as reviewed by Guido D. Corona
For two years in a row, at RMAF in Denver (CO), I have been regaled with the magnificent sound of the Bel Canto suite, where a pair of REF500M monoblock amplifiers completed the well matched chain of all-Bel Canto electronic components assembled in room 586 of the Marriott Tech Centers Hotel. My undisguised excitement for the musicality and resolution of the entire system during the 2008 show prompted me to request review samples. Some early concerns—recently proven to be unwarranted--about the power requirements of my Vienna Mahler V1.5 speakers, caused me to opt for the marvelous, and nominally twice-as-powerful, REF1000 Mk.IIs (recently renamed REF1000M), which I reviewed for Positive Feedback in Issue 43. Yet, upon my return to RMAF 2009, it dawned on me that the entire system was once again driven without an ounce of effort by the allegedly less powerful REF500M monoblocks. I was intrigued; would the little brothers of the REF1000Ms, with half the nominal power output, still be able to make real music through my power hungry and impedance-wilding Mahler speakers? Furthermore, I was extremely curious about what might be the perceivable sonic difference between the REF1000M and REF500M amplifiers, whose outward appearances are essentially identical. Hence, while sinking contentedly in my listening chair, I proposed to John Stronczer—Bel Canto Design's president—that I write a review of the REF500M monoblock amplifiers.
Soon after, just like their bigger brothers the previous year, the REF500Ms were delivered to my home, solidly packed in dense foam, and individually double boxed, with each inner shipping case sporting a side handle, reminiscent of the shipping valises that protect many notebook computers.
With a size of 8.5 x 3.5 x 12 inches, each chassis weighs 15lbs, which is 3.5 lbs lighter than REF1000M mostly because of the REF500M's more highly integrated ICEpower 125ASX2 power conversion modules. Similarities with REF1000M continue: each REF500M is a compact but hefty device sporting an understated retro industrial elegance, thanks to the contrasting geometries of its heavy gage front silver aluminum fascia whose angular outer lines frames a large recessed central black ellipsoid. A blue LED in the very middle of the front face illuminates when units are operational, while a substantive power switch is rocker mounted on the back plate. A 15amp IEC inlet sits just below the power switch, while the balance of the back panel features a set of twin WBT 5-point non-shrouded binding posts, a single Neutric XLR socket, a single high-copper RCA connector by Sound Connections, and a pushbutton selector for switching from balanced to single ended operation. All fuses are internal to the amplifier and can be accessed and replaced only by authorized technicians.
At a time of rising energy costs and growing social concern for husbanding the finite resources of our planet, I welcome the REF500M's extremely high power conversion efficiency, ranging between 81% and 86%. After the several weeks of continuous music making that I forced upon them during break in, the amps remained barely warmer than skin temperature. The low standby power consumption of only 9.5W per chassis bespeaks the amps' class D switching design, based on the exceedingly efficient ICEpower 125ASX2 power conversion modules, a technologically newer and more advanced device than the ASP series modules employed by the REF1000M and by other class D amplifier designs of that barely older generation.
In each REF500M chassis a single ICEpower 125ASX2 module operates in balanced bridged mode, to yield 250W over 8 Ohms, and 500W over 4 Ohms. Power rating remains unchanged further down to 3 Ohms, while applications to speakers whose input impedance dips to 2 Ohms or below is not supported nor is even advisable. The peak current yielded by 125 ASX2 is 30A. While this value is 15 Amperes below what the older 1000 ASP is capable of, I would discover eventually how surprisingly minor the impact of this lower specification has on my musical enjoyment.
According to John Stronczer, the ICEpower 125ASX2 sports a more advanced architecture than its ASP series predecessors with fully integrated custom control ICs, which yields lower distortion, improved low noise levels, and yields a wider bandwidth.
At least under bench measurements, the 125ASX2 easily bests the performance of the equivalent module in the older ASP series—the 500ASP, still employed by many well respected class D power amplifiers of the previous generation, such as the Rowland Model 201 monoblock. For example, while the harmonic distortion of 500ASP measures a modest 0.006% THD+N at 1KHz 1W, the newer 125ASX2 rates even lower, at a mere 0.003% under same conditions. 500ASP's noise floor is rated at 80uV, while 125ASX2 measures less than half, at only 35uV. Raw dynamic range measurements are particularly interesting because they may help to explain some of my surprising findings in the key domains of staging, imaging, and authority. The older generation 500ASP yields 115dB, while 125ASX2 extends dynamic range to 121dB. The measurement is 1dB higher than the outwardly more powerful 1000ASP module, which is deployed inside such class D power houses as the Bel Canto REF1000M, the Chapter Audio Duet, and the most recent iteration of the Rowland Model 301 monoblock amplifier.
