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NuForce REF 9 - Class D mono power amplifiers

Evolution...we cross the Rubicon!

[NuForce REF 9 - Class D monoblocks]
[Italian version]

Product: NuForce REF 9 - Class D (mono) power amplifiers
Manufacturer: NuForce - USA
Approx. price: 2700 $/€ (pair)
Reviewer: Lucio Cadeddu - TNT Italy
Reviewed: July, 2006

New Force or, rather, NuForce...kinda like NuMetal. The name says it all. NuForce is a branch (spin-off?) of NPhysics, a Company which holds several international patents in electronics and digital control applications in particular. The mind behind NPhysics and NuForce is Tranh Nguyen, former chief engineer of the Tomahawk cruise missile power system, now chief technical officer at NuForce. The other minds behind NuForce products are Casey Ng, VP of product development and Jason Lim (PR).
I won't bore you with details you can retrieve directly from the NuForce site. Anyway, before I start with my review, let me tell you the REF9 power amps have been awarded as Amplifier of the Year 2005 by the US magazine The Absolute Sound. That should say something. NuForce amplifiers have received many other awards and rave reviews worldwide. For a "new kid on the block" that is meaningful, mostly because I haven't seen NuForce buying huge advertising spaces on traditional HiFi magazines.

[The small NuForce team]
The men behind the plan (from L to R): Tranh Nguyen, Casey Ng, Jason Lim

Sooo, what are these REF9 amplifiers? Briefly, they are Class D monoblock amps. Even better, they are revolutionary Class D amps. Proprietary technology and patents aside, it is their sound that makes them so...special. So much that I dare to consider them a kind of Rubicon(1) of modern audio.

I've been lucky to follow the whole evolution of NuForce amplifiers, since their first REF8 monoblocks were released. I've tested various different versions of REF8's and finally tested their top of the line REF9's. Now they have released a "Special Edition" of these that I will test sooner or later. I just tought time was come to take a snapshot of the evolution of this Company, though I'm pretty sure products will get improved very soon. This is the main reason why this review took so long to get published...I was just trying to keep up with the (quick!) evolution pace of NuForce products.

Tech specs

As said, these are Class D amplifiers equipped with switching power supplies. More or less, the same technology is applied by UcD/Hypex and IcePower, though the guys at NuForce prefer to call it Analog Switching Technology, just to differentiate it from the Class D applications that use digital modulation (a la Tripath). Switching frequency of mosfets, on NuForce amps, is the highest available at the moment: 500 kHz (versus 340 and 380 of IcePower and UcD respectively). The pros of this kind of approach are well known: extreme high efficiency (80%), low heat build-up, small dimensions and weight.
Just to state things clearer, NuForce and Tripath (T-Amp, if you prefer) have very little in common. Thinking all Class D amplifiers equal would be a big mistake and a pure technical nonsense...since no serious audiophile worth his weight in toroidal transformers would ever claim all Class AB amps are equal. The "Class" of an amplifier just grossly indicates the technology in use...but says NOTHING on the quality of the component.

The NuForce REF9's are two small monoblocks, which deliver 160 watt RMS on a 8 ohm load, 350 watt RMS on 2 ohm and more than 1350 watt of dynamic power (still on 2 ohm). Hence, don't let size and looks fool you. To get the same power output from a standard amplifier you should use units which are ten times heavier and bigger.
The switching PSU, that helps keeping costs, size and weight down, can be connected to ANY kind of mains supply: froom 84 to 264 Volts, 50 or 60 Hz..it doesn't really matter. This is a big "plus" since the amps can be bought anywhere in the World and used everywhere without mains adapters, trasformers etc. Buying US-Spec'd electronic components, here in Europe, is a no-no...and viceversa, as you may know.
The REF9's have both balanced (XLR) and unbalanced (RCA) inputs, which can be selected alternatively by a switch in the rear panel. Still in the rear you have the on/off switch, the IEC mains cord inlet and the speakers binding posts, of a very user-friendly kind (gone are the good, but hard to use Cardas posts, now available on request).
Up front there's just a blue led that indicates power on...and that's all. Internally, everything seems well designed while the layout is neat and clean. Passive components are of good quality.

