Solid State Power Amp Reviews

Like the 2110, the 2150s produced stable images and clearly defined soundstages with familiar recordings. Inevitably, this meant I heard surprises from some of those recordings—some sounded better, more intense than I'm used to; others sounded more shrill, and/or less enticing in other ways. For instance, Markus Schwartz and Lakou Brooklyn's Equinox, a remarkable digital recording (24-bit/96kHz WAV files, Soundkeeper SR1002) that I wrote about in my review of the Boulder 2110, was intensified by greater transparency, and better edge definition and background "blackness"; other recordings—ones that could use a bit of sonic "slack"—became more difficult to listen to.

For example, a recent reissue of Ella Fitzgerald Sings the Cole Porter Songbook (3 LPs, Verve/Analog Sparks 00007), remastered, all in analog, from the original tapes, sounded wonderfully transparent for a 1956 mono recording, but a bit forward through my reference amps—and that was through either my reference Ypsilon VPS-100 phono preamp or the Dan D'Agostino Momentum Phonostage (see this issue's "Analog Corner"), neither of which is bright or ruthlessly revealing. Through the 2150, this album sounded a bit too forward at what I thought was the appropriate volume. Not that Lyra's Etna SL is at all bright, but a cartridge with a more burnished top end would solve that problem.

On the other hand, the Electric Recording Company's vinyl reissue of Leonid Kogan's recording of the Brahms Violin Concerto with Kiril Kondrashin conducting the Philharmonia Orchestra—a 1960 recording—sounded tonally and spatially just right, taking into account the stereo recording techniques of the day.

Another great recording that was reproduced remarkably well by the 2150s is a reissue of Mahler's Symphony 3, with Zubin Mehta conducting the Los Angeles Philharmonic, Los Angeles Master Chorale, California Boys' Choir, and contralto Maureen Forrester, recorded in 1978 in UCLA's Royce Hall (2 LPs, London/Decca/Analogue Productions APC 117). The beginning of the first movement features rumbling timpani that are supposed to represent organic matter bubbling up from primordial ooze. The 2150 didn't clarify the rumbling, which was good, though it did somewhat better define the timpani compared to my reference amps. The big brass climax at the end of the movement didn't harden or lose its pleasingly warm, burnished sound through the 2150s, but compared to my reference amps, it sounded somewhat drier.

However, when I switched to a somewhat more analytical phono preamp that's currently under review, the sound of it and the 2150s leaned too far in the direction of starkness. Were that my reference combination, I'd have to be more in the Koetsu cartridge camp: that's just the way these component recipes work.

The 2150s didn't add grain or grit to the sound, but neither did they hide what's in the recordings, which other electronics might not so clearly reveal. Still, I felt the Boulders' overall sound, while tonally neutral and utterly transparent, was somewhat dry. Can you measure "dry"? Probably not—but you can hear it.

Swapping Boulder's stock power cords for AudioQuest's Hurricanes
The above listening impressions were heard with Boulder's stock power cords. Think power cords can't possibly alter the sound—especially of an amplifier such as the 2150, with its overkill power supply? Sorry, but when I replaced the stock cords with AudioQuest Hurricanes, there was an immediate improvement: The Boulder amp could now better accommodate a wider range of recordings.

While not radically changed, the sound was now less ruthless—like replacing a fluorescent bulb with one of a lower color temperature. Was the AQ cable acting as a filter that actually decreased the 2150's transparency, and thus its performance? I don't know. What I do know is that sonically compromised recordings now sounded more ear-friendly. When I replayed Equinox, it was somewhat less pleasingly immediate, but it didn't exactly recede into the background or become soft. The same held true with the phono preamps mentioned above. The unidentified one, which sounded more analytical, remained so, but even it was now more listenable. I swapped the power cords back and forth more than a few times; the differences remained consistent.

