– Jörg Dames, fairaudio.de

Translated from German.

The sun shines, its light breaks in the dense foliage of the maple tree in front of the open window of my listening room and envelops it in a golden shimmer when the doorbell rings. The parcel carrier. I press the buzzer. He enters the inner courtyard full of energy and in a good mood, throws me a smile to the first floor and immediately two packages – “Today it’s just power amplifiers, I don’t have to come up there!” – which I’m sure to catch.

What is unrealistic about this story? a) Good weather this summer. b) A cheerful parcel carrier. c) 2 x 290 watt power amplifiers that can be thrown.

Well, c) is not that far-fetched: our silver (black is also available) projectiles measure just 24×28×5.5 cm (W×D×H) and weigh just under five kilograms. By the way, if you want, you can have just a single package slung by the courier: the NuPrime STA-9 offers its owner the choice between mono and stereo operation via a toggle switch, i.e. between 2 x 290 and 2 x 120 watts of output power – we will of course listen to both variants.

Running stereo power amplifiers as monos does not have the best reputation with many listeners: Usually the necessary bridge operation – typically operating with inverted phase and larger voltage swing – produces unfavorable distortion values ​​and is also more sensitive to low loudspeaker impedances . Disadvantages, which NuPrime wants to counteract with a circuit concept called “single-ended direct-inject bridge technology” (the marketing department of NuPrime is definitely sufficiently creative), in bridge operation the sound is therefore “livelier and more dynamic”, according to Jason Wei Min Lim insured by NuPrime. Well, we’ll see…

Incidentally, Jason Wei Min Lim is one of the co-founders of the manufacturer NuForce, which was launched in 2005 and has specialized in Class D technology from the start , whose mobile and smart audio division was sold to the projector manufacturer Optoma in 2014. With the then launched American-Taiwanese brand NuPrime, Jason Wei Min Lim is now concentrating exclusively on high-quality high-end products, so that both providers, who are legally and economically completely independent of each other, get in each other’s way as little as possible.

And of course – how could it be otherwise with this history and the size specifications mentioned at the beginning – the heart of the NuPrime STA-9 also beats in the class D switching frequency clock. Which is set above average at 550 kHz, which on the one hand involves components such as transformers or capacitors works more efficiently or enables smaller component dimensions, but also has a positive effect on the achievable bandwidth. The STA-9 should be able to transmit linearly up to 50 kHz. Not a bad value, but compared to many classic amplifier circuits, which often reach well into the three-digit kilohertz range with their bandwidths, anything but remarkably spacious. In fact, conventional circuits often sound more airy and finely dynamic if they are designed with a broad bandwidth and fast rise, which also allows the negative feedback to work more error-free and in turn favors lower output impedances or higher damping factors .

NuPrime attaches importance to the fact that no external, i.e. supplied, modules such as those from ICEpower or Hypex are used in any of the company’s products. Rather, both the power supplies and the amplification circuits are developed in-house and manufactured in-house.

Incidentally, the energy supply of the NuPrime STA-9 is not the responsibility of a switching power supply, as is the case in the sophisticated Reference 20 , for which a comparatively expensive variant was implemented due to the high performance requirements. With regard to the STA-9, a power supply with a toroidal transformer provides a solution that is both cost-effective and electrically clean, according to NuPrime. Which is not least more robust in daily handling: The small NuPrime can also be active without speaker load or switched on source and they apparently also take less offense at changing the cable when switched on (although of course you shouldn’t provoke this unnecessarily with any audio component).

Class D or not – the input stage of the NuPrime STA-9 works quite conventionally in Class A. And that quite broadband up to 300 kHz and with the conscious use of a “condiment” for the most pleasant sound possible: Odd-numbered distortion components (which have a reputation for sounding unpleasant for the human, at least western-conditioned ear) should be minimized as much as possible, but expanded space allow for harmonic distortion, which is what good tube amplifiers do. According to Jason Wei Min Lim, reminiscent sound design is possible. A development goal that is opposed to a circuit design based on operational amplifiers – such a design cannot be fine-tuned enough to hit the desired dose of spices with pinpoint accuracy. After all, this is the main reason why, despite the higher costs, the decision was made to go for a trickier, discrete circuit design consisting of individual transistors.

Before we pick up both amps and lead them into the listening room, a few words about practice: a STA-9 draws just 13 watts from the socket when idling – commendable – although a certain amount of heat development can be observed during operation, which I would rate as over hand warm. The workmanship of the amplifiers is unpretentious, with the exception of the chamfered front panel typical of NuPrime, there are no “design gags” to be found. On the housing side, the devices manage without any material battles: the side walls and cover plate are made of a continuous, one millimeter thick piece of sheet metal, and the two millimeter thick front panel looks more massive than it actually is due to its bevel. But: All connections and switches leave a pleasingly dignified, DAC-10H – saved at the right end.