Yet, the Bel Canto REF500Ms are far from being mere raw 125ASX2 modules hidden inside handsome boxes. Like the power conversion components imbedded in so many advanced amplifiers based on other underlying technologies, the ICEpower 125ASX2s are but central building blocks of a comprehensive design, optimized to yield astonishingly high musical performance.
To help dispel a hopelessly stale urban legend about the allegedly flawed pedigree of class D amplification, it is worth pointing out that like other amplifiers of their kind, the REF500Ms are not digital devices. At their core, the analog switching technology of the B&O's ICEpower 125 ASX2 power conversion modules is essentially analog, and so are the partially integrated and active-feedback regulated power supplies (SMPS): "We use our proprietary low noise input rectification and filter stage," Stronczer explains.
The power input stages are very similar to the REF1000M using high-speed, low-noise rectifier, high-voltage film filter capacitors and high energy storage capacity of 400 Joules—twice the energy storage capacity of the original REF1000 [Mk.I]." The advanced design of the power input stages convert the 120 AC 60Hz power from the largely unavoidably noisy public electric grid into a clean, filtered, and buffered current at approximately 320V. The resulting direct current is then fed to the active-feedback regulated and custom filtered switching mode power supply of the 125 ASX2 module, which is compatible with DC input voltages as high as 385V. According to Stronczer, the converted DC is filtered and buffered by "Low Noise rectifiers with FWB voltage doublers into large filter C bypassed with polypropylene capacitors."
The optimization of pre-power-supply rectification is a general methodology that has gained favor among several manufacturers of switching mode amplifiers, including Bel Canto, Spectron, and Rowland. The newest generation of class D amplifiers—equipped with highly customized power rectification circuits—exhibits extremely low power line grunge, while yielding a heightened refinement of the musical reproduction, uncharacteristic of more basic switching amplifier implementations.
More designed refinements introduced by Bel Canto on the REF1000M have been applied to REF500M: according to John Stronczer, "In prototyping we found 400+ joules of energy storage, low noise rectification and additional filtration on the RAW DC supply made a big improvement". Then he continues: "as did an input stage with higher CMRR/input impedance and lower output impedance to drive the amp modules".
Moving to the signal input stage, the fully balanced design of the REF500M maximizes "high common mode noise rejection and wide dynamic range. While not transformer coupled per se, the 500M's custom input stage employs high speed low distortion differential amplifiers and Caddock resistors: they pre-condition the input signal to drive optimally the power conversion module, in a modality largely invariant to input load, which also results in the amplifier's common mode noise rejection of 60dB.
Stronczer explains that REF500M's "Dynamic range is 121dB 20-20kHz A weighted, and Noise floor is over 150dB down from Max Level in FFT analysis. Furthermore, internal impedances have been optimized to reduce noise and distortion. Measurements reveal a full 6dB improvement in signal-to-noise ratio when driving the output amplifier section with low-impedance loads." With an impressive impedance of 200kOhms balanced, and 100kOhms unbalanced, the REF500M is comfortably more than fully compatible with all but the most bizarre preamplifiers on the market.
The foregoing also implies that, combined with their very high damping factor of 1000 below 100Hz, the Bel Canto REF500M monoblocks are engineered to handle the challenging loads presented by moderately sized bipolar and electrostatic speakers, as well as—to my delighted surprise--by fairly large dynamic speakers with complex impedance curves, and with deceptively demanding power handling requirements, like my own wild and wonderful Vienna Mahlers V1.5s.
Finally, John points out a slight difference in the fully balanced output stage of the two power amplifiers: "The output of the REF500M is at ground potential while the REF1000M is at 60VDC common mode above ground."
Break-in And Setup
Please allow me a bold statement at the onset: the Bel Canto REF500M monoblocks reward patience and unhurried love of the hobby, while they frustrate any seekers of quick thrills. The amplifiers require—nay, even demand—a very long and very protracted break-in to come into their own. Audiophiles bent on obtaining instant gratification from brand new components may best look elsewhere. By contrast, the steadfast and unhurried lover of music will inevitably discover that finely broken-in REF500M monoblock amplifiers create veritable gardens of sonic and musical delights.
Thus, unlike my easy encounter with the REF1000Ms fresh from the factory, an early listening test on the brand new REF500Ms fell moderately short of an instant love affair. Rather, my premature impressions were fully consistent with that ever-familiar ligation and congestion of sound common to my listening experience with so many unsullied new amplifiers of various classes, technologies, and designs. In other words: dry and cardboard-like to a grizzling fault, accompanied by the ever-unwelcome propensity of new amps to emit sudden yet predictable bursts of shrill harshness at the mildest transient, from what appears to be a toy sound stage, consisting of a lumpy object emanating from the general middling region between my speakers.
