NuPrime AMG STA stereo power amplifier @totallywired.nz

NuPrime AMG STA stereo power amplifier – NZ$2850

This is a distillation of all NuPrime’s and NuForce’s best work in amplifier design. An amazingly transparent and liquid sounding, bridgeable stereo power amplifier that will both relax and thrill you with masses of previously unheard detail, a warm tonality and what is quite probably the best value/performance ratio of any amplifier on the market today.

After a comprehensive two part review of the NuPrime AMG PRA analogue preamplifier let’s investigate the matching power amp. Equal to the far more costly flagship Evolution Monos in many areas of specification and topology with 700kHz switching speed and 1M ohm input impedance, the AMG STA steps out and defines a character of it’s own.

Understanding how NuPrime have achieved this is a great story. It’s about doing things differently. Most amplifier designers stick to a formula, making small, incremental gains. NuPrime aren’t afraid to embrace new concepts and the STA is the product of this – they have made many significant changes over previous NuPrime designs which in turn are radically different from conventional amplifiers. The end result is an affordable power amplifier that’s not just better – it’s a real ear opener.

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Because the STA is a bridgeable design, you are able to select your required power and performance at the outset – either a single stereo 130 watt or a pair of 300 watt mono power amplifiers. Or upgrade from one to two at any stage in the future. With both RCA and balanced inputs, the STA will deliver fantastic performance in any setting.

The NuPrime AMG component range is enigmatic to say the least – while the compact form factor suggests a replacement for the affordable ‘9’ series, the moment you dig a little deeper, you’ll see that the internals reflect work done with the flagship Evolution models. 

Peter Walker of Quad once famously suggested that all competently designed power amplifiers should sound the same – that in an ideal world they would be a ‘straight wire with gain’. We now see that increasing competence in amplifier design has led to significant improvements, more varied designs and they very definitely do not all sound the same.

We also now understand that there will be interactions between the power amplifier, source components and speakers. And mains power which is too often taken for granted. The AMG PRA is quite possibly unique in the way it addresses every one of these issues and I’m going to tease out all of these threads.

The rationale for a separate power amplifier is as valid today as it ever was – it is about the power to make your speakers deliver their best performance and that to build with purpose, and without compromise makes sense, especially when understanding that high current and voltage are an intrinsic part of any design.

The most obvious break from convention with the NuPrime AMG is size – for too many years we’ve associated size, weight and associated high cost with power and quality. As you’ll now be well aware all  NuPrime amplifiers are a variation of ‘Class D’ and in the case of Nuprime is a label for their self described ‘analogue switching amplifiers’. But the STA is much more than this – it’s a unique hybrid with elements of Class A operation combined with Class D to give the best of all worlds.

One of the advantages of Class D is cool running – so the AMG STA doesn’t need big heatsinks despite the power rating – 130 watts into 8 ohms or 200 into 4 ohms. This alone gives a huge saving in cost and weight. And the power supply can be much smaller and of higher quality as it’s not wasting 80% or more of its capacity in heating up your room.

But even within this particular class there are multiple ways to configure and design and what NuPrime have done with the AMG STA is really special. Take a look inside the STA below – you’ll see that there is a lot going on in there – the large toroidal transformer, a high component count, short signal paths with no wasted space and surprisingly little heat sinking – the black alloy block towards the rear transfers what little heat is generated to the outer casing.

Inside the NuPrime AMG STA Power amplifier.

Most Class D amplifiers are effectively OEM designs using modules originally made by either Hypex or ICE.

NuPrime are different because they are been pioneers with Class D – they design from the ground up and have been constantly pushing the envelope on both specification and performance.

Two of the key numbers that tie the AMG STA to NuPrime’s flagship product – the Evolution One mono power -amplifiers are the switching speed – 700kHz. And the input impedance – 1M(illion) Ohms. On the face of it the AMG is a mini-Evolution.

If it were technically easy to do either of these two things then everyone would – but it’s not. In fact it’s an absolute minefield and requires highly advanced design skills, painstaking component selection and some serious listening. No matter what the type of amplifier, every component choice, and every design decision has an effect and sometimes a cascade of unintended consequences.

