The NuPrime STA-9 stereo power amplifier is a hybrid design – Class A for warmth and musicality, Class D for power, efficiency and speed. The amplifier is rated at 120 watts per channel into 8 ohms stereo or 290 watts in mono. Never before has this level of sound quality and power cost so little. NuPrime allow easy bridging to more than double the power output and turn the STA-9 into a mono amplifier which means you can run two power amplifiers.
All about the NuPrime STA-9 Bridgeable stereo power amplifier. The NuPrime STA-9 is a stereo power amplifier which seems simple enough. But for many, the question actually is ‘what does a power amplifier do?’ The answer is that it makes speakers sound much better.
Never before has this level of sound quality and power cost so little. To expand upon this, we look at what speakers require to make them deliver all that they are capable of. A speaker is effectively a motor – you feed electricity in and the cone moves. Music happens. The trick of course is that the cones move at a wide range of speeds and distances to cover the full frequency range of music.Not so long ago we though of music as existing in a range from 20 Hz to 20 kHz to arbitrarily cover our hearing range. But with high resolution audio we now know the range is much broader. The frequency response of the STA-9 stretches from 10Hz to 50kHz – which gives both deeper bass, a far sweeter and more detailed top end with wickedly fast transient speed.
Speakers need a mix of both voltage and current (wattage is simply volts x amps) – so while the STA-9 is rated at a generous 120 watts, this is a dynamically changing mix of both voltage and current. By now you can probably see there is a lot more to power amplifier design than first meets the eye.
The STA-9 is the latest expression of a long line of power amplifiers from both NuForce and now NuPrime. NuForce built a reputation on what they called ‘switching analogue amplifiers’ that made a virtue of light weight and cool running with their switched mode power supplies and an original take on what is known for want of a better terms as Class D (for digital).
The latest NuPrime models differ in two important aspects. This is made obvious them moment you pick on up. They may not be large but they are now heavy. The power supplies are now toroidal transformers rather than SMPS units. This power amplifier also runs slightly warm – while this may not be unusual for any other designer, it is significant for NuPrime. That said it’s still only going through a modest 16w while idling so the case work is barely warm.
The STA-9 amplifier is derived from the ULCAM (Ultra Linear Class A Module)+Class D design first introduced in the IDA-8. By joining a powerful high impedance singled ended amplifier circuit in the preamp stage with a well integrated Class-D power stage, the STA-9 is intended to deliver a rich warm sound. But with the remarkable speed and clarity of NuPrime Class-D design.
In other words this is a hybrid design – Class A for warmth and musicality, Class D for power, efficiency and speed. You can read the full specification on NuPrimes own product page here.
The STA-9 is small by any standards – half normal component width, half the depth of the ST-10 and slim in profile. There are no external heatsinks or front panel switches. All the action is on the rear panel with a set of substantial speaker cable connections, and both balanced and RCA inputs with a selector switch. And for the first time with NuPrime a switch for either stereo or bridged mono operation. The amplifier is rated at 120 watts per channel into 8 ohms stereo or a stonking 290 watts in mono.
As a power amplifier, the NuPrime is simply great value by any measure – derived directly from their very best designs, it hardly skimps on power or connectivity. But given the swing towards integrated amplifiers, where does it actually fit in the scheme of things?
Obviously there is the pairing with the NuPrime DAC-9. There are clear benefits in having separation between digital and analogue – both electrical and physical. Separate and optimised power supplies likewise. The original reasons for having separate components have always been valid – and despite NuPrime making two exceptional integrated models, they really are at their very best with the separates – each design shows a clarity of focus and lack of compromise that delvers substantially better performance than the all in one solution.
What actually makes it all work so well for NuPrime is the small size – in the past we have always looked at power amplifiers as being large brute force designs – heavy and often hot. The NuPrime hybrid model changes this and also brings costs down dramatically – smaller more power efficient designs cost less to build, less to package and less to ship – yet do more and sound better. Also factor in the fact that modern speakers tend to be more efficient, wider bandwidth and easier to drive – the STA-9 is clearly a product of its time.
We can draw an immediate comparison between the sound of the amp section in the IDA-8 integrated and the STA-9 – it is immediately more open and expansive, with better bass and drive. Although there is only a 20 watt difference in specification, the STA-9 clearly has much more going on.
