{"id":23712,"date":"2020-09-23T09:43:26","date_gmt":"2020-09-23T07:43:26","guid":{"rendered":"https:\/\/www.hifi-advice.com\/blog\/?p=23712"},"modified":"2021-04-19T20:46:37","modified_gmt":"2021-04-19T18:46:37","slug":"atm-audio-epm-700-class-d-power-amplifier-with-korg-nutube-triode-input-buffer","status":"publish","type":"post","link":"https:\/\/www.hifi-advice.com\/blog\/amplifier-reviews\/power-amplifier-reviews\/atm-audio-epm-700-class-d-power-amplifier-with-korg-nutube-triode-input-buffer\/","title":{"rendered":"atm-audio EPM-700 class-D power amplifier with Korg Nutube triode input buffer"},"content":{"rendered":"

Review samples supplied by atm-audio<\/a><\/h4>\n

Retail prices in Europe ex-VAT:
\nWith Sonic Imagery SIL994 input buffer: 2.919 Euro pp (5.838 per pair)
\nWith Sonic Imagery SIL990 input buffer: 2.969 Euro pp (5.938 per pair)
\nWith Korg Nutube triode input buffer: 3.100 Euro pp (6.200 per pair) (as reviewed)<\/h4>\n

Available in Black, Silver, Red and Gold at no extra cost
\nAvailable in Chrome for an additional 150 Euro<\/h4>\n

Starting with the ICE-power Jeff Rowland model 201’s<\/a> via other brands that used either ICE or Pascal modules, interlaced with various other designs that I heard over time, I have plenty of experience with Class-D amplification. While I always admired them, I never loved them. The arguments for this are by now so worn out that I will not repeat them here. The Audio Physic Strada Monos<\/a> and the Naiu Labs Ella<\/a> that both use unique and proprietary amplification techniques are worth mentioning as positively deviating alternatives but, still, I was not prepared to trade in my Class-A\/B amplifier for any Class-D variant. My opinion changed when I heard the Mola-Mola Kalugas (Ncore) and the NuPrime ST10 (Proprietary circuitry). The because of the superb resolution and refinement along with treble air and refinement that I had until that time only heard from Jeff Rowland Class A\/B designs. The latter because of its incredible bass solidity and “alive” sound combined with remarkably acoustically convincing, non-synthetic timbre. If only these two amplifiers combined their strengths!<\/p>\n

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Mola-Mola is a high-end branch of the same company that makes Hypex and Ncore. Based in my birth town Groningen, Hypex has been producing Class-D modules since 2003. The NC500 module which is at the basis of the EPM-700’s is closely related to the internals of the Kalugas. No wonder, then, that the atm-audio EPM-700’s show some similarities with the Kalugas. But the interesting thing is that they also have similarities with the NuPrime ST10.<\/p>\n

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atm-audio EPM-700<\/h3>\n

The EPM-700’s are encased in a beautifully-shaped anti-vibration aluminum chassis made from a solid block of aluminum with individually-milled compartments for the power supply and the amplification section.<\/p>\n

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The case is available in Black, Silver, Red, Blue and Gold at no extra cost and in Chrome at a 150-euro surcharge. All connectors are Furutech. The basis of the EPM-700 Mono consists of a Class-D Hypex NC500 module with NCORE technology powered by a 1200W switched-mode power supply and a customized input stage. By default, the amplifier has a Sonic Imagery Labs 994 input buffer with\u00a0 Enh-Ticha low noise operational amplifiers and very low noise Sparkoslabs voltage regulators.<\/p>\n

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Korg NuTube Triode valve on custom atm-audio input board<\/em><\/p>\n

Optionally, the amplifier is available with the Korg NuTube triode valve or the Sonic Imagery Labs\u00a0 SIL990 Enh-Ticha Operational Amplifier. In both cases, this concerns only the input voltage stages, the output stage is the same for all versions. The version reviewed here contains the Korg Nutube triode input.<\/p>\n

Incidentally, the buffer is also compatible with the Purifi Audio 1ET400 module which, according to the manufacturer, would obtain the same results.<\/p>\n

With a specified load tolerance of 1-8 Ohms and hundreds of Watts (up to 500W at 4 ohms), the amps should be able to drive the vast majority of speakers in existence. Still, it is worth noting that I found that the way the amplifiers are voiced does favor the partnering with sensitive and lively sounding speakers. With the EPM-50 Class-A mono’s this was no different, but then, I could point at the low damping factor and presumably low feedback. With the Class-D modules these should not factor in and thus it must come down to the voicing of the amplifiers.<\/p>\n

