NuForce Reference 18Reviewers' ChoiceThese days, there seems to be quite the tug-of-war going on between analog and digital. Not that this hasn’t been going on for some time already, but depending on where you look, the lines between them seem to be blurring and/or widening. One example is the resurgence of vinyl, in which a bit of nostalgia seems to be mixed with a genuine preference for the sound of LPs, to the point where many audiophiles, once again, now listen to nothing but vinyl, their sometimes very expensive digital rigs sitting mostly idle. Another example is the ultra-expensive tubed or solid-state analog amps pitted against more efficient, quickly improving digital versions of the same. Despite many glowing reviews I’ve read of various nonglowing digital amplifiers, and how they compare favorably with, and even improve on, some well-respected members of the old guard, they don’t seem to be unseating the established and entrenched analog monoliths in numbers commensurate with the extremely positive comments I’ve read. Could it be that old perceptions, combined with a touch of snobbery, are conspiring against what could be, or may well become, the real greater good? 

This was the quandary I found myself in when two NuForce Reference 18 mono amplifiers ($7600 USD per pair) appeared on my doorstep. I had some preconceived notions about how taut yet characterless their bass would be, and how clear yet sterile the rest of their sound would be, with a forward, dry, and probably harsh treble. It took me quite a while to dispel my inherent biases and accept these NuForce amps for what they are. 


When I uncrated the NuForce Reference 18s (yes, they come in an actual wooden crate with substantial padding), I was surprised at their size and heft. Although together they seem to weigh no more than my old-school solid-state McCormack amp, I actually expected something far less substantial. These “digital” amps weigh 16 pounds each. But that erroneous preconception was merely the first of many. 

At least part of the Reference 18’s surprising heft is accounted for by the size of its aluminum case: 17"W x 2.95"H x 15"D is much larger than the cases of earlier NuForce amps, which are only half the standard rack width. Also contributing some poundage is a capacitor bank that’s new in the 18, and which NuForce calls a Cross Matrix Array (CMA). The company told me that this is one of the most important elements of the 18’s design -- at least, it’s what they mostly talked about when I asked what separates the Reference 18 from NuForce’s previous flagship amp, the Reference 9 V3SE. In a nutshell, the capacitors vary in value and polarity, in an effort to reduce unwanted resonance and increase the speed of power delivery, the claimed sonic benefits being increased speed (duh) and better tonality. They also mentioned deeper bass, though they said that could also be attributed to some of their refinements of the amp circuit board and/or chassis. 

NuForce Reference 18

NuForce also states that their amps are not “digital,” but analog switching designs. According to them, analog switching is “built on the principle that a power oscillator can be modulated by an audio signal so that it produces an amplified audio signal via a reconstruction filter without the bandwidth limitation of a conventional, fixed-frequency, carrier-based PWM control. Our design employs a high-performance analog modulation technique and closed-loop control system, which is why we refer to our product as an analog switching amplifier.” This analog switching is also said to avoid phase shift, and to result in lower distortion and wider bandwidth than other class-D designs. Indeed, the 18’s spec sheet indicates a frequency response of 20Hz-120kHz, -3dB, a damping factor of >4000, and an output impedance of <10 milliohms. When I read such specs I tend to be skeptical, but in this case, the bass response and clarity I heard from the Reference 18s would have led me to expect some good numbers in these departments. 

NuForce also claims that the Reference 18 puts out 175W RMS into 8 ohms, or 335W into 4 or 2 ohms, with respective peak power ratings of 325W, 650W, and 1300W (with a hold time of 20 milliseconds). During my listening the 18s never ran out of power, and I never felt that the amps had to strain (although, admittedly, my speakers are not terribly difficult loads). 

NuForce Reference 18

The front panel has a technologically sleek, tasteful, modern look. The unique (in my experience) power switch is not a switch at all, and might confuse those of you who don’t read manuals: The Reference 18 is turned on by swiping a finger from left to right across the black strip at the center of the faceplate. This done, the NuForce logo glows red, the speakers emit a small click, and you’re in business. To shut the amp down, swipe the strip from right to left. Although I found this neat at first, I tired of it -- it didn’t always work at first swipe, and made me wonder just how robust the 18 would prove over time. Still, the amp is meant to be left on at all times, so this shouldn’t be as big a deal for the owner as it is for reviewers. This was the only operational hitch I experienced with the Reference 18s. Otherwise they performed flawlessly, and no matter how high I cranked the volume, they never got more than moderately warm to the touch. 

