Music in the Round

Last spring, when I was listening to Bowers & Wilkins's 802 D3 Diamond loudspeakers, Classé Audio offered a pair of their new Sigma Mono amps for the review. They claimed a synergy—B&W's D3 series had been developed using Classé amps. I declined, only because using unfamiliar amplifiers would add to my assessment an uncontrolled variable. Now that the B&Ws have settled in—three 802 D3 Diamonds across the front, two 804 D3s at the back—it seemed time to hear what they could do when driven by the Classés.

Unlike most other recent class-D amplifiers, the Sigma Mono ($8000/pair) is not based on an OEM module made by Hypex, Bang & Olufsen, or anyone else. The entire circuit was developed in-house by Classé and, in addition to the Mono, is offered in two- and five-channel versions (Classé's Amp2 and Amp5, respectively, footnote 1). Although it uses a Bridge-Tied-Load (BTL) topology like the Amp2 and Amp5, the Mono is not a bridged version of an Amp2, but a larger, more powerful version of a single Amp2 channel. It has an upgraded, more costly input circuit, and the timing of the output stage has been tweaked to optimize the performance of the more powerful output stage. The Mono also adds a line-level output that passes the input to another amplifier, and it's completely buffered: It doesn't matter if you go in balanced and come out single-ended, or vice versa.

Although all Sigma amps occupy the same case (17" wide by 3.75" high by 14.6" deep) and are of similar weights (22–23 lbs), the Mono develops nearly twice the power (350W vs 200W into 8 ohms), has lower total harmonic distortion (0.012% vs 0.018% at 1kHz), and lower noise (>109 vs >100dB at peak output). These are not huge differences, but they're significant. Much the same might be said in a comparison of the Sigma Mono and Classé's CA-M300 monoblock ($5500): The CA-M300 has marginally lower THD, intermodulation distortion, and noise, but also has marginally lower power output while weighing more than three times as much.

Compared to the silvery brawn of Classé's Delta series, the look of the Sigma models is dark and cool. The front surface includes an ornamental, softly curved inset panel, but the LED-illuminated pushbutton at the upper left is functional: Push it to toggle between Operation (blue) and Standby (red). Flashing indicates Initialization (blue) or Protection Circuit Engaged (red). The rear panel has pairs of RCA and XLR inputs as well as RCA and XLR signal outputs—all extremely useful in bi-, tri-, or even quadamped systems, or to drive a subwoofer. The Mono supports easy biwiring via its two pairs of multiway speaker binding posts. In addition, the Sigma Mono offers: RS232 and CAN-Bus communications and amp control through RJ45 ports; IR control and DC trigger in/outs; and a USB port for firmware updates. There are also an IEC AC socket and an external power fuse.

Setup consisted of connecting each Sigma Mono to my Audio Research MP1 preamp with an AudioQuest Earth/DBS XLR interconnect, and to a B&W 802 D3 Diamond with AudioQuest Oak/DBS biwire speaker cables. AudioQuest and Kubala-Sosna power cords provided the juice. When these were connected, the Sigma Monos switched on, and each power-button LED glowed red (Standby). I pressed the buttons, and a few seconds later relays clicked, the LEDs turned blue, and the Monos were ready.

For the past year or so, as I've investigated power amps for my three front speakers, I've found that every good amp nevertheless has a subtle but definable audible character that's often easily apparent. Sometimes, even if the character was benign, it seemed that something was missing or slightly emphasized. Anticipating hearing another such character, I began playing music.

I was gripped: The music was all there, but I was—I still am—at a loss to define any detectable character. Across the audioband, there was no part that stood out from the rest, and so, in that regard, there was nothing worthy of comment. My reference recordings all sounded disarmingly natural and devoid of distractions. That drove me to select at random stereo and multichannel recordings, and every time, there was a sense of discovery—a sense that this must be the way it should sound.

Powering the 802 D3 Diamonds was a piece of cake for the Sigma Monos. Demonstrating the new system for a friend, I played Saint-Saëns's Symphony 3, "Organ," in the recording by Christoph Eschenbach and the Philadelphia Orchestra, with organist Olivier Latry (SACD/CD, Ondine OSE 1094-5), at a nice high level. The organ-pedal notes were firm and body-throbbing. The Sigma Monos and 802 D3s shook the room without any smearing of the notes. My friend saw my pair of JL Audio Fathom f113 subwoofers and thought they sounded impressive—until I told him that this was a 5.0-channel recording with no bass management: the subs were not engaged.

Equally but differently impressive was Biber's Missa Salisburgensis, with Jordi Savall and Hespärion XXI (SACD/CD, Alia Vox AVSA9912). Even before the music started, I could hear and feel the immensity of the space. Somehow, that space never completely filled—even Savall's large forces were precisely incorporated within it. Marc Minkowski's inventive Une Symphonie Imaginaire, based on selections from Rameau's orchestral and ballet music and performed by Les Musiciens du Louvre, led by Minkowski (SACD/CD, Archiv 00289 477 5578), fairly leapt into the room, from the opening bass-drum tattoo through the wildly varied and mostly dance-like melodies. Instrumental detail and image placement were qualitatively different from what I heard with the Biber, but were equally convincing.

Voices and instruments were consistently reproduced with their full tonalities intact, throughout the audioband and regardless of their positions on the soundstage. From the grit of B.B. King and Eric Clapton's Riding with the King (DVD-A, Duck/Reprise 47612-9) to the suavity of the Guarneri Quartet's performance of Ravel's String Quartet in F (DVD-A, Surrounded-By Entertainment SBE-1004-9), everything was effortless and engaging.

Most excellent power amps—the McIntosh Laboratory MC-303, the Parasound Halo A 31, the Theta Digital Dreadnaught D—are so heavy as to be nearly unmovable. What I want is one—or three—small enough to be hidden behind the speakers, not too heavy to move around, yet powerful enough to do the job: Bel Canto Design's REF600M, NAD's Masters Series M-22, Benchmark's AHB-2. The Classé Sigma Mono now leads that list.

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Footnote 1: For descriptions of the underlying engineering of Classé's Sigma power amps, see Classé's website and my review of the Sigma Amp5 in the May 2015 issue.