With conventional amplifiers almost all the possible permutations have been tried and progress is at best slow, iterative and costly. Class D allows for much more development and hence NuPrime’s astonishing rate of progress.

Now – here’s the really interesting bit; I believe NuPrime have made progress since the introduction of the Evolution and the AMG STA is the result of this. The STA has an astonishing level of resolution and I’m going to go out on a limb and say that I think it is even better than the Evolutions in this. I believe I am simply hearing music on my recordings that I have never heard in any other setting or with any other NuPrime power amplifier. Or for that matter any other brand I’ve listened to.

I think the STA does this in part because it is smaller and has a lower (but still generous) power rating than the other options in NuPrimes range. Improved topology, shorter signal paths, reduced internal RFI and the provision of higher quality components for the same end price as a side benefit of the aforementioned savings, all play their part, and together make a large and clearly audible improvement.

The STA has an immediately apparent ease in which it lets more music through – never forced, strident or strained. The first impression is that in stereo mode the STA is really subtle. I found myself listening into the music in a way that I’ve not before, at lower levels, yet hearing much more in terms of detail. It’s obvious that the STA does things differently.

Combined with this ease and flow is a definite impression that the STA gets things right in terms of timing and coherence. But this is really just the start. As we’ve repetitively pointed out, all electronics and cables do have a warm up period and with NuPrime this process is becoming both more obvious and more extended.

If you look at the internal pics of the STA you’ll see that it is relatively complex when compared to both their other NuPrime models and even more so with conventional designs. There has been a school of thought that suggests less is more with circuit design – ie fewer parts in a design means more music gets through but this is overly simplistic as I’ll go on to show. The complexity and high component count within the AMG STA are (in my experience) responsible for this extended run in period but also for the far greater gain in performance that this process yields.

As great as the initial impression of the AMG power amplifier was for me, the months in which I since spent listening to it have shown that it’s a very special component. In simple terms the detail just keeps coming – there have been a continuous succession of ‘I’ve never heard that before’ moments and the longer we live with the AMG STA the greater the gap between what we thought an album sounded like and how it does now. For tracks we haven’t heard for a few months the difference is astonishing – essentially like having a completely remastered, or even different recording. There is no downside as everything is sounding better.

Putting words to this is difficult as the AMG takes us into uncharted territory. Balance and poise are my best descriptors so far.

When we try to describe sound, it can be quite a few different qualities – sound-staging, bass, treble, detail, immediacy and many others. What the AMG achieves, is to find what I’d describe as a natural and organic balance between all of these qualities. Everything is in proportion and no particular part shouts ‘look at me’. Which takes a while to fully appreciate but you find yourself continually drawn in. The poise comes from it’s even handed approach to every genre but also the accuracy with which the STA delivers.

This has been highlighted by a recent sale where we traded in what we would have described as a pair of reference quality amplifiers from some years ago on the AMGs. Originally far more costly than the NuPrimes and certainly much bigger, their sound is like a disparate grab bag by comparison – some things they do almost as well but in other areas they sound way off track relative to the NuPrimes. The lack of coherence and weird time effects as music seems to change pace, a smearing of the leading edge of notes and a flatness were completely at odds with my memories of when I first heard them. But in retrospect much has changed over that time – both my own experience, quality of recordings and all the surrounding components and cables that I now take as part of a system.

NuPrime MCX2 or AMG STA?

Now here’s a really interesting comparison – Nuprime’s similarly priced (actually a bit less) 550 watt MCX-2 stereo power amp vs the AMG STA. I really like the sound of the MCX – there’s a muscularity and liveliness that makes it a compelling choice for modern genres. Technically, there are big differences between these power amps. The MCX-2 uses a high power 1000 watt switched mode power supply whereas the STA features a more conventional 380 watt linear toroidal transformer supply. The MCX switches at 550 kHz, the STA at an appreciably higher 700kHz. The specified noise floor performance of the STA is also better. 

The AMG is a lot more laid back than the MCX – it just lets things flow in their own time and quietly draws you into the music. The MCX makes it literally jump out of the speakers. And I have to admit I’m torn between the two. 

