Solid State Power Amp Reviews

A year or so ago, in my review of the Pass Labs XP-30 preamplifier, I wrote that the heart of an audio system is the preamplifier, in that it sets the overall quality of the system's sound. But it is the power amplifier that is responsible for determining the character of the system's sound, because it is the amplifier that must directly interface with the loudspeakers. The relationship between amplifier and loudspeaker is complex, and the nature of that relationship literally sets the tone of the sound quality.

I first became aware of the significance of a system's amplifier with one of the first reviews of mine ever published: of the Krell KSA-50 amplifier, in the August 1983 issue of British magazine Hi-Fi News & Record Review. By May 1988, when I reviewed the awesome Mark Levinson No.20 monoblocks for Stereophile, I had become convinced that while a great amplifier can wrest great sound from modest loudspeakers, a modest amplifier will not do the same for great speakers. For the past few years, therefore, I have had on long-term loan an arsenal of monoblock amplifiers to try with the speakers I review: Lamm M1.2 Reference, Classé CTM-600, MBL Reference 9007, and Pass Labs XA60.5 (footnote 1); and, every couple of years, I drag out that original KSA-50 that I purchased following my 1983 review.

The four monoblock models are all great amplifiers, but each imposes on the system's sound a different character. The Pass Labs XA60.5s excel at soundstaging and image palpability, but lack the iron grip on a loudspeaker's bottom end exerted by the high-dynamic-range Classés; the Lamms' forceful sound lacks some of the MBLs' subtlety, though the 9007s' low frequencies lack a little by comparison in terms of ultimate authority. Which amplifier I prefer has depended on the loudspeaker I'm reviewing.

The MBL 9007 was reviewed for Stereophile by Michael Fremer in September 2006—an eternity ago in amplifier years. So when, at the 2013 Consumer Electronics Show, MBL North America's Jeremy Bryan showed Larry Greenhill and me the German company's new Corona C15 monoblock ($25,000/pair), I put it on my "must review" list.

The Corona Line
"Corona stands for the ideal fusion of high end sound & and innovative design concept. . . . [a] perfect match for almost any type of room interior," says MBL in its literature. The Corona C15 certainly looks very different from the high-end norm. There are none of the usual black heatsink fins, as the C15 uses a cool-running class-D output stage. Instead of the ubiquitous rectangular box, this modestly sized amplifier comes in an elegant white- or black-gloss–finished case, its center section finished in gold, or a chrome-like finish MBL calls Palinux. No fasteners are visible. Although its case is made of 4mm-thick aluminum (the vented rear panel is steel), the C15 is surprisingly heavy for a class-D design, at 48.5 lbs. This is due to the use of extensive internal magnetic shielding of mu-metal, an alloy of nickel, iron, copper, and chromium (or molybdenum).

A turquoise, fluorescent, alphanumeric display is set into the 16mm-thick front panel. In conjunction with three small buttons above it, this display allows the user to choose the input, access the setup menu, and switch the C15 in and out of Idle mode. A button at the bottom left of the front panel brings the amplifier out of Standby mode. At the front of the top panel, a circular, 40mm-diameter button bearing the MBL logo and illuminated with a white ring allows the display to be dimmed in three steps or turned off completely.

On the rear panel are RCA and XLR input jacks, two pairs of high-quality binding posts, an SD card slot for firmware updates, a master on/off switch, the IEC AC input, and two RJ45 jacks for MBL's SmartLink, to allow communication among system components. The C15's rated output power is 280W into 8 ohms or 480W into 4 ohms (footnote 2), and its gain can be switched between 20dB balanced, labeled "XLR" on the display; and 26dB balanced or single-ended, respectively indicated by the display as "XLR High" and "RCA."

Design
Although I've said that the Corona C15 has a class-D output stage, MBL's chief engineer, Jürgen Reis, refers to the design as a "Linear Analog Switching Amplifier Design" (LASA). As he explained in an e-mail, "the only thing our LASA concept has in common with typical class-D is the low heat radiation. Nothing else. LASA stands for the complete concept, not only for the switching core module." The C15 uses a version of the well-respected Hypex class-D module, designed by Bruno Putzeys, but Jeremy Bryan, clarified via e-mail that "the boards that contain the UCD modules are made for MBL to our proprietary specification. They are not standard boards available to any consumer or other OEM customer." Reis confirmed that MBL's OEM modules "differ from the regular one in parts, measurements and sound . . . This work with Hypex modules started about 7 years ago. Those [first modules] were good, but it needed time [for me] to understand what parameters influence the sound . . . and to change the good basic concept [to the point where it matches] our 'house sound.' . . . [W]e changed the modulator behavior in order to get our requested THD performance and changed the input parts to get the tonal balance required."

The LASA concept includes using a linear power supply rather than the switch-mode supply that many companies use. "Our power supply consists of a toroidal power transformer that has mu metal shielding to prevent magnetic disturbance from the mains [from entering] the unit," Reis explained, adding that the transformer also has electrostatic shielding, "to prevent any stray coupling between mains and the unit." There are separate power supplies for the control and audio circuits, and complete galvanic isolation between all the supplies. MBL also uses low-drop, fast-recovery rectifiers to give a low level of power supply-generated noise.

In terms of sound quality, of primary importance in the LASA concept is that, unlike a conventional class-D design, its level of distortion doesn't rise as the frequency increases (see my "Measurements" sidebar). The distortion level is also claimed to vary very smoothly with output power. Together, these factors mean that the character of the amplifier's sound should not change with different musical instruments and/or at different loudnesses. The C15 also features "soft clipping," said Reis: "even in the moments when the signal goes into clipping, the sound doesn't get aggressive or too bright and always remains in the balanced tonal character."

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Footnote 1: The Classés have since been returned—I felt guilty for having hung on to them for so long. I've offered to return the Lamms, which I've had for almost as long, but Vladimir Lamm has told me that he's okay with extending the loan period. I held on to the Pass Labs and MBL 9007 amps specifically to use as references for this review.

Footnote 2: While I was preparing this review, Jürgen Reis e-mailed me to let me know that the specifications in the Corona C15's manual were based on initial production, and that two things have since been changed: "We improved the power transformer behavior to have a better common- and differential-mode suppression against the mains (which improves the sound), but that reduces the power output a tiny bit from 500W into 4 ohms to 480W. We also made the current protection a bit safer and reduced the maximum output current (depending [on] the time width of the impulses) from 36A to 28A. This reduced the steady-state output power into 2 ohms, but not for impulses shorter than about 100ms."