These are the main (claimed) tech specs:

[NuForce REF 9 - inside view]

I've spent more than 1 year with NuForce amplifiers, from REF8B's in various releases till these REF9's under test. During this long evaluation period I've tested them with 4 different set-ups and into two different listening rooms. For this reason I think I've been able to get a precise idea of their "family sound". What follows is a review of the REF9's but at the end you'll find a brief comparison with the smaller REF8's.

Evolution...we cross the Rubicon(1)

I can still remember a good album by George Michael, titled "Listen without prejudice". This is EXACTLY what audiophiles should always do (and how rarely they do that!). Prejudices build up during the years, stratify and then fossilize. They quickly become fossil fuel for our beliefs.
One of the most popular golden rules is that power amplifiers have to be big, heavy and insanely expensive. Moreover, they should produce heat like a race car engine. Everything which differs from this cliché is seen with suspect, to say the least. HiFi components, and power amplifiers in particular, stimulate our macho side: we need to exhibit a BIG HiFi system to impress friends, like it was a substitute for our...penis. A big amplifier looks reassuring and makes one think that if something goes wrong with the HiFi system...well, certainly the culprit isn't the amp!
It is not uncommon to read of audiophiles who, describing their system, proudly remark the weight of their amplifier. More or less the same we did when teenagers, claiming to have the bigger penis in our classroom. Centimeters (or inches, if you prefer) have been substituted by kilos (or lbs).

Why are you reading this preamble, you may ask. Well, it is because the NuForce REF9 power amps don't follow the rules above, they are small, lightweight and, considering the power, not expensive. At 2.5 pounds each they can't be defined heavy! There are cables that weigh much more than that ;-)
Their cabinet isn't built like a tank ready for another war in Iraq. You don't need two friends to move these amps, the palm of your hand will suffice. The only thing they share with their bigger competitors is the power output, which is really impressive, especially in dynamic terms. I'll write something about this later, by now just let me try to describe you the way they sound.

The first thing that comes to mind when you hook up the REF9's to you speakers is T R A N S P A R E N C Y. Just imagine the most powerful magnifying glass pointed directly at your system. They are utterly analytic, every nuance or detail gets exposed in a way that makes you wonder where these were hiding before...
Don't think, not even for a moment, that this means the sound is fatiguing or unnaturally hyperdetailed. Quite the contrary: what they do appears like the most natural thing in the World. Music flows through them and they seems to just don't care. There's nothing "in your face", no overbright high range, typical of those HiFi components that tend to impress with tons of stinging, unnatural detail. They don't sound like "Did you want HiFi? Here it is, now be ready suffer!" :-)
No, this is real High Definition, dudes.

The tonal balance tends to be on the neutral side, just a bit "clear", if I may. But please try (again) to understand correctly! If you use "open and clear" as synonyms for "dry, lightweight and clinical"...you're on the wrong route. Better do an U-turn. The REF9's are crystal-clear, I'd say even luminous but they possess a bass and mid-bass range that are simply stunning for weight, power, impact, depth, control and articulation. Their control over the woofers is so extreme that it extends till the first octave where most amplifiers tend to amalgamate everything, mixing attacks with decays, fundamentals with harmonics.
The bass range is nothing short of spectacular: the extensions reaches the inner depths of Hell, while the control makes everything tight and tuneful. If you have already read some review of mine, you should be aware I'm a kind of a bass freak. It is not easy to please me, there's always something to criticize, in this area of the audio spectrum. This time I have no complaints to make: pace, rhythm and timing are at their best, like on good Naim Audio amplifiers...plus you have a "drive" that leaves you speechless. At any frequency, no matter how low, any loudspeaker, any (loud) listening level, these amplifiers shake the woofers like never before but keep them "in control", without any overhang or tendency to the ubiquitous one-note attitude.
In order to be able to properly evaluate the behaviour in the first octave you need speakers with appropriate frequency response and recordings with deep bass, for example the old but always thrilling Cantate Domino (Proprius) or Massive Attack (Mezzanine, Blue lines etc.).