Despite the slower switching speed I think the SMPS does a lot for the pace of the MCX and obviously allows for the massive power rating. The rating is also attributable to the use of bridged modules within the MCX – if you look inside you can see two separate amp boards but each board is in itself a stereo unit. This means that the gain and sensitivity of the MCX is a lot higher – effectively twice – that of the AMG STA. While the noise floor performance of the MCX is greatly improved over previous NuForce models that have also featured an SMPS, the combination of this, high gain and power also means that it is still not quite up with the AMG.

And this is where the AMG pulls ahead – it’s the really fine, low level detail that comes through – there’s space, sustain and decay that I’ve not heard before in any setting – even with the Evolution Monos. You can be listening to tracks you know really well and suddenly there are whole passages that simply weren’t there before. It’s subtle, but it’s definitely there and it really shows you what a good production can do. And you can hear this at very low listening levels – if anything the AMGs will have you listening more quietly whereas the excitement generated by the MCX points you in the other direction.

Bridging.

This is one of the big advantages of the AMG STA – It can be bridged with the flick of a switch – of course this makes it mono so you’ll need two.

Do this and the AMG STA moves to a whole other level. Suddenly you have 300 watts per channel on tap. That’s Evolution One territory. The sound immediately expands – there’s a much bigger more clearly defined soundstage that advances metres from the speakers. Bass digs deeper and the top end extends and opens out. Every advantage the MCX-2 (or earlier ST-10) might have had over a single AMG STA is gone and in its place an astonishing mix of both dynamics and finesse.

The increased power is obviously part of this but with bridging you also double the gain and improve the sensitivity – this translates to greatly improved dynamics.

We’ve had plenty of power amps we’ve been able to do this with in the past – in almost every case it’s a positive and worthwhile upgrade. But there’s something about the AMG that makes it more compelling than anything we’ve heard before. Conventional bridging can sometimes be a double edged sword with greater power but also less tolerance of speaker loads or increased distortion. But the AMGs are not conventional amps. The internal design of the NuPrime amps actually reduces distortion levels when bridged.

’The standard method for bridging a stereo amp into mono employs a series of op amps (OPA) in order to invert the input signals to one of the stereo amp, thereby creating an opposite phase, which can then be combined to increase power. Conversely, the AMG STA’s innovative bridging circuit utilises a design we call Single-Ended Direct-Inject Bridge Technology, in which the mono mode, in addition to acquiring more than double the power, sounds livelier and more dynamic.’ 

– NuPrime

We’ve experienced this before; the standard internal design of many other NuPrime power amps is a symmetrical pair for each channel (we’ll talk about balanced shortly). But in bridged mode there is no comparison at all – the AMG simply cleans up. The improvement made by bridging with the AMGs is far greater. This is because you will start at a much higher level and so have far more to work with – it’s transformational.

Yet the AMG looses none of it’s subtleties when bridged – draw the volume down and the fine detail, natural decay and tonal accuracy are all still there but enhanced by a greater sense of space and extension at both ends of the frequency spectrum.

Anyway – short story; if you try a second AMG STA there’s no going back. While I can identify some limitations with a single STA relative to far more costly and bigger amps, a bridged pair do pretty much everything I could wish for. 

This makes the AMG STA an option for twice as many systems – it’s the best sounding cost effective 130w power amp for the vast majority of people – fantastic with both small speakers and reasonably efficient floor standers. Or as a pair it’s perfect for the more demanding listeners and harder to drive speaker systems in larger rooms.

I know a few people are going to underestimate the AMG STA – it’s compact size, relatively low cost and perhaps being less well known than older brands may count against it. But I’ve been in this game for over 30 years and have seen and heard more than a few amps – notably having close involvement with all of the big New Zealand made brands. I’ll have the AMG over any of these – there’s not one thing I think they can do as well and you can’t even pull the cost argument as the NuPrime comes in at substantially less than anything you might see as comparable in terms of specification.

Much of this just comes down to smarter engineering and efficiency – less wasted energy and the need to get rid of heat (and the power supply to produce it in the first place), shorter single paths with less interference as a natural consequence, less material cost allowing higher quality componentry. 

Balanced or Single ended?

