wonderful. I hope JA will repeat the measurements with fixed modules.

John Atkinson's picture
Submitted by John Atkinson on December 22, 2024 - 4:58am
mocenigo wrote:
I hope JA will repeat the measurements with fixed modules.

Buckeye is sending a pair of amplifiers with the corrected Purifi modules for me to measure and Kal to audition. Our follow-up coverage should be published in the April 2025 issue.

John Atkinson
Technical Editor, Stereophile

JRT's picture
Submitted by JRT on December 21, 2024 - 9:15am

pma commented, "My take is that the high level of CCIF distortion measured by John is most probably the result of iron steel amplifier case, thus the filter coils in the module get core non-linearity."

No. I recall Dylan Launder of Buckeye Amplifiers mentioning that the enclosure case material is aluminum alloy sheet (not steel). He also mentioned that those are sourced from Protocase, and are manufactured in Nova Scotia, Canada.

David Harper's picture
Submitted by David Harper on December 20, 2024 - 6:49am

Good review Cal. The high end subjectivists always say "judge with your ears not measurements". Which is exactly what you did. I guess that advice doesn't apply in the case of a low priced component like this one. Only in the case of vastly overpriced gear.

Anton's picture
Submitted by Anton on December 20, 2024 - 3:17pm

;-D

I trust him, but others don't 'see' it his way.

The best and worst thing about the hobby is a person can bloviate in any direction and claim special abilities.

I wonder what color 0.0001% distortion looks like vs. 0.00001%?

supamark's picture
Submitted by supamark on December 20, 2024 - 5:23pm

You don't have it but I do, so you really should shut up about something you will *never* understand or experience. You will never know what it's like to be encased in a golden honey-like radience as you listen to a good soprano in a good hall. You are nothing more than an ignorant bigot.

Bruce Swedien, who had synesthesia, said he knew his mix was tonally right when it was "golden" colored. I mean, he only won 5 Grammys for his engineering/mixing work so he was probably just bloviating right?

Oh, to answer your stupid question - for me distortion is a jagged texture more than a color. You made the assumption that it's only visual, and that it's the same for everyone. Only the ignorant make such assumptions.

I also didn't say he was wrong, I said he has different aural tastes. At least learn to read before spouting off your bigotry.

justmeagain's picture
Submitted by justmeagain on December 20, 2024 - 7:55pm

but there's no need for that kind of attack on someone just because they don't understand your particular sensory responses.

Anton's picture
Submitted by Anton on December 20, 2024 - 11:42pm

Is Kal unable to perceive audio performance to any less of a degree than a LITERAL ‘golden’ ear?

If only Kal could ‘see’ the flaws?

I am dubious regarding claims of ‘superior’ sonic sensation here.

Kal Rubinson's picture
Submitted by Kal Rubinson on December 21, 2024 - 10:17am

I wonder what color 0.0001% distortion looks like vs. 0.00001%?

I wish I knew. Because I suffer from R/G deuteranopia, I must rely on what I hear. :-)

georgehifi's picture
Submitted by georgehifi on December 21, 2024 - 12:04pm

"I wonder what color 0.0001% distortion looks like vs. 0.00001%?

Sweet FA when the speaker distortion your listening through has the decimal point on the other side of the 1

Cheers George

Ortofan's picture
Submitted by Ortofan on December 22, 2024 - 4:53pm

... "second harmonic sauce" has a specific aroma or taste and what cross-modal interaction or neurogastronomical link it would have with the auditory system?

Laphr's picture
Submitted by Laphr on December 21, 2024 - 10:56am

Which means that to the synesthesiaurulist, neither has a hue.

I sure wish we knew what did have a sound, and thus a color, because reliably, it sure ain't the associated data. I happen to somewhat see sound, but I also see stereo images, which is a point of stereo.

Yes, I know it's not a word. To be fair, in the service of better sound neither is commentary about data. That's more of an oxymoron.

mhardy6647's picture
Submitted by mhardy6647 on December 20, 2024 - 12:06pm

Kal, thanks for the review! Nice to see it. I do find these amplifiers quite intriguing. I'd like to hear one (ahem... well, two, I suppose) sometime. Way too expensive to buy on a lark just to hear (and to compare to SE 2A3 amplification on high-sensitivity loudspeakers), though -- so, unless someone in my neighborhood happens to pick up a pair...

mrh

Strat56's picture
Submitted by Strat56 on December 21, 2024 - 8:22am

... I am now a happy owner and listener of (diy) monoblocks based on Purify 1ET400A driving my Monitor Audio PL200 II and Allison Ones.
No regrets having sold the PassLab and I would never step back to it after having experienced these class D. I fully share the brilliant KR review report... 100%.

mocenigo's picture
Submitted by mocenigo on December 21, 2024 - 8:43am

I am one of the very first DIY builders of a 1ET400A stereo amp, at a time when the hypex power supplies were on backorder (expected lead time 3 to 5 months) and so I used a Connex PSU for the modules, and self built superregulated linear supplies for buffer (the Neurochrome Universal Buffer) and the modulators. It is a splendid ampifier and it should be common knowledge that not even shelling the price for the most expensive Dagostino wiill give you a better amp, except for power. But stuff like the 1ET9040BA are now fixing that aspect as well.

Strat56's picture
Submitted by Strat56 on December 21, 2024 - 8:53am

Nice to read your comment. So far I have re-designed and built 6 monoblocks, all based on 1et400 and linear PSU. I have never looked for kg of useless alu in my amps, .. not needed to play music... and these class D are very easy to be integrated in ultralight cases. They are not fatiguing even after hours of listening, very detailed even at low volume, they have revitalized my old Allison Ones and Twos, and almost reach perfection coupled to MF Platinum 200.

David Harper's picture
Submitted by David Harper on December 21, 2024 - 10:30am

I would very much be interested in your opinion of the Schiit Vidar amp.

Kal Rubinson's picture
Submitted by Kal Rubinson on December 21, 2024 - 11:46am

I have no opinion on the sound of the Schiit Vidar amp because I've never heard it.

georgehifi's picture
Submitted by georgehifi on December 21, 2024 - 2:12pm

"should be common knowledge that not even shelling the price for the most expensive Dagostino wiill give you a better amp, except for power."

"Power" when asked in audio circles usually means wattage, but in the D'Agostino case it's more "continuous amperage" they can give into very low speaker impedances which many of the esoteric ones usually have, this is where they shine compared to "any" Class-D into these loads.

Imagine this is what happens to them when driving something like Wilson Alexia Mk1
The bulge is the Class-D in the speaker cable https://i.ytimg.com/vi/G4xiGplJHEE/sddefault.jpg

Cheers George

Brent Busch's picture
Submitted by Brent Busch on December 22, 2024 - 2:37am

Purifi has posted a statement on their Facebook page about the distortion and linked it to a bach of early modules, and has since been fixed.

georgehifi's picture
Submitted by georgehifi on December 22, 2024 - 10:53am

JA: "Buckeye is sending a pair of amplifiers with the corrected Purifi modules for me to measure and Kal to audition."

Wow!!! owners of any amps (not just Buckeye) with these Purifi modules in them now are going to freak out and have second thoughts about what they are listening to!

Cheers George

mathompson@gmail.com's picture
Submitted by mathompson@gmail.com on January 12, 2025 - 9:07am

If you retest amp maybe you could check out the SpeakerOn connectors and comment.
https://www.buckeyeamp.com/shop/amplifiers/options/connectors

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