Bel Canto e.One Ref600M power amplifier

Has it really been more than seven years since I reviewed Bel Canto's REF1000M monoblock? According to the Bel Canto website, that model, based on Bang & Olufsen's ICEpower class-D modules, is no longer available. But now, like so many manufacturers, Bel Canto has adopted for its new models the NCore class-D module from Hypex—although the REF600M monoblock ($4990/pair) is not Bel Canto's first product to use it . . .

Last year, Michael Fremer reviewed Bel Canto's nearly all-digital Black amplification system, which comprises one ASC1 Asynchronous Stream Controller (essentially a Master Clock/preamplifier) and a pair of MPS1 Mono PowerStreams (monoblock D/A power amps with digital and analog inputs, connected to the ASC1 via ST-optical links). The MPS1 is based on an NCore amplifier module and Hypex SMPS power supply board, supplemented by Bel Canto's own AC power pre-conditioning circuitry. The sound of this $50,000 system ($20,000 for the ASC1, $15,000 for each MPS1) impressed the fastidious Fremer, and John Atkinson's measurements (and comments) were highly complimentary. After spending an unseemly amount of time with it at the 2015 Consumer Electronics Show, I, too, was impressed with the Black system.

I couldn't help noticing that the power-output specs of the REF600M monoblock—300W into 8 ohms, 600W into 4 ohms—are identical to those of the MPS1, suggesting that they share similar NCore and power-supply components. So, even taking into account the fact that a basic power amp such as the REF600M has no need for the MPS1's digital inputs and DAC, the new amp's price is eye-openingly low. Yes, there are cheaper NCore monoblocks out there (and Hypex modules are available to DIY-ers), but this amp comes with a pedigree. I had to have a pair.

Déjà vu
When the REF600Ms arrived, I felt as if I were in the movie Groundhog Day: Everything seemed the same, but not quite. The REF600Ms' shipping cartons, outer and inner, were the same as were used for the REF1000M and REF1000M Mk.II. The User's Guide was spare but adequate. The amp itself is similar in appearance to Bel Canto's previous little brick, and 3.1 lbs lighter. The REF600M, however, is finished with smoother corners and edges—and, in all black, it's quite reminiscent of the Black system. The only thing on the front panel is a single green power-on LED at dead center, but the rear panel is all business: IEC power inlet and switch, two sturdy WBT NextGen five-way binding posts, RCA and XLR input jacks with pushbutton selector, and a 5V/12V trigger input. Just like its predecessors.

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Inside are three circuit boards: a Hypex NC500 amplifier, a Hypex SMPS1200 power supply, and Bel Canto's input conditioning board with their Impedance Optimized Input Stage, for balanced, high common-mode rejection and to provide a low output impedance to the input of the amplifier board. The active element on the board is an LME49720 Dual High-Performance Audio op-amp in what appears to be a low-pass filter configuration. I was interested to note that Bel Canto has chosen not to include their own power supply, as they did with the ICEpower-based REF1000M—which probably accounts for the REF600M's lighter weight. Note, too, that while the power specs of the REF600M and Black are similar, the earlier REF1000M has a bit more oomph, at least on paper.

Installation of the REF600M was trivially easy. I pulled the left and right XLR interconnects and speaker cables from my Parasound Halo A 31 power amplifier and inserted them in the Bel Cantos, which I plugged into the wall with Kubala-Sosna Emotion AC cords. I flipped on the REF600Ms' power switches, the green LEDs lit up, and off we went.

Potent Little Bricks
Like other NCore-based amps I've tried—specifically, NAD's Masters Series M27 and M22, and Theta Digital's Dreadnaught D—the REF600Ms sounded smooth, powerful, and without significant faults. Their sound was detailed without being notably bright, they provided full bass extension and excellent imaging between the speakers, and were capable of driving even somewhat difficult speakers—the only kind I have—without breaking a sweat. Still, despite their common genealogy, each of the NCore implementations has unique characteristics, and the REF600M was no exception.

Despite its small size, the REF600M is a powerhouse, outputting 600W into 4 ohms, which is the approximate impedance of my B&W 802 D3s in the most demanding part of the audioband. It never ran out of steam with any sort of music at any listening level, and remained absolutely consistent in tonal balance and clarity. In "Mars," from Dean Peer's Think . . . It's All Good, electric-bass virtuoso Peer and percussionist Ty Burhoe produce lots of energy from about 100Hz down (CD, Turtle 8713606599008), yet their individual contributions remained unobscured through the Bel Cantos, whether I played this track for my own musical enjoyment or pumped it up to annoy the neighbors. Inspired—or, rather, provoked—by this, I popped on Yello's One Second (CD, Mercury 832 675-2) and let rip. "Oh Yeah"—Ferris Bueller would have loved how the REF600Ms delivered the big thrills.