The mono variant
Let’s put the NuPrime STA-9 on a leash via XLR – initially as monos. So at least 2 x 290 watts should be available, which increases the chance that you might even be able to send them into the ring together with big power amplifier heavyweights like my NuForce Reference 20 or the AVM Ovation SA 6.2 – which I’m particularly curious about. Of course, I will also go into the operation as a stereo power amplifier below.

And if you are particularly curious to see what this flyweight duo with its – at least on paper – almost impostor-looking output power actually delivers to the loudspeakers in terms of thrust and control, other character traits catch my eye during the first listening rounds. Well, the myth of the harsh digital sound of Class D amplifiers – which, like the STA-9, mostly work in analog anyway – should hopefully have survived anyway: In line with this, the NuPrime STA-9 are definitely one of the types that are absolutely suitable for the long term, since almost conspicuously hissing and hardness-free amps. Don’t worry, the STA-9 can’t be considered a fabric softener either, more on that in a moment when we take a closer look at the topic of “dynamics”. I feel the sound of the STA-9 – rather difficult to describe, Unfortunately, I can’t think of any other words – rather than slightly “oily”. In any case, it doesn’t go so far that the NuPrime would somehow artificially round off what’s on offer. I don’t want to use the often cited special “musical flow” in this context either. No, the STA-9 does have its own, albeit admittedly subtle, note in that regard.

This note may also be related to the clarity and low level of interference that the NuPrime STA-9 exude, which is commendable for this price range: instruments and voices are defined on the one hand wonderfully free of grain, true tonality and (also spatially) pleasantly transparent. On the other hand, this means that the STA-9 arrives without the slightest dose – I hope not only the photographers and Photoshop users among you can understand what I mean – sharpening from: instruments/voices artificially hard or severely outlining or texturing to The STA 90 simply don’t need to suggest precision despite a rather restless sound pattern because of their fundamentally “calm” or better: clean sound pattern.

In terms of space, the NuPrime, as indicated, are exemplary in terms of transparency and location-sharp. But also beautifully three-dimensional – the sound image is involved and separates from the loudspeakers towards the listener.

Some deep bass drum beats and e-bass impulses, isolated cornet horn parts, not particularly dense, partly echoed percussion as well as e-guitar string plucking and harmonica tones: using these ingredients to create an illusion of a well-structured, plastic and therefore the listener-absorbing sense of space, get the illusion Listen to the piece “Floratone” (project/album of the same name, an atmospheric mixture of jazz and dub, which the collaboration of guitarist Bill Frisell and drummer Matt Chamberlain, among others, brought about there, on Amazon) at least also my much more expensive Reference 20 or the AVM Ovation SA 6.2 no better. Yes, the stage depiction is also clearly one of our test subjects’ favorite qualities.

“Floratone” also shows that the STA-9 doesn’t let the listener miss out dynamically: the deep black, sustain-rich bass drum and the rather sporadically rumbling electric bass have the NuPrime just as firmly and controlled in the coarse dynamic grip as they have the abruptly ethereal interspersed guitar sounds, the shaker that continuously rips off the eighth notes or the fine hi-hat impulses in a fine-dynamic, nimble and light-footed manner. I wouldn’t go so far as to claim that a stupendous dynamic behavior is one of the first things you notice when listening to the STA-9 – for example the NuForce Reference 20 “over everything” seem a little more energetic to me – , but there is also nothing to criticize here, the STA-9 are simply free of pick sides when it comes to dynamics.

Speaking of bass: When it comes to depth, the STA-9 are undoubtedly on par with significantly more expensive power amp solutions – integrated amplifiers in or even above the price range of the NuPrime will hardly be able to keep up with them on a regular basis. How the STA-9 makes the abysmal sound parts in downloads “Outafter” (Album: The Eyes of Stanley Pain, listen on Amazon ) more tactile than audible overmy Sehring 902 resounds in the listening room, is definitely worth it – yes, fans of official power amplifier solutions will get their money’s worth here. On the other hand, they have to accept compromises when it comes to level stability: the sound image loses stability on my very low-efficiency Sehring 902 (84 dB) and begins to smear a bit when I switch from high room volume to real party levels. In terms of level stability, the STA 9 do not show any real weaknesses, but the 2 x 290 watts read as more than what the NuPrime actually bring to the voice coils – the big boys from the power amplifier guild, such as an AVM Ovation SA 6.2, are capable of they don’t hold a candle here.

What I haven’t mentioned yet, but which is definitely one of the most important criteria for many: the NuPrime STA-9 are tonally walking on virtuous neutral paths. As already mentioned, the deep bass integrates perfectly with the sound, the mid-range doesn’t leave vocals, such as the bell-like voice of Ross Jennings in Haken’s “Because It’s There” (listen to the album: The Mountain, on Amazon), neither particularly warm nor present / appear bright and the treble is also largely inconspicuous.

