[Listening test] NuPrime ST-10M power amps

Product: Monoblock Power Amplifiers
Manufacturer: NuPrime - USA
Recommended Retail Price: 1895€ each at June 2022
Reviewer: Piero Canova - TNT Italy
Reviewed: July, 2022

Introduction

When, in 2005, Jason Lim and his team filed their patent for a class D amplifier and subsequently they introduced, to the market, the first NuForce power amplifier, it was a revolution in the design of power amplifiers. For the first time, a high power amplifier weighing only a few kilograms, didn't get hot like a stove, and didn't have any problem in driving any kind of speaker. Suddenly, output powers of 150W into 8 Ohms, dynamic power greater than 1000W, and damping factors greater than 4000 were at hand, and the amplifiers weren't too expensive, since a pair of Ref 9 sold for 3000€. They won the "Amplifier of the Year" award in 2005 and the sector press wrote very positive comments about this tiny pair of amplifiers; read our Editor's article of June 2006 and you will understand what I mean.

My conversion to this type of amplifier happened a bit later when, in 2008, I bought my pair of Ref 9 SE which have been with me for several years of listening and tests. Meanwhile, NuForce decided to focus on more low cost consumer products and the original NuForce team decided, in 2014, to create a new company and buy the Hi-End branch naming it NuPrime, which continues until today. If you remember my test of the ZeroZone IRS 2092 monoblocks, I had in mind to test some class D last-generation reference amplifiers to fully understand the evolution that had happened over recent years and what they had to offer today. So, I made a call to the Italian importer Audio Graffiti, where I explained the article I wanted to write. After some discussion, the Audio Graffiti team advised me that the best model to test was the ST-10M since they are the direct evolution of my Ref 9 SE, and after a short while I had them with me to start the test.

What's inside

While waiting for the delivery, I started to look for more information on the web on this specific model. Looking at the NuPrime site, I found a section that alone would be worth a review. The name of the section is “Amp comparison and sonic characteristics”. Scrolling down the pages, a first table gives the sonic characteristics of the different NuPrime models allowing easy comparison of the sonic performance of each model (example: considering the bass extension, MCX1 monoblock is rated equivalent to Evolution one and so on). There follows a section where it is described, in a simple way, how a class D amplifier is built and finally there is a second table that summarizes, according to NuPrime's view, the main sonic characteristics of each model. For the ST-10M they write: “neutral, dynamic, smooth and refined sound”. Each model has a specific sonic characteristic so, if you are looking for a tube-like sound, you should choose the STA-9. The best part of the section is a sentence that says: “We advise customers to make their selection based on sonic preference and usage scenario. In particular, a higher price model does not necessarily mean that it is better”.

Allow me to say that, if 15 years ago NuForce showed that it's not always the case that “good watts = many hot kilograms”, today they do the same to the phrase “the more you spend the better it will sound”, resulting in happiness for many audiophiles who spend many hours improving their systems by listening instead of spending money.

Coming back to our test, I received two nice white boxes, and inside, two amplifiers and the instructions. Immediately you notice the quality of the mechanical construction, which is quite superior to that of the old Ref 9 SE. Furthermore, there is, as usual, the main power switch on the back, but there is also a small standby switch on the front: an excellent idea since, in this way, I don't need to fall to my knees each time I need to switch them on. Here below are the two models side by side.

One of the first things you notice is that they are quite heavier: they are only 6 Kg each, but compared to the 3,2 Kg of the old model you notice the difference. The dimensions are roughly the same: width and height have differences of few millimeters, while the depth is larger by 40mm. On the back panel, the connections are the same so you retain both balanced and single-ended connections. Connectors are always among the best available on the market: they were WBT while now they are ETI.

I opened both amplifiers to compare the construction and I have to admit that these class D amplifiers have circuitry that is quite difficult to follow without a schematic, but at least two things can be noticed immediately. The first is the change of power supply that moves from switching to linear with a nice 300W toroidal transformer. In the picture below the Ref 9 SE is on the left side, while the ST-10M is on the right side. You will notice that the power supply section is much larger and this can partly explain the higher output of the new model. In the NuForce, the switching power supply was split to give the analog input section a cleaner supply; now with the linear power supply, this isn't necessary anymore.

The second thing you get from the NuPrime is the introduction of a pre-amplifier stage before the class D section. In my view, this is the biggest change in the new class D amplifiers. The first class D amplifiers didn't have any stage before the class D module, then they moved to buffers (unit gain circuits) and, finally, now to true pre-amplifiers, with gain greater than 10 dB. The introduction of this circuit offers the line stage a better interface, allows you to fully exploit the potential of the main gain stage and and to change the sonic character of the amplifier.

Do you want your power amp with a warm sound and plenty of even order harmonics? If you design a nice pre-amplifier with these characteristics you will get a tube-like sound. The same if you want a Mosfet class A sound like in the Evolution One. The ST-10M has the pre-amplifier working with op-amps and this gives a very neutral and transparent sound. The main class D gain stage is the V4 model, so it's the fourth generation of the original NuForce class D stage.

How do they sound?

Having almost the same size, the swap between the two amplifiers is very easy. To have a realistic and honest comparison, the NuPrimes had 100 hours of burn-in through a CD player on repeat. As sources, I used the best I have available: a Sony TT 2500 turntable with 12" tonearm and a Dynavector Karat 23R cartridge with a Parasound Halo JC3 phono stage. The second turntable was a Sony TT 4000 with a 13" tonearm and a Shelter 901 cartridge with a BMC MCCI phono stage.

