Solid State Power Amp Reviews

Of equal importance, said Reis, is the fact that the C15's output impedance doesn't increase with frequency (again, see "Measurements"). Because a class-D amplifier generates high levels of ultrasonic and RF energy, a typical class-D amplifier uses a passive low-pass filter between its output devices and output terminals. This filter needs to be tuned to a single load impedance, meaning that with speakers that have a different impedance, the amplifier either prematurely rolls off the top octave or peaks above the audioband. The LASA design maintains a consistent output impedance, meaning that the rolloff above 20kHz is identical into all loudspeakers, and that there is no ultrasonic peaking.

Sound Quality
I used the Corona C15s for several months of loudspeaker reviews, alternating them with the amplifiers mentioned earlier. Unless otherwise stated, my comments on sound quality are a portmanteau of my experience with all of those speaker models: Vivid Audio Giya G3, Joseph Audio Perspective, and Rogers BBC LS3/5A, the last two also used with the EnigmAcoustics Sopranino supertweeters I review elsewhere in this issue. MBL North America's Jeremy Bryan felt that the C15 sounded its best after being left on for several hours and with its front-panel display dimmed. I concur with him on both counts.

The C15's intrinsic character was all about control, especially the tight control of low frequencies. The Vivid Giya G3's woofer alignment is on the rich side, and benefits from being driven by an amplifier that can keep a tight grip on the bass. In this respect the C15 was as good as the Classé CTM-600—the combination of the Giya G3s and C15s proved optimal in my room, whose lowest-frequency mode coincides with the tuning of the Vivids' ports.

With this setup, my bass-guitar tracks on Editor's Choice (CD, Stereophile STPH016-2) had the perfect combination of leading-edge definition and tonal weight. But the low frequencies were too tight when the C15s drove the Joseph Perspectives, whose low-frequency performance, I suspect, was voiced with softer-sounding amplifiers. Charlie Haden's rather subdued-sounding double-bass solos in Live at Birdland—with Lee Konitz on alto sax, Brad Mehldau on piano, and the late Paul Motian on drums (CD, ECM 2162)—were superbly defined with the MBL C15s, if a little lacking in weight on these speakers.

However, the balance now tended to be on the lean side, though it was very clean. This was not a problem with well-recorded music, which the Corona C15 had no trouble playing loud without strain. A track I've become very familiar with in recent years, from MBL North America's use of it at shows, is "Walchensee, Mondnacht," from Martin Vatter's Klangbilder (24-bit/88.2kHz ALAC file, Martin Vatter label). Recorded by MBL's Jürgen Reis, this track features a close-miked piano that Vatter manipulates by damping the strings with his hands to produce harmonics and various strummed and percussive effects. The C15s and Vivids produced an enormous sound with this recording. But on the rather overcooked recording of Buck Owens's "Foolin' Around" on Bakersfield, Vince Gill and Paul Franklin's tribute to Owens and Merle Haggard (CD, MCA 80018655-02), the C15s sounded somewhat relentless. Great music, however—my thanks to music editor Robert Baird for giving me a copy of the CD.

So, yes—the Corona C15 had wide dynamic-range capability without its intrinsic character changing. But I kept remarking in my listening notes on the amplifier's evenly balanced low frequencies. I had problems mixing Jerome Harris's Taylor acoustic bass guitar when I engineered his Rendezvous (CD, Stereophile STPH013-2, now out of print). Jerome plays very melodically and with an empathetic touch, but if I brought up the level of the bass guitar too much in the mix, it started to compete with rather than reinforce the solo instruments. Some listeners have criticized this album, which made through the first round of votes for the 2000 Grammy awards, for the bass being too quiet. So several years after its release, I tried remixing it—and ended up with almost identical bass-guitar levels.

I suspect that Rendezvous is sensitive to speakers and systems that lack lower-midrange clarity. With the Rogers LS3/5As driven by the MBL C15s, there was an extraordinary evenness to the sound of Jerome's bass. Though lacking the ultimate weight it had through bigger speakers, it spoke cleanly and clearly, to the benefit of the music. If ever there needed to be an example of the thesis I laid out at the beginning of this review, this was it—the expensive amplifiers allowed these speakers to transcend their age and their humble size and price.

The Rogers-MBL combination's superb imaging definition and stability made for a synergistic match with the EnigmAcoustics supertweeter. In March, I recorded New York alto saxophone player Rocco John Iacovone, empathetically backed by Philip Sirois on double bass and John Pietaro on percussion, at the Goodbye Blue Monday club in Williamsburg, Brooklyn. I used a Zoom H4n SD-card recorder running at 24/96, but used an outboard pair of DPA 4011 cardioid mikes rather than the Zoom's own mikes. Yes, this recordist got lucky that night, but the MBL-Rogers-EnigmAcoustics system took me back to the club—not only were the images of all three players solidly positioned in three-dimensional space, but, to a surprisingly lifelike degree, I "heard the walls" of the intimate club surroundings. It is experiences like this with a two-channel recording that make me reluctant to deal with the extra system complexity mandated by surround-sound playback.

The C15's stability of imaging worked with the other speakers. As I write this review, I'm listening to the MBL amplifiers and Joseph speakers reproducing David Oistrakh's 1969 recording of the Brahms Violin Concerto with George Szell and the Cleveland Orchestra (16/44.1 ALAC files ripped from CD, EMI Classics 9 55978 2). (This recording had been sent to me in a pitch-corrected version by John Marks. The A above middle C now equals 440Hz, whereas the SACD transfer he recommended in his 2014 "Records to Die For" was pitched closer to A=446Hz, perhaps due to faulty speed control on either the original recorder or the playback machine; John will be writing about this recording in a future "The Fifth Element" column.) The orchestral image was well delineated, but when the world's largest violin entered at 2:50 in the first movement, my disbelief became impossible to suspend. With the Joseph Perspectives driven by the Corona C15s, everything in this musically extraordinary but sonically compromised 1969 recording was laid bare.

But when the recording was more naturally balanced—eg, Iván Fischer and the Budapest Festival Orchestra's superb 2004 performance of Rachmaninoff's Symphony 2 in equally superb sound (DSD64 files, Channel Classics 21604)—the artifice of both recording and playback could be forgotten.

Comparisons
The MBL C15s didn't produce the same degree of imaging palpability as the Pass Labs XA60.5s. But I'm beginning to believe that these Nelson Pass–designed amplifiers are really quite special, and especially so for their relatively real-world price of $11,000/pair. However, the XA60.5's bass is softer and warmer than the German amp's, and it offers 210W at clipping into 4 ohms compared with the MBL's 480W, which might be a problem with less sensitive speaker and larger rooms.

In direct comparisons, the Corona C15 didn't have quite the silky-smooth high frequencies of the considerably more expensive MBL Reference 9007 ($42,800/pair), but the C15's grip on the low frequencies was superior. This was an advantage with the Vivid speakers but not with the Josephs, whose presentation of Charlie Haden's double bass mentioned earlier benefited from the additional body imparted to his instrument by the 9007s.

Summing Up
The unique selling propositions of a class-D amplifier are that it is efficient, lightweight, and inexpensive. MBL's Corona C15 offers only the first of these, but justifies its shortfalls in the two other areas by offering very high power, and sound quality on a par with what you'd expect from a high-end amplifier with a conventional linear output stage. While it may well sound too lean with some sealed-box speakers, such as some of the Magicos, it will be a synergistic match with MBL's own speakers, with their coupled-cavity low-frequency alignments. That it is also a beautiful piece of gear will be relevant to all but the hairiest-shirted audiophiles. Recommended, especially to those whose speakers need a firm fist on their woofers.