Largely? Well, with the wonderful “No One’s Fault” by Triosence (album: Turning Points, intelligent as well as emotional jazz from Germany, which sometimes reminds of EST, definitely give it a listen ) the ethereal shimmer of individual piano sounds becomes a little less towards the very, very top clearly worked out as if it were pure teaching, the fine cymbal webs lack the very last bitairiness. No, the treble does not sound coarse or clumsy at all, as described, it is also pleasantly smooth and suitable for a long time, but on its way to the very, very top, it simply “stops” earlier and therefore does not dissolve quite as subtly as with some other amplifier. The NuForce Reference 20 – also not overly airy, but still sounding fresh – seem a bit more present and appealing in a positive sense. The difference becomes even clearer with the new Bryston 4³ power amp (approx. 6,000 euros), which also serves extremely delicate treble events accurately and is exemplary straight and clear in the high registers.

The stereo variant
The question remains as to what will change if we take a NuPrime STA-9 out of the race and switch the remaining one to stereo operation.

I made this listening comparison with a Wilson Audio Sabrina, which on the one hand is more efficient (87 dB/W/m) and on the other hand is more electrically critical (impedance minimum of approx. 2.5 ohms and phase shifts in the lower registers) than my Sehring 902 (impedance minimum approx. 4.5 ohms). I think two things are crucial: Firstly, the STA-9 plays a bit more openly in stereo mode, a bit more present and rhythmically a bit livelier and more direct than in mono mode – it tends a bit in the direction of the big Reference 20. A trait that varies depending on the pending loudspeakers, but also the tonality of the source can be ticked off under questions of taste and did not surprise me particularly, since a developer friend who uses the STA-9 as work equipment alongside a few other amps,

For me, however, it was surprising that the stereo version on the Wilson Audio Sabrina played with a higher level. On the Wilson, the STA-9 start as monos, dynamically compress from just above high room volume and sound more uncontrolled, wired as a stereo amp, you can almost let the cow fly with an STA-9 in the listening room in terms of volume before the sound of the Sabrina showed first artifacts. Which leads to two conclusions: The concept of the bridge circuit of the STA-9 (we remember: “single-ended direct-inject bridge technology”) does not seem to be more robust against more difficult loudspeaker loads than is usual with other concepts. In addition, it is apparently not possible to generalize whether two STA-9 monos (worked excellently on my Sehring 902 and sounded a bit more sovereign,

Test conclusion

Yes, you can go incredibly far with these small blocks. In combination with my Sehring 902, I found them in mono operation to be damn close to large, several times more expensive power amplifiers. The tests with the Wilson Audio Sabrina show that, in addition to the level stability, the biggest difference to the big transistor power amplifiers is that the heavy guys are usually real universal tools and usually have a wide range of speaker types under control without any problems, while the NuPrime STA 9 reach their limits with electrically more difficult loudspeakers despite their quite impressive power of 2 x 120 or 2 x 290 watts on paper.

Basically, the NuPrime are something like if-then machines: If it fits or if you take the appropriate care when choosing the loudspeakers (although it is probably less the efficiency that matters than the impedance curve and the phase behavior), then the STA-9 can do it to swing up to veritable price-performance hits that will only arouse a few desires for more expensive amps. Whether it should be two STA-9s or even a single stereo STA-9 is the better solution can also be tested on your own loudspeakers.

The NuPrime STA-9 are characterized by …

• an exceptionally clean, gritty-free, clear sound, which last but not least promotes a very pleasant, organic sound representation.
• tonal neutrality. Also depending on the speakers connected, it tends to be a bit livelier and fresher in the treble in stereo mode, and a bit more relaxed/sovereign in mono mode. Either way, the sound is absolutely consistent and coherent.
• Good fine and coarse dynamics, but the STA-9 are only suitable to a limited extent for very high volumes.
• an impeccable resolution, meticulous listeners might only miss the last little bit of high-frequency reproduction – it is commendable that the precision of the STA-9 is not associated with any severity, a pleasant long-term sound image is delivered.
• a well-balanced vocal and midrange reproduction.
• a sufficiently open, stress- and hardness-free treble range, the very top of the mountain air cannot be sniffed quite so distinctly with the STA-9.
• excellent three-dimensionality: localization sharpness and plasticity are in no way inferior to much more expensive amps.
• rather sober, but impeccable processing quality. The mechanical quality of the switches and interfaces seems reliable.
• Efficiency: The STA-9 consumes almost 13 watts when idling.
• due to the stuff at the price-sound hit, if the pending speaker fits, it is advisable to experiment with both the stereo and mono versions.

Link to the review --> https://www.fairaudio.de/test/nu-prime-sta-9-test-endstufe-verstaerker-class-d-1-dwt/