The first difference you notice is the complete silence without any music. The Ref 9 SE was very quiet, but when switching on there was some hiss and, when entering the room, you noticed the system was on. With the ST-10M, there is only silence. I invited some friends to listen to them and they confirmed to me that the noise floor was almost zero, which allows us to better exploit the qualities of the source components. The greater output (230W against 140W into 8 Ohms) is immediately evident and to keep the same listening level you need to reduce the volume. You immediately notice they both come from the same family, but also that they are quite different. I can summarize this by saying that ST-10M does everything like te Ref 9 SE, but much better. It is an improvement through all the listening areas, and clearly noticeable.

Let's start with the virtual sound stage: in the width there is a small improvement; in the depth, there is an evident improvement, but more than that, it is in the way the virtual stage is populated where the improvement is greatest. Each instrument and voice is much better separated and occupies a part of space that's much more precise and contoured. My impression is that with the Ref 9 SE there is the sound of the instrument, while with the ST-10M there is the illusion of having the instrument in front of you.

To illustrate this, take the album “Crazy Eyes” by Poco. In the homonym track, there are several parts played by a solo banjo. With the NuPrime, the instrument is right there in the middle of the room (which for me means 3m from the back wall and 2,5m from each side wall) and you have the temptation to reach out to touch the instrument. Sound layers are well separated in the depth; I never had any effect of collecting different instruments or voices toward the rear wall as can happen with inferior components.

Bass frequencies have a similar extension, but with the NuPrimes, are much tighter and controlled without losing dynamics. Highs are more open and faster; cymbals are natural and well-rendered, both in the percussive part, and in the decay. I left to the last the mid-range, where probably the best improvements have been made. The understanding of words and rendering of voices is much better. Even more impressive is the reproduction of an acoustic piano: it is a very complicated instrument to reproduce, since a note has a first part that is percussive followed by a resonating part where the harmonic richness characterizes the sound of every single instrument. I have invited friends who play piano professionally and they could recognize the manufacturer and the type of instrument being reproduced. Complex orchestration is reproduced, separating perfectly, the different sections; the limiting factor is the resolution of the recording, not in the amplifier.

The sound is the most transparent I have heard; also PRaT depends on what you have before them so with the Parasound it is a bit more sustained while with the BMC is a bit more relaxed. They work fine with average recordings without flattening the good ones.

Some advice

The old NuForces were a bit temperamental and from time to time they had some reliability issues. NuPrimes have never had any problems after installation. They get very mildly warm, possibly less than NuForces. I never experienced any problems with interference or disturbance from other electrical domestic appliances or other audio components. After burn-in time you can leave them on stand-by since I haven't found sizable improvements by leaving them always on. Standard feet are OK, but you can always experiment with some soft ones.

Conclusions

In every test I made, the superiority of NuPrimes against NuForces was evident, but so far nothing really exciting. Normally you expect that, after 15 years of improvements, the new model would be better than the old ones; at least this is what every manufacturer claims. What is unusual is that the improvement is so evident, and all this comes at a lower price than it was 15 years ago. Normally, the MK II or Plus or Signature version costs much more than the standard one, but in this case, in 2006 two Ref 9 traded for 3000€ and, considering average inflation in the last 15 years they are 4200€ in today's currency. The SE version was more expensive and their price today will be above 5000€. The ST-10M pair trades at 3790€ so we have better performance and spending one third less. Do you know other components that by improving become cheaper?

I can't exclude there are better amplifiers, but so far nothing has sounded better in my system and they have evidently improved my setup. Considering that their price is still "reasonable" I am warmly recommending you should listen to them or, better, to listen to an appropriate NuPrime amplifier. I don't know your system, your room, and your taste so perhaps another model in their range will fit you better, costing even less.

Another comment I would like to add is slightly off the review. This test has been made possible thanks to the courtesy of the Italian importer, Audio Graffiti, that has made available to me these two amplifiers for more than a month. The added value was in the discussions that lead to the choice of the right model to test. I am fully aware of the continuing discussion about physical media distribution channels vs digital distribution, and I am personally convinced that the future will be an integration of the two, but in any case, without a contribution of professionalism, the risk of wasting your money is always there.

Post Script

If you have a similar component to hand, it becomes obvious to make a comparison between the two. I am well aware of the price and the construction quality difference, but how do the NuPrimes compare to the ZeroZones? The first two things evident are the output power and the noise floor level. The ZeroZones are much more powerful on paper and this comes out immediately, even if not in the proportion of 3:1, as the data sheets suggest.

The noise floor level of the ZeroZones is much higher; I am not one of those people that put their ear next to the speakers to hear the noise level, but I can't see the point in using a super silent phono stage or line stage to have everything spoiled by the noise from the power amps. In my test on the ZeroZones, I wrote that they could be compared in equal terms with the NuForces, and in some areas, they were better. Compared with the NuPrimes the differences are quite evident and not in favor of the ZeroZones. The virtual stage has a similar width and depth, but the focus on the single instruments is of a different level. Dynamics are similar, but transients are much better handled by the NuPrimes. Bass frequencies are tighter in the NuPrimes, the mid-range has a harmonic content far superior and highs are less digital and harsh. The piano test with the ZeroZones gives an instrument missing many harmonics; horns sound harsher and cutting. With complex orchestras, the ZeroZones tend sometimes to mix everything together. Long listening sessions are a bit tiring with the ZeroZones while much more relaxing with the NuPrimes. From a manufacturing point of view, they are a world apart in favor of the NuPrimes. In short, the ZeroZones are still an incredible bargain but, for me, they were a side step or at the best a minor step forward against the NuForces, the NuPrimes are a substantial step forward.

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