Yet, mindful of my own recommendations to anxious audiophiles, I exercised the amps 24/7: music during the day, and FM inter-station hash from my old NAD tuner during nights, recycling power at least every second day, to enhance demagnetizations of circuits, components, and dielectric materials. Ever so gradually, the distorted transient edges retreated. The sound stage broadened, deepened, reached and sometimes exceeded the physical span of the speakers 11 feet apart. Instrumental images acquired believable solidity an existence in the virtual space. Harmonics started to ring, sustain, and decay throughout the audible spectrum--initially mere evanescent hints in the retreating gray, and then ever clearer and free of the haze of residual dust-bunnies. The bass region, thuddy and uninvolving during the first few hundred hours, deepened, extended, and then acquired a pitched assuredness perhaps even superior to that of the REF500Ms' bigger brothers. The mid-range became gradually textured, and lost for good a certain forwardness that waxed and waned with periodic irregularity during the early weeks; REF500M's treble region, that problem child, initially akin to a young one suffering of an unhappy combination of pulmonary insufficiency with whooping cough, happily healed from its youthful malaise whence the amplifiers approached 1000 hours of operation, and commenced to bloom into a delightfully lyrical maturity, almost devoid of residual constrictions.
Totally unforeseen, the steadily growing authority of the REF500M remained my greatest surprise. Well past the 1000 hours mark, at least until 1500 hours of music making, REF500Ms kept maturing in ease, a general sense of expansiveness, a surprising comfort in handling complex and intense dynamics, and a considerable solidity even in the most demanding fff (fortissimo) orchestral passages. Fully expecting to witness that dreaded and sudden flattening of the sound stage into a disturbing 2-dimensional glassy surface upon the least dynamic excitement, so rather typical of older designs based on the 500ASP module, such as the Rowland 201 monoblocks, I was astonished by the REF500M's steadfast stamina, much more akin to the seemingly endless reserve of power found in twice as powerful amplifiers, based on the 1000ASP module, such as the Bel Canto REF1000M monoblocks, and the Rowland Model 312 stereo. By the time this astonishing rise to power tapered off, between 1500 hours and 2000 hours of music making, the REF500Ms had acquired an authority incommensurate with my early modest expectations. More than capable of inducing a convincing and stable image, stage, and satisfying musical experience in synergy with the Mahler speakers, the relatively moderate power of REF500Ms can only be gleaned when contrasted with the apparently endless reservoir of energy yielded by their bigger REF1000M brothers driving the same system.
Since their arrival, the amplifiers have been connected to the balance of my reference system: TEAC Esoteric X-01 Limited, Rowland Criterion twin-chassis battery power preamplifier, and Vienna Mahler V1.5 speakers. The REF1000M have been frequently used for comparative purposes, while the Rowland Model 312 stereo has been kept consistently offline throughout, and has been factored out of my listening analysis. Power to the entire system is supplied by a dedicated 20A circuit terminated by twin Furutech High End Performance FT-D20A duplex outlets for maximum electric contact. Once REF500M break-in tapered off, I commenced to experiment with wiring and power distribution. I found the most musically goldielockian configuration, to consist of custom Furutech High End Performance (HEP) interconnects, shotgun runs of Furutech HEP speaker wires, and Furutech HEP power cords being fed by the fabulous Furutech ETP-309 power distributor. I preferred the reference-level Furutech PowerFlux cord for connecting ETP-309 to the FT-D20A outlet on the dedicated AC circuit, because PowerFlux's greater gage, agile transient speed and energy created an almost ideal synergy with the slightly softer presentation of the Furutech HEP PCs. Replacing the single length of PowerFlux with a recently received but well broken-in Elrod Statement Gold created a fascinating alternative: the Elrod cord yielded even greater authority to the amps, and perhaps even greater low level information and staging than the PowerFlux, at sole cost of marginally softer macrodynamics. In this particular context, I enjoy both presentations, as two cords yield differently charming versions of musical beauty.
As the reader may have already noted, the Bel Canto REF500M amplifiers having so much in common with their more powerful REF1000M siblings, this article is patterned on my review of the REF1000M (Mk.ll) published on PFO Issue 43.
In Antonin Dvorak's the Ruins of the Old Castle, a work spanning 7 octaves performed by talented pianist Inna Poroshina (Brilliant Classics) the REF500M create a sizeable soundstage, which comfortably spans and fills the 110" that separate the Mahlers center to center, just shy of the expansive staging induced by REF1000M. The bass is extended, and perhaps more linear than what their bigger brothers generate. I detect a significant amount of harmonic development, with gracefully prolonged harmonic ringing and decay. An occasional foreshortened stress in the treble, matched by a hint of glassiness, reminds me that the REF500M yields less power than its larger siblings do. The authority yielded in the bass region of the Boesendorfer Imperial Grand piano is impressive, and prompts me to check if perhaps I have connected the wrong amps: after all, the REF1000Ms are resting just on top of REF500Ms. Yet, I need not be concerned: the REF500Ms are the ones creating this magic.