So, what is this voicing I speak of? It’s the way that music is produced in a totally unforced and effortless manner, flowing freely and without a hint of hardness or dryness. One might expect such qualities in a Class-A or A\/B design but not in a Class-D design. I suspect that the Korg Nutube triode tube input section plays a large role in this and, of course, this is where the special atm-audio expertise comes in.<\/p>\n

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While the Class-D NuPrime ST-10 (on top of the CH A1.5) has no problems driving either the Magico’s or the Logans and sounds mighty powerful and lively while doing so, both speakers do make it very obvious that this amp lacks resolution, refinement, liquidity and air – all areas in which it is royally outclassed by the atm-Audio EPM-50’s. Would the Class-D amplifiers offer the best of both worlds with these speakers?<\/em><\/p>\n

Listening – Magicos<\/h3>\n

The EPM-700’s only offer an XLR input but since that is a format that I generally prefer I have plenty of cables to choose from. If you want to use cinch cables then you will have to use an adapter. This reduces the gain by 6dB but otherwise works just fine.<\/p>\n

My first impression of the EPM-700 amplifiers, connected using Driade Flow 405<\/a> speaker cable with the Magico S1 MkII loudspeakers in the main system with the Aqua La Diva CD transport into the APL DSD-AR<\/a> DAC with its built-in volume control after having used the NuPrime ST-10<\/a> amplifier, is that they are most certainly not your typical Class-D amplifiers. For starters, they do not have the vice-like grip and overly clean sound of regular switched amps. Actually, they sound similarly relaxed and sweet as I remember the EPM-50’s with these speakers, with a similarly low-feedback-like, smooth and utterly relaxed delivery. Further, the EPM-700’s are a lot more sophisticated, airier and more refined than the ST10.<\/p>\n

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Meanwhile, the EPM-700’s low-level resolution and fluidity are several notches above the ST10’s and this is where they are more similar to the Mola Mola Kalugas. The EPM-700’s timbre is remarkably natural and highly convincing and this is one of the things they have in common with the ST-10. There really is no trace of the lingering synthetic nature that I still heard with the Mola Molas.<\/p>\n

The EPM-700’s also have a more voluptuous soundstage with an involving wrap-around effect. The Spanish dua sounds so good, in fact, that I feel that they operate at the level of an outstanding Class A\/B amplifier while their relaxed bass and midrange sweetness makes one almost suspect listening to a Class-A amplifier.<\/p>\n

So far, the atm-audio amplifiers are pretty amazing! However, as a price to pay for their utter lack of edge, they do not sound quite as expressive and impactful or dynamic as for example the NuPrime ST-10. That amp performs less well in a whole range of parameters but it does<\/em> sound more exciting. Also, while the EPM-700’s don’t sound slow, the other transistor amplifiers that I have available do sound nimbler and more immediate. Now, I should note that all this is noted in combination with the Magicos.<\/p>\n

The Magico Q-range is notorious for being unforgiving as well as difficult to drive but the S1 mkII’s that I use are much easier to drive (although they, too, are current-hungry) and have a much more forgiving sound. They have spectacular resolution but are also gentle and refined and thus they work best with amplifiers that are very neutral and lively. Surprisingly, they even work pretty well with amplifiers that sound a little rough, as is the case with the NuPrime. In my experience, sweet-sounding amplifiers are not ideal for these speakers.<\/p>\n

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After having substituted the APL DAC for the even more astute-sounding Weiss DAC502,<\/a> the pacing does pick up and so does the dynamic differentiation but the transients remain a little soft and the feeling of a sweetened and overly relaxed sound remains. The Weiss is one of the most direct sounding DACs that I know but in this combination, it’s hardly recognizable as such. The delivery sure is highly sophisticated and certainly not possessing of the usual Class-D downsides but it’s leaning over to the other side where it becomes so smooth that much of the excitement is ironed out in the process.<\/p>\n

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Evidently, the Magicos are not ideal partners for the Spanish Duo. So, let’s do a swapperoo.<\/p>\n

Listening – Xavians<\/h3>\n

In an effort to try something completely different while remaining in this listening space, I grabbed the Xavian Perla Esclusivas<\/a> and hooked them up in the main room. Dropped in-between the Magicos and retaining the rest of the system including the Weiss DAC, the EPM-700’s turned in a completely different<\/em> performance!<\/p>\n

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The Weiss’s neutrality and excellent impulse behavior teamed up perfectly with the Xavians’ bold and infectiously dynamic behavior to turn in an immediately captivating and immensely entertaining performance. Propulsive and impactful, enormously dynamic and highly lyrical, the system was now truly rocking!<\/em><\/p>\n

