– Dick Olsher, theabsolutesound.com
In case you’re wondering, NuPrime Audio was founded in 2014 by Jason Lim, who previously co-founded NuForce Audio, with the express goal of continuing NuForce’s high-end product line. The Evolution STA amplifier is a prime example, nudging forward the state of the art in Class D amplification.
Right out of the box it was clear that this $4995 stereo amplifier meant business. Heavy for a Class D amp at a net weight of 23 pounds and sporting a power rating of 230Wpc into 8 ohms, it was ready to rock. The front panel is rather sparse looking with a small power LED and a power switch on the left that’s well camouflaged and hard to pick out. No sex appeal here. Both RCA and XLR inputs are provided on the rear panel. Note that the amp is shipped with the voltage selector switch set to 230V. Be sure to slide it fully upward, if like me, you’re in a 120V country.
In a Class D amplifier the power MOSFETs only have two states of operation, ON and OFF. The audio signal is embedded in a stream of square waves, whose width is proportional to the signal’s amplitude. The width of the square wave determines the MOSFET’s ON time. The longer the ON time, the closer the signal is pulled to the voltage rail of the amp. GaN MOSFETs have become popular in the last few years for Class D switching applications, and while NuPrime has experimented with these devices, it concluded that the familiar silicon MOSFETs allow better control of an amp’s sonic character. The Evo STA uses NuPrime’s proprietary low-loss switching technology, which is said to effectively reduce dead-time losses with increasing MOSFET temperature. This allows silicon-device switching at a frequency of 650–700kHz, approaching GaN device territory. The power stage topology is full bridge, providing push-pull differential operation with NuPrime’s proprietary MOSFET drive design.
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. According to NuPrime, it was selected on the basis of its natural, tonally dense, and warm sound character. “Other op-amps might have better specs, but they seem cold or emotionless when compared to the 8820.”
The linear power supply benefits from a 550W, ultra-thin, toroidal power transformer and a total of 52,000uF filter capacitance. It has dual windings for dual outputs—to allow the left and right channels to be configured as mono amps. Kudos to NuPrime for opting to use a linear instead of a cheaper switching power supply. I haven’t been a fan of the latter and tend to blame it for shortcomings in Class D amplification. The entire power path is compatible with 40-ampere current flow and features low-resistance terminals.
Class D amplifiers typically use feedback to improve performance. Feedback here is taken after the low-pass output filter, basically including the speaker in the feedback loop. Dual feedback pathways are used—not only to maintain stable switching at 650kHz, but also to reduce distortion over the audio bandwidth of 5Hz to 50kHz. The resultant harmonic distortion products are said to measure 0.0025% at 5 watts at a signal-to-noise ratio (SNR) of 95dB, and 0.01% at 100 watts at a S/N ratio of 110dB. In addition to this feedback, the STA derives an error signal after the output stage and filter that is part of a proprietary distortion-canceling circuit that reportedly reduces noise. Ultimately, a successful, high-end Class D amplifier requires some tradeoffs in terms of efficiency, distortion, and noise floor. It seems to me that in this case, NuPrime has made the right choices, opting for reduced distortion and noise.
Of course, these test bench measurements typically use a resistor as a load and do not reflect the impact of real-world speaker loads. Therefore, it was my objective to evaluate the NuPrime in the context of three dissimilar speakers—to assess any potential sonic issues.
First up was the Fleetwood Sound DeVille loudspeaker, a fairly benign 8-ohm bass-reflex load. Several sonic characteristics became evident rather quickly. First was the exceedingly low noise floor. Backgrounds appeared to be eerily quiet, more so than with the average solid-state amp and, obviously, without the tube-rush noise that I’ve become accustomed to. Second, resolution of low-level detail was superb. In particular, violin vibrato was exquisitely resolved with the sort of timing precision I’ve only experienced from far more expensive amps. But what surprised me the most was the Evolution STA’s ability to flesh out a believable depth perspective. In fact, the soundstage was exceptionally transparent, wide, and deep, with believable image focus. So now I’m thinking to myself, can a Class D amp really dish out this sort of imaging? And then it got even better. Microdynamics were served up with emotional expressiveness. Orchestral crescendos were unfurled with tremendous power and control. And bass lines were punchy and tightly defined. It began to occur to me that this amp was going to be difficult to criticize.
As I was experimenting with front-end changes, it dawned on me that the Evo STA’s personality was much like that of distilled water, lacking intrinsic flavor. As a consequence, it simply reflected the character of upstream components. It became a question of tailoring the front-end’s tone to achieve an appealing sweet and warm system personality. For me that meant trying out several tube preamps to get the voicing just right. Throughout this process the NuPrime was exceptionally revealing of tube-rolling nuances. This scenario was much different than that presented by, say, a single-ended triode amp, whose distortion spectrum is rich in even harmonics and as a result dominates a system’s voicing. The NuPrime proved to be the total opposite. In this respect, it turned out to be a reference tool by providing a truthful sonic snapshot of upstream components.
Next in line was the Innersound Isis 3.5 hybrid electrostatic loudspeaker. This load has proven to be problematic for some Class D amplifiers due to its capacitive impedance and 2-ohm minimum through the upper octaves. Again, there was nothing of significance to criticize. A minor finding was a slight loss of treble air—an issue I’ve observed with other amps, as well. But otherwise, the NuPrime’s strong suits were still on full display; soundstage transparency and depth perspective being first class. As you would expect from an electrostatic midrange and treble, transients were supremely well controlled, while the bass range was reproduced with control and full extension.
Finally, it was time for the Analysis Audio Omega. This quintessential planar speaker, an Apogee look-alike, has been a staple in my reference system for many years. It’s a 4-ohm nominal load, though it dips to around 3 ohms and demands a lot of current. In this context the NuPrime was able to generate a panoramic soundstage and believable depth perspective. Its level of microdynamic finesse, purity of tone, resolution of individual voices in a complex mix, and rhythmic drive kept me listening late into the night.
On a challenging piece, such as the “Intolerance” track from The Mighty Wurlitzer: Music for Movie Palace Organs (New World Records 80227), the NuPrime was able to nicely capture the tonal complexity of a four-manual, 34-rank Wurlitzer Special that was built in 1928 for the Fisher Brothers (of auto-body fame). Here, but not with the Fleetwood DeVille, I was a bit disappointed with its limited macrodynamic reach. I expected more from an amp that is said to generate instantaneous power in excess of 360W into 3 ohms. It sounded like it was just stopping short of fully scaling dynamic peaks. For a better perspective, I switched over to a recent acquisition of mine, the SMc Audio DNA-1/GT-21 Ultra. This Class AB amplifier represents a complete overhaul of the venerable DNA-1 and incorporates Steve McCormack’s latest circuit refinements and tweaks. On the Omega, the GT-21 Ultra was just crushing it with stupendous bass slam and macrodynamics that reached Olympian heights. It also became clear that the GT-21 Ultra offered better image focus, and as a result, more incisive spatial resolution. However, the NuPrime did have an edge in terms of soundstage dimensionality. Both amps resolved low-level detail exceedingly well, though the edge goes to the GT-21 Ultra.
The NuPrime Evo STA caught me by surprise. I didn’t expect any amp at this price point, let alone a Class D amplifier, to be capable of such exquisite musicality. Mated with a compatible speaker load, it is deserving of an enthusiastic endorsement. It will not be leaving my system any time soon.
Link to the review --> https://www.theabsolutesound.com/articles/nuprime-audio-evolution-sta-power-amplifier/