This is not to say that the REF600Ms lacked subtlety. Every participant in the fairly complex combinations of instruments, solo voices, and choruses of Le Jardin de Monsieur Rameau, a collection of 17th- and 18th-century French music performed by William Christie and Les Arts Florissants (CD, Les Arts Florissants AF002), was exquisitely discernible within an ensemble setting that was believably arrayed across the soundstage. I had thought this recording just a bit too lively, but the Bel Cantos tamed its edginess to reveal a well-balanced sound—an ability that did not diminish with scale. I was particularly taken with a recent and surprisingly effective recording of Stravinsky's The Rite of Spring, with David Bernard conducting the Park Avenue Chamber Symphony (CD, Recursive Classics RC2057001; download from ClassicsOnline.com). The PACS bulked up to 100 players to record this in a relatively small space at the DiMenna Center for Classical Music, in Manhattan. While the bass in this acoustic is a little ripe and the perspective is, of necessity, somewhat close, the recording provides an opportunity to hear into this icon of 20th-century music. The REF600Ms made sure no detail was lost, and Bernard leads this semiprofessional band in an affecting performance. Local guys make really good.

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Solo voices and instruments had satisfying presence and warmth through the REF600Ms. In Gerald Finzi's "Come Away, Death," Marianne Beate Kielland's mezzo-soprano was nicely balanced and of appropriate depth, while the sound of Sergei Osadchuk's piano was full but a bit lacking in ping—as if its lid were closed (24-bit/192kHz PCM, free stereo download from SACD/CD, 2L 2L-064-SACD). Robert Silverman's brilliant pianism in Rachmaninoff's Piano Sonatas 1 and 2 (CD, Stereophile STPH019-2), was less subject to that lack, which was not apparent with solo guitar, whether electric (as in Dean Peer's Think . . . It's All Good) or acoustic, as in Francisco Tarrega's Capricho árabe, from classical guitarist Stefano Grondona's La Guitarra de Torres (CD, Divox CDX-29701).

The lack of ping was elusive with bowed strings, perhaps because the performer is constantly shifting the position of the instrument with respect to the microphone and, thus, the listener. Overall, I suspect that it has to do with the different overtone structures of various instruments, but it led me to suspect that the attractiveness of the REF600M's warm sound might be due to some rolloff of the very high treble. That might also have contributed to the inky "blackness" of the backgrounds, as well as the Bel Canto amp's kind treatment of less than tip-top recordings. Moreover, there was none of the somewhat parched treble that bothered me with earlier ICEpower amplifier technology.

That the REF600M didn't sound identical to some other amps was neither surprising nor a criticism of any of them. Of three recent amps of my experience, the REF600M was the warmest, the NAD Masters Series M22 the most detailed, and the Theta Dreadnaught D somewhere in between. Why should various amps based on circuit boards of the same technology (NCore) and made by the same company (Hypex) sound different? I don't know, but the amplifier boards aside, I could see physical differences inside these three amps. The big Theta uses a big linear power supply or two, while the more compact Bel Canto and NAD use a Hypex SMPS. Theta and NAD use the input circuit on the NCore amp board, while Bel Canto supplements it with a proprietary circuit. These differences must affect the sound quality, but to link them with what I heard will take testing procedures that control more variables than I can.

Conclusions
Bel Canto Design's REF600M has the power to impress. It never ran out of watts, and, with recording after recording, its sound was musically satisfying—so satisfying that it encouraged, even demanded, listening with complete engagement. During those listening sessions, the REF600M never gave me any reason to question its performance, even with the recordings mentioned above. As they used to say in the British press, the Bel Canto REF600M could not be caught out.

COMPANY INFO
Bel Canto Design
221 N. First Street
Minneapolis, MN 55401
(612) 317-4550
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COMMENTS
georgehifi's picture

JA you never used to use this bench test filtering with Class D's, and in the past we got to see just what was coming out of these Class D speaker outputs.
But now with the Audio Precision's auxiliary AUX-0025 passive low-pass filter we're left screen shots and measurements of something that not representable of the product being tested.
Were the Class D manufactures complaining that the 1khz or 10khz square waves shots were looking more like a timber band-saw than a proper square wave?
In my view not a valid representation of the product on test.

