My previous close encounter with a NuPrime power amp, the Evolution STA, was quite memorable in that it made me realize that the performance gap between Class A and Class D amplification had shrunk considerably. In fact, I had no doubt that in specific system contexts, I would have preferred the NuPrime to much more expensive conventional amplifiers. NuPrime’s Jason Lim is quite proud of the fact that his affordable (sub-$5k) amps can compete with amps costing many times more. According to Jason, a very good amp has to sound “right” with a synergistic balance between detail, dynamics, texture, power, speed, and warmth. During its research and development effort, NuPrime experimented with various design ideas, including GaN devices operating at a switching frequency above 1GHz. This resulted in pushing some sonic characteristics to the extreme, but at the expense of others. Fine-tuning a Class D design is apparently no easy task, but it seems to me that NuPrime is most definitely on the right track.
Currently, the Evolution STA and the Evolution Two (a pure mono version of the STA) sit atop NuPrime’s amplifier lineup. Think of the AMG One as the Evolution Two’s little brother, offering about half the power delivery in a similar circuit topology, incorporating NuPrime’s proprietary “Only Distortion Cancellation” (ODC) feedback circuit. As with the Evo Two, the design was tuned to run around a frequency of 650kHz, as this operating point was determined to be optimal. The front end is fully analog. Both the preamp stage and Class D pulse modulation circuit use the NJR MUSES8820 op-amp, which was specifically designed for audio applications. The input impedance is 47k ohms, which should be satisfactory for most preamps. The main circuit differences between the AMG One and Evolution STA are said to be the power transformer and the op-amp used in the ODC circuit.
My first set of listening impressions was obtained with a system based around the Fleetwood Sound Company DeVille loudspeaker. As it so happened, the AMG One replaced a pair of very good sounding Tube Fantasy 300B SET monoblocks. I was most curious to discover just how well the NuPrime would communicate the music’s inner tension, something that the 300B SET excelled in. This is a top priority of mine, and a perceptual attribute that in my experience only a handful of solid-state designs have been able to fully convey. Of course, the perception of musical tension is a combination of the score’s structural elements such as harmony, melody, tempo, and dynamic markings together with the performer’s interpretation, which in the end grants the music its expressiveness. When it came to dynamic shadings and microdynamic inflections, I was surprised by how well the NuPrime held its own. A CD such Jazz at The Pawnshop 30th Anniversary still sounded uninspiring after all these years, while a nearly 80-year-old recording of Dexter Gordon, Dexter Rides Again, totally came to life.
The AMG One features exceptional specifications when it comes to harmonic distortion and signal-to-noise ratio. However, I’m always cautious and often skeptical when it comes to accepting amplifier specifications as the final arbiter of sound quality. To echo none other than Nelson Pass: “There has been a failure in the attempt to use specifications to characterize the subtleties of sonic performance. Amplifiers with similar measurements are not equal, and products with higher power, wider bandwidth, and lower distortion do not necessarily sound better.” This should not be surprising given that processing of the auditory stream is quite complex and involves billions of neurons to create a perception of the external stimulus. That said, sometimes measurements do correlate with perception, as demonstrated by the AMG One. It floated a soundstage with a remarkably low noise floor and sterling transparency. It was easy to retrieve low-level detail aided by the almost spooky silence between notes. The music ebbed and flowed with exemplary textural purity and without any obvious tonal bias.
For someone like me, in search of a warm balance, maximal tonal color saturation, and, to be totally candid, some tube-like textural liquidity, the NuPrime on its own was just too honest to satisfy my needs. One obvious way of tilting the presentation in favor of tubes was to use a tube preamp. I tried several before settling on the Wyetech Labs Coral with upgraded output coupling caps. The Coral is pretty neutral, but it did add a touch of tube warmth which is essential to my enjoyment of reproduced music. During this process it became clear that the NuPrime accurately reflected the sonic character of the associated preamp and the flavor of the front end. Good to know, since this makes it easy to mold its sonic personality to suit specific system needs.