Interestingly, in spite of their clearly non-ideal placement, the soundstage still filled the entire room with sounds coming from seemingly everywhere except straight from the speakers. Actually, a friend who was listening in had to do a double-check to confirm that it was really the Xavians producing this room-filling sound.<\/p>\n

The toe-tapping factor was off the charts and, in spite of the speakers’ higher coloration, the tonality came across as highly lifelike in all the right places, especially with vocals, piano and guitars. Put short, there was some mighty fine<\/em> music-making going on here.<\/p>\n

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This was a truly rare case of amazing lushness combined with boundless dynamics and great PRaT. The amps’ rich tonality and gentle smoothness are still present undiminished but now perfectly balanced with solid and impactful bass and an overall highly energetic and expressive delivery. How is it possible that these amplifiers can sound so different depending on the speakers? Well, as I mentioned, the Magicos are highly refined and already gentle themselves and they don’t need any sweetening. The Xavians are just very different beasts. They are much less linear and refined and they are certainly not as neutral or as transparent, and yet, they turn in an absolutely lifelike performance.<\/p>\n

With the Xavians, the EPM-700’s bass is also truly superb. It’s well-proportioned, solid and full-bodied, yet articulate and precise, just like it is in real life. In spite of lower apparent resolution, instrumental textures are now also laid out beautifully and temporally, the performance is absolutely perfect and dynamically everything is just right.<\/p>\n

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<\/p>\n

Interlinks<\/h3>\n

To get the best overall balance from the EPM-700’s with the widest range of loudspeakers, they are best coupled with transparent and open-sounding interlinks. In that category, the CH Balanced Link was a match made in heaven.<\/p>\n

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Foilflex<\/a> poses an interesting alternative that is every bit as neutral and transparent and unveils a similarly impressive amount of information at a much lower cost. The main advantage of the CH cable over the FoilFlex is more of a free-flowing nature and a more organic feel in the midrange.<\/p>\n

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The Vermouth Reference<\/a> interlinks also worked surprisingly well, adding some extra color and substance as well as a more sonorous bass to combine into a very musical and engaging delivery. The Vermouth is not short on detail but its voluptuous sound can<\/em> push the EPM-700s’ already relaxed sound too far, depending on the rest of the system’s balance.<\/p>\n

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Compared with EPM-50 using Kromas<\/h3>\n

Now that it was established that the EPM-700’s do a very good job of sounding like a Class-A\/B amplifier and, in some aspects, even a Class-A amplifier, it was time to compare them directly to the EPM-50’s which are, of course, actual<\/em> Class-A amplifiers. For this comparison, I changed scenes to the secondary listening room where the EPM-50 Class-A monos were still set up and connected to the Kroma Audio Carmen<\/a> loudspeakers. After having listened to a few well-known tracks, with much anticipation, I swapped the EPM-50’s for the EPM-700’s and cued up the same tracks. Would they sound similar? Or actually quite different?<\/p>\n

As it turns out, the amplifiers do have similar signatures but they are also different. Surprisingly, the upper midrange and treble are remarkably similar. This is surprising because one amp is Class-A while the other is Class-D and the latter class has never been very good in the treble. Of course, Hypex Ncore is to credit for this. As I already heard with the Mola Mola Kaluga monos, this technology produces the very best treble that I have heard from a switching amp. The major difference is in the bass and the lower midrange. To my ears, the EPM-700’s have a neutral response with a relaxed and smooth touch and a pleasant warmth. As such, I would absolutely not label them as cool or factual. Still, the big Class-A beasts sound considerably creamier and warmer and even more lush and relaxed. At first, this made me wonder which of the amplifiers I preferred but after a moment of accustomizing the EPM-50’s seduced me with their lusher and richer sound and their more natural flow. There’s just an immensely organic quality to these amps’ delivery that the EPM-700’s cannot match.<\/p>\n

The EPM-700’s counter with speedier transient behavior, better articulation and more responsiveness to speed changes which all combines into great PRaT. Apart from speed, the EPM-700’s bass is just also incredibly convincing in its stature, agility, stability and tonality. With the EPM-50’s, the bass is richer and sweeter which for some people may be precisely what they are after when in pursuit of “typical” Class-A sound. Personally, however, I feel that the EPM-700’s more articulate low frequencies sound more realistic. And as regular readers will know, for me, realistic bass translates to a more realistic overall delivery. Other listeners may focus more on timbral richness and bloom. And yet another group may strive for getting tube-like-sound from a transistor design. While it is debatable what constitutes tube sound, for many people this means a big, sweet and relaxed sound. And these people will find what the EPM-50’s do to be more realistic or more desirable. These differences aside, both models achieve the very same goal of putting the music first.<\/p>\n