The AMG STA has both balanced and RCA inputs. The resolution of this amplifier will enable you to clearly hear the differences between interconnect cables in a way that you won’t have before. It appears to be remarkably even handed in bringing out the best with these two formats. For lower cost cables I’m leaning towards balanced options – the lower noise and separate paths for left, right and ground plus the well shielded plugs make something like the entry level Kimber Tonik Balanced a great choice. An added advantage of balanced is that if you want to have your power amps close to speakers but distant from source, this format is both cost effective and immune to interference with the longer interconnect often costing a lot less than speaker cables.

The internal layout and bridging capabilities of the STA leads me to conclude that it is a balanced design.

On the other hand the the KLEI analogue interconnect range takes sound quality a lot further within the RCA terminated format – that’s a story in itself and with some 1m cables that actually cost more than the STA, we’ll have to point you in that direction. But we should get back to the actual subject in hand.

Rather than the question of which cabling format is best, it’s the extremely high input impedance of the STA which actually makes the greatest difference. Without getting too technical, this enables the upstream components – be it preamp or volume controlled DAC to send much more information into the STA and in a more coherent state across the full frequency range regardless of the characteristics of the cable. But it also highlights the abilities of any particular cable.

Any discussion of cables and electronics has to include mains cables. Within the AMG STA there is particular attention paid to mains filtering. There are components in there that look to be lifted straight out of NuPrime’s own high end mains filter and distribution products the Pure AC-4. Rather than render the concept of a better mains cable redundant, the higher resolution produced by this filtering magnifies the response to a better mains cable. I wish I could provide a full explanation as to why this is so – my best shot is that the filter gets rid of noise but the cable enhances transmission and quality. Again with the AMG the higher resolution produced by the lowered noise floor also allows you to hear more of what the cables can do.

One thing the AMG series does do is alter the hierarchy of performance gained relative to cost. This applies as much to the STA power amps as it does to the PRA analogue preamplifier. While in most systems, cable is the most cost effective upgrade, in the context of AMG I conclude that you are better off actually adding components. Two STAs are vastly better than one, the PRA offers an equivalent scale of improvement. And the performance of all 3 together is stellar. Yet per component they are less than some of the cables we sell. This doesn’t mean cables should be ignored or aren’t worthwhile – just if you have the choice, adding another AMG component delivers the greater bang for the proverbial buck. 

The same applies with speakers – very few people have heard their speakers running at anything like their best and by their very nature, changing speakers is inevitably costly and fraught with uncertainty. The benign nature of the STAs means that there is minimal risk of damage to even quite small speakers when driven by a pair of STAs. And for the truly committed a bi-amped system with 4 mono AMG STAs delivering 600 watts per channel is by no means out of the question – this will still occupy less space than most other power amplifiers, generate less heat and most alarmingly, cost less than many single chassis models.

And it’s in this context the AMG-STAs can actually take on NuPrime’s own flagship Evolution Ones head on. Equal in switching speed, with the advantages of multiple separate and isolated power supplies, and twice the rated power for (at the time of writing) around NZ$2500 less. The connections for the AMG option are obviously a bit more complicated so the elegance of the Evolutions should not be ignored.

The key difference between the two options is an identifiable character of sound – the Evolutions make a virtue of neutrality and accuracy – they are absolutely transparent to the components around them whereas the AMG STAs do have a signature. 

This is not a bad thing. Sonus faber speakers are intentionally voiced in a particular way and even within their own range, individual models have clearly defined characters that each deliver a different interpretation of music.

It all makes sense now…

Like the STA, the AMG PRA preamplifier is a leading component in terms of resolution and fidelity regardless of cost. And it’s also got a particular set of sonic qualities. Combine the two and we suddenly unleash the full potential of both. One builds upon the other, amplifying the combined virtues to give you a compact, cool running pre/power system that will outperform both integrated and separate options at a similar cost and a great many for much, much more.