Through the little REF500Ms, the piece has assumed a contained majesty. The lowest octaves are clearly tuneful and well pitched, with the ringing and decay of deep harmonics extending upwards, well into the treble region, reinforced by complex resonances in the soundboard. There is no obvious loosening in the mid bass, while the midrange has a pleasing grace without euphonic glow. Thankfully, there is no stereotypical black background made opaque by missing sonic cues. On the contrary, I discover a wealth of low-level information in the recording, which contributes to filling the air around the pianoforte with a sense of realistically sized living space. The Bel Canto REF500M deliver very low frequency thumps from the sostenuto pedal action, and a subtle rumble of air conditioning or perhaps road traffic, detectable only during the final ppp (pianissimo) decay comes through, while soft swishes emanate from the piano action felts, and occasional faint creaks can be heard.
The often-percussive mix of Diana Krall in "Stop This World" from The Girl in the Other Room (Redbook Verve) is rhythmic and punchy; the overall percussion section sounds nimble with fast attack and decay. Even the transients from the kick drum have an enviable agility. The hint of a warm bloom in the structural foundation of the plucked bass is more in control than on REF1000M. A virtual semi-circular stage of instruments surrounds the clean centered image of the voice of Krall, barely smaller than on REF1000M. Exactly like on the larger amps, Krall's diction remains engaging, crisp and incisive, without those regrettable signs of hot miking which seem to plague this album through far too many amplifiers, while stray excesses of energy on sibilants and plosives are kept largely under control.
The REF500s are even more arresting than REF1000Ms in Jan Garbarek's live soprano and tenor saxophone improvisation on the Hilliard Ensemble's sensitive performance of Regnantem Sempiterna, a Gregorian chant on Officium (Redbook EMI). The REF500Ms yield a palpable three-dimensionality through the interplay of the saxophone and vocalists softly reverberating throughout the church venue, the circular ring of harmonics exposed by the cantus firmus of the bass singer, the amps evidencing a subtle pass band filter effect—almost a Tuvan throat song. Only the high treble of the soprano saxophone breaks in moderate and brief glassy raggedness during the most extreme fff overblowing, while the hiss of Garbarek breath through the reed of the tenor sax in pianissimo is exposed by the low-level detail prowess of the amps.
The contained authority of the amps shine in Lorin Maazel's performance of Antonin Dvorak's cello concerto Op. 104 in B minor, with soloist Yo-Yo Ma and the Berliner Philharmoniker (CBS Masterworks, 1986). The dynamic contrasts are exciting: the amps reveal the most intimate solo passages with all the delicate expression of chamber music—Ma's cello is anchored center stage in a musical world ranging from soft vibrato, to lyrically charged thematic expositions, to eerily glassy passages sul ponticello, while the well separated images of the woodwinds remain spread clearly behind it. During fortissimo orchestral tutti, the REF500Ms reveal a surprising authority only moderately bested by their more powerful kin's. The lower brass section in particular, deliver a foundational growling character, which forces me to sub-vocalize along with the bass trombone, while I lose myself in the magic of the music.
The Bel Canto REF500Ms continue that Bel Canto tradition of sonic excellence and subtle musical expression, which I already enjoyed so much in the REF1000Ms. John Stronczer has harnessed the extraordinary potential of the 125ASX2 power conversion modules in bridged Mono configuration, to yield a surprising authority and frequency extension that barely falls short of a 1000ASP powerhouse. Harmonic exposure, development, and congruence is wholly convincing, and bass coherence is perhaps even more successful than on REF1000M. Tutti seldom become glassy even on high SPLs, while raggedness is limited to only occasional treble overpressure. Even on demanding large speakers like the Vienna Mahlers, the REF500M remain seductively agile, and fill my large listening environment (18 x 30 feet) with a magnificent sense of life, harmonic refinement, and a rare sense of space and 'being there'. The sound stage spans the distance between my speakers instead of exceeding room boundaries. Admittedly, I have the occasional impression that the amps are sometimes breathing through their task, instead of delivering by sheer magic an apparently infinite amount of power. It is worth reminding ourselves that these capable devices yield but half the power of the REF1000Ms.
Doubtless, these great price performers will rise to even greater heights when driving speakers that exhibit gentler impedance curves than my charmingly wild Vienna Mahlers, or when they are perhaps applied to the Vienna Strauss', less power-hungry brothers of the Mahlers. Do not be deceived by the REF500Ms' conservative size and weight, or by the trite and obsolete urban legends surrounding class D amplification, which are barely kept alive by a shrinking hyper-nostalgic crowd. There is not a trace of 'digititis' ever to be detected in their great performance. Give the Bel Canto REF500Ms a long audition: you will be amazed by their surprising authority, seduced by their agility, and will fall in love with their superior musicality. Guido D. Corona