After all the tweaking was complete, I became fully aware of a highly transparent, wide, and deep soundstage, featuring well-defined image outlines. On a well-recorded choral track such as “Shenandoah” by Cantus, I was startled by the 3D presentation of individual voices. It became clear that the AMG One could deliver a spatial impression that most solid-state amps can only dream of. Bass extension and impact were superb. Upright bass pitch definition was spot on, while tympani strikes took off with satisfying punch. Even in a fairly large room, power reserve was adequate for the DeVille. Transient speed and control were also noteworthy, which accounted for vinyl surface noise being non-intrusive.
I should point out that the NuPrime sounds its best about 30 minutes after being powered on, which brings up the obvious possibility of simply leaving it on to cook continuously. Unlike my Pass Labs Aleph 5, which dissipates 300 watts continuously in Class A and gets warm enough to fry an egg on top of its chassis, the AMG One dissipates a mere 8 watts of idle power per monoblock. Such an Earth-friendly amp is one major advantage of Class D.
It was time to switch listening rooms and mate the AMG One with the MoFi SourcePoint 10 loudspeaker. I was a bit concerned about the possible outcome, since the Evo STA had not turned out to be a good match, sounding too crisp and lacking sufficient midrange textural sweetness. The story was much different with the AMG One. Harmonic textures were much sweeter sounding relative to the Evo STA, eliciting lovely violin overtones, though still not to the same degree as a good tube amp. For example, in contrast with my Quicksilver V4 monoblocks, harmonic textures were somewhat less liquid and rich sounding, most likely the result of the Quicksilver’s distortion spectrum with its much higher levels of second-order distortion products. But as compensation, the AMG One offered far better bass definition and extension, as well as enhanced soundstage transparency and immediacy. Overall, I found the AMG One to be a synergistic match for the SP10, highlighting its best sonic attributes and making for an absolutely enjoyable coupling.
Over the years I’ve been accused of being quite fussy about solid-state power amps, routinely finding fault either due to a lack of textural purity or expressiveness. In particular, solid-state sterility had driven me to seek musical solace in tube gear. That is definitely not the case with the AMG One. It is an enjoyable amp, checking all the boxes on critical sonic metrics. The AMG One is currently not only my favorite Class D amp but also one of my favorite solid-state amps. Retailing at $2900 per pair, I find it to be competitive at even a $10k price point. Plenty of power and Class A-like finesse in an affordable package make for a no-brainer recommendation.
Specs & Pricing
Power output (RMS): 170W @ 8 ohms; 220W @ 4 ohms
Power output (peak): 210W @ 8 ohms; 260W @ 4 ohms
Sensitivity (for rated power): 1.7V rms
Input impedance: 47k ohms
THD/S/N (typical): @5W (0.003%/95dB), @50W (0.004%/105dB), @100W (0.03%/110dB)
Frequency response: 10Hz–50kHz (-3dB)
Standby power: 0.8W @ 115VAC
Idle power: 8W @ 115VAC
Dimensions: 285mm x 55mm x 235mm
Weight: 5 kg
Price: $2900 per pair
NUPRIME INC.
Taipei City, Taiwan
nuprimeaudio.com
SUMMIT HI-FI (U.S. Distributor)
summithifiusa.com
Associated Equipment
Speakers: Fleetwood Sound Company DeVille & MoFi Electronics SP10
Preamplifier: Wyetech Labs Coral line preamp
(upgraded)
Phono front end: Kuzma Reference turntable, Kuzma Stogi Reference 313 VTA tonearm, Clearaudio daVinci V2 MC Phono Cartridge; Sound Tradition MC-10 step-up transformer; K&K Maxxed-Out phono preamp
Digital front end: Qobuz Streaming via Audirvana Studio; Denafrips Terminator & Schiit Audio Yggdrasil+ OG DACs
Cable & interconnects: Kimber Select & KCAG interconnects; TARA Labs RSC 500 Prime speaker cable
Accessories: Sound Application CF-X & TT-7 power line conditioners; ALLDAC USB 3.0 & Topping HS02 USB Isolators; Herbie’s Audio Lab tube dampers
Tags: POWER AMPLIFIER NUPRIME
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By Dick Olsher
Although educated as a nuclear engineer at the University of Florida, I spent most of my career, 30 years to be exact, employed as a radiation physicist at the Los Alamos National Laboratory, from which I retired in 2008.
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