Above the first octave you get a whole brave New World of exciting performance: the electric bass and the drums get reproduced with sheer power and violence, with a sense of "pace" and rhythm that forces you to stomp your feet like crazy. For example, take Rage against the machine's debut album. This is a real torture track for woofers and power amps. Drums & bass, played with all the genuine violence of this crossover band, EXPLODE literally into the listening room. Just crank up the volume and pray for your woofers to survive. If they will, your neighbours won't, I'm sure. Well, when such a high power demand is required to the amps normally they run out of steam in the mids and highs...you get a distorted and compressed sound, with voices that jell like a Cheshire cat.
It becomes difficult to follow each instrument separately, everything becomes confuse and mixed. Not so with the REF9's: even at call-the-police-now! listening levels you can distinctly follow each and any instrument, completely separated from the rest. Each note, each chord...it's like having a recording mixer and moving a slider up and down to isolate any track you like: now the drums, now the guitar, then the vocals and so on ad infinitum.
This process is completely effortless: just decide which instrument you like to follow and concentrate on it. It's that simple. Only, you don't really have "sliders" to move. Quite the contrary, all instruments mix together beautifully, if you just decide to listen to them as a whole. Frankly, this is a totally new experience: I've enjoyed the same phenomenon with other HiFi components, but never this way. Now you can understand when I say these amps are TRANSPARENT. They let you decide to enjoy the full picture or to concentrate on details, effortlessly.

Let me digress for a moment. Many listeners are used to amps that I use to call "mincers": they take the musical flow, cut it into small pieces and then mix everything. As you may know, when you mix too much, you miss the flavour of the single ingredients. This kind of amps give a strong, personal flavour to the Music they reproduce, hoping for the listener to say "yeah, that's the sound I was looking for!". Either everything is turned into a warm and soft mixture or into a dry cocktail. Either way, you lose information.
These listeners, when facing the sound of these REF9's, might claim "There's too much information!". Yes, they might not be wrong: the amount of information retrieved from the source is simply stunning but, hey!, it was THERE, just waiting to be picked up and displayed. An amplifier can't create information, it can just try not to lose bits of it.
Giorgio (Pozzoli, of TNT-Audio) and me have discussed for a long while about the necessity to mercilessly display everything is hidden beneath the pits or the grooves of our discs. Someone might say that not everything is necessary...provided you can enjoy the Music as it was ...real. Personally, I've always preferred a no-nonsense approach. If it has to be called "hifi" then it should get as closer as possible to the Music, i.e. to the way this has been recorded. Any HiFi component that retrieved more information from my discs ended up being much more natural and realistic. Even badly recorded discs might sound better through an extremely revealing system. This is my experience. You might prefer "my-fi" to "hi-fi" but, since the differences from recording to recording are HUGE, I think this fact should be underlined from any component that aims to be referred as "hifi". End of digression.

Finally, let me concentrate on the performance in the mid range. This is the area where the NuForce REF9's perform at their best. Take voices, for example. They do possess a sense of realism that is hard to find on other amplifiers, regardless of price. Modulations of intonation, particular inflections, accents...everything gets displayed with a sense of presence which seems unbelievable. Lyrics are easier to follow, different voices in a choir are easier to detect as are singers grasping for breath. And, of course, every kind of electronic voice manipulation become evident.
Two brilliant examples of heavily manipulated female voices are Celine Dion and Mariah Carey. If you have listened to one of their recordings on a good HiFi system you certainly know what I'm talking about. With lesser systems (particularly, amplifications) you get an unpleasant sensation of distortion and a gross mixture of unnatural effects. Now, with the REF9's in action each and any artificial effect becomes extremely easy to spot and isolate from the rest. It is like the voice engineer (Mrs. Carey has one, for example) brings you by hand and explains what he did, where, when and why: some echo here, a tad of reverberation there, a highly-pitched double-voice in between...you can see them all and understand how they concour to create the final effect.
The overall sensation is much less unpleasant than one might expect. Even highly modified voices become bearable, confirming once again my "more information is better than less information" theory. Since our ears are extremely sensitive to human voices, some extra information in this area can't do any harm. Actually, it will help improving realism.
Male voices need some extra comment. Many amplifiers, trying to give the listener the sensation of a warm and caressing sound, do boost the mid-bass region. As a result, male voices appear warmer and "bigger" than they should be. In real life, singers don't always have warm voices! There are nasal inflections and even dry and harsh intonations.
If a listener is used to this kind of "boost" in the mid-bass he might find male voices a bit "cold" when reproduced by the REF9's. I prefer them this way, since they seem closer to reality. Again, one should choose between "my-fi" and "hi-fi". If you prefer to listen to singers with "warm" voices, even if these aren't warm in real life...well, it's up to you....but then don't call it "hi-fi".