I did note, however, that despite the 18s being a bit more powerful than my reference amp, a 100Wpc McCormack DNA 0.5 Rev.A, I had to scootch my Bryston BP 6 C preamp’s volume knob to about 1:30 rather than the usual 1:00 position. No biggie. One of the oddities of the Reference 18s -- again, this pertains more to reviewers than to potential owners -- is that you apparently can’t leave them powered up without their being connected to speakers, lest the 18s’ passive components overheat. This meant that when I switched to the McCormack for comparisons, I had to completely power down the NuForces, which obviously caused them to cool down and thus be less ready for a direct A/B comparison when again hooked up. Again, not a big deal, but something to mention. 

NuForce Reference 18

On the Reference 18’s rear panel are both RCA and what NuForce claims are fully balanced XLR inputs, a 12V trigger, an IEC receptacle for an aftermarket power cord, WBT speaker binding posts, and a master power switch. 


It took me a long time to get a handle on the Reference 18’s sound. Though it seemed very good tonally and dynamically, with completely natural detail, at first I wasn’t very impressed. But when I invited Rich, a bandmate and fellow audiophile, to drop by and give his opinion, he took to the Reference 18s immediately. What was I missing? 

It was fairly easy to tell that the speed of the Reference 18’s bass was one of its strengths, as one might expect from an amp of this ilk. There was quite a bit of tonal color along with the quickness I hadn’t expected. The treble was refined yet well defined, and not too airy; along with this, backgrounds tended to be more of the black-silence than the clear-as-air, transparent type. The midrange was very neutral yet full and expressive, again more so than I would have thought for a class-D amp. So far, so good. 

But while these aspects of the sound were individually impressive, the overall soundstage remained a bit cloudy, with discrete elements seeming to blend together in a kind of a fog, rather than each convincingly occupying its own space and exhibiting its dimensional proportions. If the tonal, timing, and other elements are there, some listeners will be fairly forgiving of such a sound. I’m not. In addition to getting the individual notes and sounds right, I want an audio product to present me with a visual illusion of the performance (when the quality of the recording makes that possible), and I just wasn’t seeing it through the Reference 18s. 

Then, one day well into the review period, while working out, I was playing Donald Fagen’s Morph the Cat (CD, Reprise 49975). Out of the corner of my ear, I heard what I’d been missing. I sat down in my listening seat and there it was: the dimensions of both individual instruments and the recording venue were revealed. This screwed up my workout, but I sat there and listened to almost the entire CD -- the Reference 18s were unraveling and presenting this recording in a new way. I don’t associate Fagen -- or Walter Becker, his partner in Steely Dan -- with being overly concerned with the audiophile aspects of sound (although they do a lot of things right), but damn if I wasn’t hearing this disc in a whole new light. 

Why had the sound of the NuForce amps taken so long to open up? Had my ears needed to adjust to what had been there all along? Maybe. Had the amps finally finished fully breaking in? I doubted that -- at that point, I already had well over 100 hours on them. Then I remembered that hidden behind my equipment rack was a new power conditioner I’d installed not long before the NuForces arrived. Although at that point the conditioner already had a couple hundred hours on it, it’s said to require up to 500 hours of break-in before realizing its full potential. I tend to take these break-in numbers with a large grain of salt, but maybe this time there was something to it. Whatever the reason, I finally felt I was hearing what the Reference 18 could do. 

NuForce Reference 18

Wanting to try something a little more bombastic to see how the Reference 18’s newfound abilities would register, I pulled out Stevie Ray Vaughan’s Couldn’t Stand the Weather (CD, Epic 65871) and cued up the title track. Right away, I could tell that the performance seemed bigger and more enveloping, with more of a surround-sound effect going on. Nice. What really grabbed me was during this track’s opening minute, when the band pauses in silence a couple times between phrases. There was a musical tension in those silences that heightened my anticipation of the music’s erupting once more. But those silences were only relative -- during those pauses there are some very subtle rubbing and creaking sounds that the Reference 18s more fully and realistically conveyed, giving me a better sense of the musicians readying themselves to launch the next attack. 

In this way, the Reference 18s made listening a consistently more involving experience, regardless of the musical genre. The tom toms had a much woodier, thus more complete sound that conveyed a greater sense of impact, and that strength carried over to all the other instruments. Stevie Ray’s guitar hung in space as more of a wholly dimensional instrument, just as I’d experience with the Fagen CD. This ability of the NuForce amps also made it easier to “see” the positions of the other players on the soundstage. 

I’d recently picked up a copy of Steve Gadd’s Live at Voce (CD, Varèse Sarabande 403), and had been a little disappointed: This live recording sounded a bit too distant, as if I were sitting a few rows too far back to catch all the wonderful sounds, dynamics, and nuances of the performance. Given the Reference 18’s ability to flesh out the life of instruments and music, I had to hear what the pair of them could do to make this album a more enticing experience. 