We’ve often taken lower cost power amplifiers home, both for audition but also just to keep us going when we shuffle our system around – what has become obvious is that there is a very definite level at which the sound drops quality below a threshold in terms of both musicality and power that we find acceptable – there is a world of difference between ‘good for the money’ and a something that really spins our wheels. The good news is that the STA-9 has no problems at all in meeting this challenge. The original NuForce STA-100 just scraped in, but the NuPrime sweeps in. The resolution and low noise performance allows it to slot in with a bevy of far more costly components without giving any hint of compromise. This is a power amplifier that I’d be quite happy to live with. Which is all the more remarkable given it’s low cost.
The NuPrime STA-9 could find its way into any number of systems – despite the small size and low cost it would be a great upgrade for many aging NZ made or Rotel power amps out there. But you’d have too also consider the quality of the input signal. We’d suggest you’ll get more out of it in a modern system.
There is a further twist to the STA-9 story. NuPrime allow easy bridging to more than double the power output and turn the STA-9 into a mono amplifier which means you can run two power amplifiers with no extra cabling – this is an instant upgrade for any system with 290 watts per channel on tap. There are no doubts whatsoever that this works and allows you to sequentially upgrade your system for a modest cost.
The obvious question you’ll want to know the answer to is – do 2 x STA-9s in bridged mode outperform the ST-10 reference series stereo power amplifier? If you just went on the rated power this would appear to be the case but there is a lot more to amplifier design than a single number. What you do get with bridging is an increase in immediacy and control – bass tightens up while the dynamics become close to explosive. The character of the amplifier becomes faster – effectively turbocharged. It’s a lot of fun. If you have one amp that’s been in place for a while, it does take a day or two for the differences between new and run in to even out and I’ll admit to being a little worried at first as it seemed a step backwards in terms of subtlety. The soundstage initially lost coherence with the mix of new and run in, but 24 hours later things were falling into place and we were better able to appreciate what a pair of amplifiers could do. Given our experience with NuPrime I suspect that once you got a month down the track you’d never contemplate going back.
But the ST-10 is still my pick if you can make it to that level at the outset given there isn’t much difference in price between the single Reference series or a pair of ST-9s. The bigger power amplifier is richer, has a better sound stage and greater levels of detail and subtlety – the tone is more natural and relaxed than the bridged models. There are plenty of clues as to why this is so in the specification – the ST-10 has a significant advantage in gain, a lower noise floor and markedly higher input impedance. It’s switching speed is also higher which has always been one of the selling points as to why NuFore/Nuprime outperform other similar designs – so even within their own range there are levels of performance.
This isn’t to say that the building block approach isn’t without merit. Another scenario to consider the the STA-9 is bi-amping. This is a little more involved than bridging, but ultimately I think it is a better way to go. There is no reason why you can’t initially bridge and then later mover to bi-amping. What is required is an extra set of speaker cables and interconnects – and we assume that your speakers have dual terminals that would normally be used for bi-wiring.
While bridging makes an amplifier work harder, bi-amping effectively lightens the load. Each amplifier section is only driving a section of the speaker and the cable paths for bass and treble are separated (part of what makes bi-wiring work is the doubling of cable volume with the effective drop in resistance.) Of course you still get 120w +120w per channel so that’s 240 effective. You still need dual feeds into the power amplifier. This can be accomplished with the appropriate adaptors of cables and any compromise is more than compensated for by the advantages of bi-amping.
This also leads to another scenario which is actually the best of all. The NuPrime HPA-9 has dual outputs and this is partnered with a seriously capable analogue preamplifier section. This is where you find out just how capable the STA-9 is – the improvement in dynamics and overall goodness is quite remarkable. At this point I think yes – this is at the same level as the ST-10. You have more power and the benefits of separating bass and treble and also two power supplies. This makes up for some specification advantages in the bigger amplifier and provides a fantastic listening experience. Given the extra cabling, you’ll end up at about the same end cost as an ST-10 but what this does enable is an upgrade path (which is quite likely to be improved upon as time goes on).
So – in summary – if you start with a NuPrime STA-9 power amplifier in any scenario, you can progressively upgrade like this.
- 1. STA-9
- 2. Second STA-9 added, switch both to bridged mode
- 3. Add a second set of speaker cables and interconnects, switch back to normal mode and bi-amp
- 4. Add a high quality preamplifier.
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