Listening to stringed instruments I can't avoid to remark how much of their harmonic content is preseved by these amplifiers. One of the most popular "urban legends" against Class D amplifiers is that they don't preserve the harmonic content of the instruments. A quick test with the REF9's will prove how false can these statements be.
Of course, it all depends on everyone's listening habits. For example, many audiophiles are used to the presence of (extra) harmonics added by some tube amplifier (read: pleasant distortion). One can substitute the word "distortion" with "harmonic richness" and live happily. Unfortunately, we're all into HiFi here, right? Or, at least, we should.
Since I don't like distortion, even if it is "pleasant to the ear" I prefer something closer to reality: REAL instruments aren't always pleasant to hear (ever listened to a trumpet in near field?).
Actually, the natural timbre of acoustic instruments is perfectly preserved by these amplifiers so each one is displayed with its natural "color": wood, leather, brass appear to be the elements that give "voice" to violins, guitars, drums, cymbals etc.

Micro amp; macro dynamics

As you can esily guess from the tech specs (RMS power and damping factor) these amplifiers are true power plants. They possess an impressive dynamic power reserve and this, matched to an impressively high damping factor (not dependent from the load!) makes speakers driving just a walk in the park. It doesn't matter how hard the load can be or how inefficient the speakers might be, the NuForce REF9's will take the lead and get the dirty job done. Dynamic performance seems limited only by the ability of the speakers to reproduce what comes out from the REF9's binding posts. The first two adjectives that came to my mind, after the first minutes of listening, were: explosive and devastating. When NuForce's Jason Lim asked me for a first opinion on these babies, I replied with a single word e-mail: "Devastating!" :-)
I must confess that, no matter how hard I tried, I haven't been able to make these amps run out of steam. They simply take the woofers and make them dance, literally. If you are planning to upgrade your speakers because you find them slow and lazy, hook up a pair of REF9's before...chances are you'll change your mind :-)
Attacks are lightening fast while decays are precise and temporally correct. Cone membranes stop exactly at the moment they should, without overshooting (thanks damping factor!).

Dazed by this performance, I've decided to put Keb'mo's "Just like you" on my CD player. Then I've cranked the volume up till insanely high levels. Bass slam, control and articulation has remained the same even when sound pressure levels reached peaks of 110+ dBs at the listening seat. A "just like live" experience, without the usual distortion of PA systems...
Don't even try to put them in trouble: your speakers, your ears or your listening room will collapse first. These babies seem to do everything effortlessly (how many times I've used this adverb in this review?) and, despite the fact they can sound extremely loud, they get barely warm....aaaah the joys of Class D amplification! ;-)
Among others, a disc that allows a correct evaluation of impact, slam, micro and macro dynamics, especially in the bass and midbass portion of the audio spectrum, is the renowned (though old) "Sheffield Drum and Track Record", another torture field for woofers and amplifiers.
Take Jim Keltner's drum solo, for example (this is track 6 on the CD). I've always found the kick drum to be extremely fast and precise but even a bit "thin". I've listened to this track hundreds of times literally, with almost any HiFi component I've tested, at home, at HiFi Shows, dealers...everywhere. The feeling has always been the same: quick...but thin. With NuForce REF9's in action impact and speed remain unchanged but - finally! - the kick drum has a body like never before. At last, it sounds like a real, full-sized kick drum. You can detect the high-pitched transient when the membrane is hit and THEN the vibration of the full body of the drum propagating afterwards.

In the microdynamics department, as you can easily guess, the extreme transparency helps a lot. Nuances, tiny variations, small high-pitched transients...all get reproduced naturally and with extreme care, without sounding clinical.