That both models manage to have such a similar sound signature is quite baffling and this says a lot about our prejudices regarding Class-A, Class-D and amplifier design in general. atm-audio’s Jes\u00fas Alejos P\u00e9rez clearly has a vision for how he wants his amplifiers to sound and evidently he achieves this irrespective of the amplification principle.<\/p>\n

No doubt, the Korg Nutube triode input section also has a hand in this. While this section does not transform the amplifiers into EPM-50’s, what they do is to take away the last remaining hint of synthesis, that sense of listening to an electronic circuit, to leave only pure music.<\/p>\n

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Listening – Ayon<\/h3>\n

Before wrapping up this review I decided to pay audio buddy Jan, aka “Meneer Buis” a visit with an EPM-700 under each arm. At this moment, Jan uses the Ayon Falcon-S loudspeakers which are very revealing and transparent and also very easy to drive. Using an Ayon CD-T<\/a>\u00a0CD transport with\u00a0Ayon Sigma<\/a> DAC as sources, the EPM-700’s were connected via Straightwire Crescendo II interlinks and Driade Flow 405<\/a> speaker cables. Jan normally uses WLM (now owned by Trafomatic) tube pre- and power amplifiers but these use cinch connections only while the EPM-700’s use XLR only. Fortunately, the Sigma DAC can also drive power amplifiers directly which is what we did with the EPM-700’s, after having bent the ridiculously stiff FIM Gold<\/a> XLR cables into position.<\/p>\n

In this system, the Spanish duo has stunning resolution and refinement. They really are a lot<\/em> faster, more articulate and precise than Jan’s WLM combo. Although the sound was still gentle and sweet, on balance, it could do with a little bit more “meat on the bones” and the bass was now arguably even a little lean. I should note that the last situation is a bit of an “apples and oranges” comparison because I’m comparing a situation with a DAC feeding a tube pre- and power amplifier to one where the DAC feeds switched power amplifiers directly. But even so, this puts things perfectly into perspective.<\/p>\n

Perspective<\/h3>\n

With the Kroma Carmens, I obtained some absolutely magical results with a balance that was quite lush and warm. The Xavians were less lush but provided a more dynamic and exhilarating, edge-of-seat, performance while remaining utterly lifelike and musical. The Ayon speakers, finally, tilted the balance over to the slightly cool side.<\/p>\n

This tells me two things. First: the EPM-700’s can swing both ways of the spectrum, depending on the speaker pairing. Second: they definitely prefer to be matched with unrestrained\/enthusiastic and expressive speakers.<\/p>\n

With the inherently cool-sounding Ayon speakers, the WLM tube preamp and power amp sound warmer and sweeter and thus make for an emotionally more compelling performance. But they are also a lot<\/em> softer, a lot<\/em> less well-paced and a lot<\/em> less well-focused in terms of imaging and once we had heard the tighter and much more energetic EPM-700’s, the WLM’s just weren’t satisfying anymore. I have a sneaking suspicion that if we had the possibility to connect Jan’s tube preamp to the EPM-700’s, the results might have been absolutely spectacular.<\/p>\n

Jan is a real tube-aficionado who has repeatedly said to have no love for a “Torrenbak” which is a Dutch derogative term for a transistor amplifier. Yet, he had to admit that the EPM-700’s sound remarkably great, not only for a Class-D design but for transistors in general. Maybe it was the spur of the moment but I swear that I noticed Jan contemplating owning a transistor design again someday.<\/p>\n

Conclusion<\/h2>\n

While the Korg Nutube triode input section does not transform the EPM-700’s into EPM-50’s they do share a large chunk of what makes the full Class-A models so special. Both models seem to be tuned to never sound forced or edgy and both models have a smooth and absolutely non-synthetic, non-electronic delivery with a completely natural timbre.<\/p>\n

They have the superb bass and linearity that comes naturally to with the Class-D territory but when also taking the superbly fluid and refined treble into account and adding their sweetness and fabulously natural timbre, these hybrid amplifiers deliver up to 500 Watts of what closely resembles actual Class-A sound from a tiny cool-running box.<\/p>\n

Their unforced delivery does favor partnering with expressive loudspeakers but when the match is right, these amplifiers turn in an utterly lifelike and superbly involving performance.<\/p>\n

Associated Equipment<\/a><\/p>\n

External Links<\/h3>\n

Manufacturer: atm-audio<\/a><\/p>\n

Read Also<\/h3>\n

atm-audio EPM-50 Class-A power amplifiers<\/a><\/p>\n

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