I also swapped out DACs, alternating between the recently reviewed Simaudio Moon Evolution 780D ($15,000) and my reference Lynx HiLo A/D converter ($2500), which also does D/A. (I'd never before used it as a DAC.) While the Simaudio was definitely more highly resolving and closer in sound to the Boulder, the Lynx's easier-going, somewhat more relaxed sound was more inviting when I randomly "swam" in the Meridian Sooloos server, repeating the playlist through both DACs.

Separating the sound of the 2150 from the rest of the system
Listening to any audio product means that you're hearing not only that component but an entire system's worth of audio components. In this case, it was a lot of them, including three different phono preamps, two DACs, and two power cords. Nonetheless, it's possible, through all of that, to hear the sound of the product being reviewed—a good thing, since that's how we do it around here.

The Boulder 2150 is a state-of-the-art powerhouse of an amplifier designed to impart little or none of its own character on the sound of the incoming signal. Boulder has accomplished this by engineering noise to an absolute measurable minimum. The 2150 did likewise with distortion and every other sonic parameter. Its 0 ohm, nonreactive output impedance ensures that its behavior won't vary, regardless of the electrical characteristics of the loudspeaker in use.

In other words, Boulder Amplifiers has done everything it can to achieve the goal of a blank slate: a product that makes the output signal much larger than the input signal, without in any way altering that signal's electrical and, therefore, audible characteristics. I'm confident that John Atkinson's measurements will confirm Boulder's published specs.

Assuming that Boulder has succeeded in all this means that a system's analog and digital source components (as well as the cables that link them) and the speakers that ultimately transduce signals into soundwaves will have the greatest effects on your system's final sound—because these amps will contribute the least. The remaining variable is probably the biggest: the sound quality of the recordings played. With the right associated gear, the best recordings will be allowed to express themselves as well as or better than you've ever heard them. The worst ones won't be able to hide in the shadows, because these amps cast none.

Mobile Fidelity's recent reissue of Santana's Abraxas is made using MoFi's Ultradisc One-Step" process: instead of using plated lacquers to make two more generations of parts, metal negatives made from the lacquers are themselves used as stampers to press discs—in this case, two 45rpm LPs (Columbia/Mobile Fidelity Sound Lab MFSL45UD-1S-001). Played through the Boulder 2150, these discs produced some of the most dazzling recorded sound I've ever experienced. They also sounded really great through my reference amps—but not in the same league.

Conclusions
Based on having the pair of them in my system for an extended period of time, I conclude that Boulder Amplifiers' 2150 is a muscular, tonally neutral, all-controlling monoblock power amplifier capable of unrestricted macrodynamic excursions and, because of its vanishingly low noise floor, the most delicate and highly resolved explorations of microdynamics. The 2150's high level of transparency let me see into the sound as far as is possible, to pick out the smallest details of music and space.

The 2150's electrical stability produced well-controlled, ultra-solid, well-defined low-frequency response that was neither over- nor underdamped, and remarkably clean, precise, etch-free transient response from top to bottom. In fact, the 2150s' overall grip and control produced aural images and a solidity of soundstage that were, in my experience, second to none. The 2150 was "analytical" only in the sense that it added no sound of its own to the signals it was fed—but this means that the associated gear must be chosen with great care. Owners of 2150s must also be willing to accept the fact that some, or even many, of their favorite recordings don't sound so great.

But nothing made by mere mortals is perfect, and every audio product, despite its designers' best intentions, "sounds." I think that the 2150's sound does have an overall character, and that that character is somewhat dry, and lacking the ultimate in musical flow. It reproduced precise attacks and ear-popping decays, but the sustains of notes between those attacks and decays were less than fully expressed.

By comparison, my reference monoblocks, darTZeel's NHB-458s, don't grip quite as tightly as the Boulders, nor do they achieve the 2150s' levels of transparency and solidity. What they deliver so well, especially for solid-state amps, is music's liquidity and natural flow—due to their more generous sustain. You can't have everything.