Cheers George

John Atkinson's picture
georgehifi wrote:
JA you never used to use this bench test filtering with Class D's...

I have been using this filter for 10 years.

georgehifi wrote:
... and in the past we got to see just what was coming out of these Class D speaker outputs.

I still occasionally show class-D amplifier squarewaves with and without the low-pass filter. But without the filter, all you see is a wave with a large amount of RF noise on the tops and bottoms. As I have explained in the past, removing this noise with the filter allows me to see overshoot and ringing, which I regard as significant. Note that I always note the level and center frequency of a class-D amplifier's switching noise.

georgehifi wrote:
But now with the Audio Precision's auxiliary AUX-0025 passive low-pass filter we're left screen shots and measurements of something that not [representative] of the product being tested.

The AUX-0025 filter is necessary to prevent the Audio Precision analyzer's input circuitry being driven into slew-rate limiting by the class-D amplifier's ultrasonic switching noise.

georgehifi wrote:
Were the Class D manufactures complaining that the 1khz or 10khz square waves shots were looking more like a timber band-saw than a proper square wave?

No. And so what if they were?

georgehifi wrote:
In my view not a valid representation of the product on test.

And by expressing that opinion, all you are doing is revealing your own lack of insight, I am afraid.

John Atkinson
Editor, Stereophile

georgehifi's picture

Well JA I still like to see what's trying to ultrasonically fry my tweeters/ears. Especially some Class D at turn on that squeal like a stuffed pig for a second and have a very high level of HF switching noise hiss who's artifacts can be heard from the tweeter.

One of our well known reviewers that I know used his Wilson Watt 8's, reviewed a Nuforce 9se V3 for a few weeks, while reviewing them both had the turn on squeal and tweeter hiss, which Nuforce said was normal.

After the 2 week review, complaints were that the top end of his speakers were sounding a bit hard, under my instructions he got new tweeter diaphragms for me to insert (he's a reviewer, not very good with his hands) when I remove both old tweeter diaphragms I was shocked to see that the coils were blued with heat damage and the lacquer was bubbled because of either the squelch of the turn on, or the 800khz fundamentals of artefacts of the switching noise hiss that we can hear from the tweeter.

I get asked to recommend amps including Class D
This is why I like to see what's coming out of the speaker terminals of Class-D amps without it being hidden from view in test results, before recommending them and not through lack of insight.

BTW from a quick look and there must be others, it is a bit selective when this filter is applied, without as early as last year, not as of 10 years ago.

Please JA go back to showing both with and without the filter, not just with.

http://www.stereophile.com/content/theta-digital-prometheus-monoblock-po...

http://www.stereophile.com/content/anthem-statement-m1-monoblock-power-a...

http://www.stereophile.com/content/channel-islands-audio-d-100-monoblock...

Cheers George

dumbo's picture

There must be some way to see the presence of these HF anomalies (if they exist) and the use of the filter on the measurement equipment could hide the possibility of this behaviour being present? Is it possible for HF noise at the upper limits of a Class D Amps bandwidth to find its way back down into the audible range via reflections and or load instabilities at those Freqs?

Do the Hypex based amps use output filters at all before the speaker output terminals? Do most none Class D Amps use output filters in general?

georgehifi's picture

"Do most none Class D Amps use output filters in general?"

Some linear amps have HF output filters if they're subject to oscillating, but I have found that if the linear amp is unconditionally stable there is no need for them

And yes all Class D's I know have a gentle (so it won't burn out) passive high wattage output filter, if not they would fry your speakers in a micro second.

But this filter can't filter out all the switching noise without totally effecting the audio band, this is why you'll see the remminance of that noise as the dreaded sawtooth ringing noise all across the top of a test square wave, some much more than others.

The Audio Precision's auxiliary AUX-0025 passive low-pass filter that JA uses goes between the class d amp speaker outputs and the measurement device, which eliminates most all of the switching noise, so you don't see it or measure it anymore. If it were left in place and speakers attached after it instead of the measurement device, it would sound very different to what it would if it were not there, and may well burn out very quickly if the level is turned up, because when used as stated it's only used with a small signal level.

PS: Once the switching frequency is >5 x higher than it is now, then you'll see that switching noise/sawtooth dissapear, and JA won't need to use that filter to hide it, but technology is't up there yet for that to happen, Technics I have have seen have made a very expensive >$20K class D poweramp that has double the switching noise to 1.5mhz!! this is a step in the right direction, 5mhz would be much better. Once this happens, our treasured big, hot, heavy, inefficient linear amps, will become door stops or boat anchors.

Cheers George