Right off the bat, I was pulled several rows forward, and felt more as if I were now sitting in the venue’s sweet spot. Sometimes this effect comes at the expense of a recording’s perceived sense of depth -- everything sounds a bit more up front. But in this case, Gadd’s brushes on his snare drum were still firmly rooted at the rear of the stage, where they should be, and all other spatial relationships among the players were also preserved. The Reference 18s didn’t provide sharp outlines of images, but softer, more organic borders that more accurately represented what I hear at concerts. However, if crisp and sharply defined image outlines are what you crave, this might be something to watch for. 

Which brings me to the only aspect of the Reference 18s’ reproduction of the sound of Live at Voce that wasn’t a definite improvement over that of my McCormack DNA 0.5 Rev.A. The NuForces portrayed cymbals believably enough that at first I didn’t question them, but gradually I became aware that they slightly favored the sound of the metal rather than the sound of the stick hitting the metal. When Gadd fired off multiple strokes on crash cymbal, I was less able to locate the individual cymbals very precisely in space, perhaps because there wasn’t enough well-defined sound of stick striking metal to guide my ears and, thus, my aural eyes. I didn’t record this CD, and I didn’t attend the concert -- the NuForces could have been reproducing the sound more accurately than my reference. But this was something I heard consistently throughout my listening, and was the nearest thing to a character trait of the Reference 18 that I could pinpoint. 

The Reference 18 is an example of what are generically called “digital” amps, which have a reputation for excelling at bass and overall dynamic range. My Soliloquy 6.2 speakers don’t plumb the stygian depths, so I can’t comment on how the NuForces might handle the truly low stuff, but listening to Opus3’s 15th Anniversary Sampler (CD, Opus3 9277), I heard both excellent control and seemingly unfettered tonal bloom in the sound of the double bass. I’ve found that these characteristics can often be mutually exclusive, so it was impressive to hear both reproduced in spades. I also found both micro- and macrodynamics to be excellent in general, and the 18s never ran out of steam or protested in any way throughout the review period -- and I tend to listen on the loud side. 


If you’ve read this far, you have a good idea of how the NuForce Reference 18s compared to my McCormack DNA 0.5 with Rev. A upgrade (around $2500 with upgrades, discontinued). The NuForces threw a more expansive soundstage, with weightier and more believable 3D images; in comparison, the McCormack seemed to put up two-dimensional cardboard cutouts. The amps were comparable in dynamic range, but the NuForces were clearly superior in tonality and tautness of bass. Voices sounded fairly similar through both models, but the Reference 18s unearthed more of the inherent natural tonalities of instruments. I also thought the NuForces were a touch more refined in handling the upper mids, and integrated sibilants more seamlessly into singers’ vocalizations. 

It was in the treble range, and specifically the upper treble, where the comparison could possibly be considered a toss-up -- though I’d bet most audiophiles would side with the NuForce twins. The McCormack serves up a more airy, crystalline sound that was more aggressive, and that I’m sure many would feel is a bit tipped-up. But this sort of sound has its appeal with certain instruments, and it allows reverb trails to hang on and tail off a bit longer. As mentioned earlier, the Reference 18s had a blacker, more dead-silent background, the McCormack more of a clear window picture on the sound; I think this difference was linked to the different ways they portrayed those subtle little sounds that exist in the uppermost octaves. 


Never having had a switching amp in my system before, my preconceptions were to expect a harsh and/or forward treble, a sterile midrange, and excellent bass and dynamic range. I got the last part right, but the rest not so much. It’s apparent that, whatever limitations or hurdles may exist in building class-D amplifiers, NuForce has successfully engineered their way around most of them. Other than the upper treble, which could arguably be a positive or a negative, depending on the buyer’s associated equipment and taste, I can’t name a significant flaw in this amplifier. To be sure, the Reference 18 has some worthy competitors at its not-insubstantial price level of $7600/pair, but they had better bring along their “A” game in any shoot-out with this outstanding class-D amp. Biases be damned. 

. . . Tim Shea 

Associated Equipment 

  • Speakers -- Soliloquy 6.2
  • Amplifier -- McCormack DNA 0.5 Rev.A
  • Preamplifier -- Bryston BP 6 C
  • Digital sources -- Oppo DV-970HD universal A/V player used as transport, Electronic Visionary Systems Millennium DAC 1 D/A converter
  • Interconnects -- Acoustic Zen Silver Reference II, Stereovox Colibri-R
  • Speaker cables -- Acoustic Zen Satori shotgun biwire
  • Digital cable -- Stereovox XV2 coaxial, Apogee Wyde-Eye

NuForce Reference 18 Mono Amplifiers
Price: $7600 USD per pair.
Warranty: Five years parts and labor. 

NuForce, Inc.
382 S. Abbott Ave.
Milpitas, CA 95035
Phone: (408) 890-6840