The PRA brings scale, dynamics, resolution and detail to the party. The STA delivers the power with transparency and its inherent subtlety is beautifully balanced by the sweep of the PRA. It’s hard for me to articulate how well they compliment each other and indeed how the end product of both is just so seriously good. The distinct and different voicing of each combine to make a remarkable whole. While each stands up as a separate component that will improve almost any system, it’s together that you get to a level that very few other listeners will ever be lucky enough to hear. To be able to easily take this further with the option of bridging and still be less in total cost than locally made integrated amplifiers, all but defies belief.

Because the the AMG STA has a definite character, NuPrime suggest that this may suit certain musical genres but I take a converse view. Either bridging, the addition of the PRA or both will take the STA to a completely different level that transcends what few limitations the STA may have on its own.

It’s also worth noting that if you have a specific taste it’s highly likely that you have consciously selected speakers that enhance this. What I’ve found having listed to the STA in many different settings is that it is able, by virtue of it’s specific qualities, to make almost any speaker system sound much better.

In terms of tonality, the STA’s smoothness helps tame ribbon tweeters while at the same time the fine detail delivered makes the most of their higher resolution – so it’s great with the better Monitor Audios especially the demanding Studio. By way of contrast the Sonus faber range is all about midrange and the voice – here the STA enhances the ‘red wine’ characteristic yet there is an amazing clarity and openness delivered with vocals and acoustic instruments.

The ability of the AMG PRA to fine tune bass response dovetails in with the STA’s extended but light touch. With smaller speakers and brighter rooms, we can see you taking the PRA’s adjustment up two, three or even 4 steps, making both the STA and speakers sound considerably more powerful in the bass while not diminishing high end clarity. In many ways this replicates much of what bridging does.

By the numbers…

  • Stereo Power: 2 X 130W @ 8 Ohm & 2 X 200W @ 4 Ohm
  • Mono Power: 300W @ 8 Ohm & 320W  @ 4 Ohm
  • Rated / Peak Current: 10A / 15A
  • Gain : 26 (stereo) & 52 (mono)
  • Signal to Noise Ratio : 100 dB @ 10W
  • THD+N : 0.006
  • Frequency Response : 10Hz~50K Hz +-0.2dB @ 8 Ohm.  58K Hz @ -3dB
  • Input Impedance: 1M Ohm
  • Sensitivity: 1.2 Vrms @ Stereo; 0.95 Vrms @ Mono
  • NZ$2850 including GST and delivery

Conclusion

By being different and being willing to use their very best technologies in an affordable power amplifier design NuPrime have delivered a remarkable component. The AMG STA sets new standards in fluidity and detail while being completely user friendly and broadly compatible. It’s also the perfect partner for their equally accomplished analogue preamplifier – the AMG PRA. Separately or together they take NuPrime’s sound and designs to another level. The AMGs are complete vindication for the concepts behind Class D – yet draw upon many other strands with power supply filtering, Class A operation for the input stages, high switching speeds and extremely high input impedance all added to the mix. Each aspect imparts a considerable performance advantage and taken together they add up to something truely special.

Validation

All my writing on the AMG models has been in a comparative vacuum – especially so given the year from hell 2020 has proven to be. There have been no other reviews to reference and I’ve come to conclusions based only on my own listening. So just after I’d finished the article above, it’s fascinating to get a review from Christiaan Punter, the owner of Hifi-Advice.com.

Christiaan is an amazing enthusiast and an obviously astute listener who has access to a range of componentry that most of us can only dream of. Yet, like me he’s got a huge respect for the NuPrime designs and is first and foremost into listening to music for pleasure rather than being hung up on specification.

In his review it’s obvious we’ve found common ground in our assessment of the AMG STA and its qualities – ‘I will agree that the amp is unusually fluid and refined not only for a Class-D design but even if it were a Class AB amplifier. But I never had the impression that the AMG STA is in any way coloured. Rather, it is very pure and neutral and utterly linear, with just a hint of sweetness.

Playing with the big boys – the diminutive NuPrime AMG STA in the context of the Hi-Fi advice review system.

Christiaan’s comparisons with more costly designs go far further than mine as do his expectations of the AMG STA in terms of working with some seriously costly and demanding speaker systems. So you can trust what he says and add it to what I’ve written and I strongly recommend you do take the time to read his articles as I know just how much he has poured into them.

John Ransley – December 2020

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