3D soundstage and imaging

A good 3D image depends on many factors but one of the most relevant ones is the ability to extract informations from the musical flow. Especially in the very high frequency portion of the audio spectrum there are informations that help building an accurate virtual picture of the musical event. Now, just imagine a big, powerful light beam pointed at the stage: every musician or singer gets "exposed" (pun intended) without mercy. Any movement, any position, any instrument...you'll be able to detect exactly what's happening on stage! This is possible thanks to an extraordinary ability to retrieve details and put them under the right focus.
The 3D stage is big, granitic and stable: geometric proportions are correctly reproduced and the search for different horizontal planes is no longer a tiring task, requiring loads of concentration ...and imagination :-)
They are there, right in front of you, and you can look at them, walk through and around them, between and behind the speakers and everywhere around them.
Don't think this extreme virtual reality has something "artificial" in it. Simply put, you get closer to the place where the disc has been recorded. If the recording possesses some "spatial" information, it will be displayed in a very natural way. You get, when the recording allows it, a big, luminous soundstage where everything finds its place, with extreme precision. This "picture" remains stable regardless of the listening level, in the sense that it doesn't collapse at high listening levels...provided your speakers survive.

A comparison with the REF8

I've compared the REF 9's with the smaller and less powerful REF8's (various releases) for a long while and here's what I've found. To be precise I must say the REF8's have been recently replaced by the 8.5's but nevertheless the differences might prove to be meaningful, especially for previous REF8 customers willing or planning to ugrade.
The differences are NOT subtle. The first thing one notices is overall dynamics and headroom: the smaller REF8's appear less energetic and explosive and this is even more evident in the first octave. Clearly, especially if pushed hard, the extra continuous and dynamic power makes the difference here.
Then comes the tonal balance. Though undoubtedly belonging to the same "family" (loads of transparency from both amps) the REF8's are somehow warmer and sweeter. For this reason it might be easier to find a good partnership with other components. Hence, while the REF8's might sound "easier" for most audiophiles, the REF9's require some extra care. Indeed, the REF9's are for those audiophiles who are brave enough to search for the ultimate performance in every component. This search might be risky and expensive.
With respect to 3D imaging I'd say there are no big differences, at least in terms of geometrical size. The main difference becomes evident when one tries to focus his attention on details and particulars. The REF9's retrieve and reproduce more details and this greatly helps to make the soundstage more realistic and vivid.
Summarizing: if you are one of the happy REF8's customers and are planning an ugprade...look no further than to a pair of REF9's. You won't regret the move, for sure. On the contrary, if you are skeptic about the possibilities of the new Class D technology then test drive a pair of REF8's, they'll give you a precise idea without turning your world upside down (like the REF9's do).


[NuForce REF9 - rear view]

Manufacturing and finish. Though essential and minimalist, the REF9's are well manufactured, finished and built. Not exactly glamorous, they have that kind of "form follows function" elegance. 100% pure understatement. Unobtrusive, that's for sure. WAF will score high undoubtedly.
For these reasons they are not recommended for audiophiles who like to exhibit their system.
Balanced (XLR) and unbalanced (RCA) inputs can satisfy everyone's needs. The speakers binding posts are very easy to use, certainly more user-friendly than the Cardas posts that were available on the first releases of the REF8's. Now the Cardas posts are available on request.
One of the main customers' complaints is noise. The high switching rates the REF9's use create a strong electromagnetic field that surrounds them. This means you can experience some trouble with FM tuners, if these are placed near the amps or if a proper aerial antenna is missing. Anyway, different tuners can behave differently. In any case, these amps can pick up noise and amplify it quite easily. Ground loops, noisy preamps or sources, badly shielded cables...all can worsen the signal/noise ratio of these fine amplifiers.
The NuForce staff is well aware of these problems and suggests to read the FAQs on noise on their website.
Packaging and owner's manual are simple and essential, but up to their tasks. These amplifiers offer 5 years of guarantee and an interesting "30 days trial - money back guarantee" just in case a customer isn't satisfied with their sound (I'm pretty sure not many REF9's have been sent back to Nuforce...). Check their website for details.

Sound. And now for the hardest part :-)
I'm somehow embarassed since I can't find real complaints to make. One thing is for sure: these are not everyone's cup of tea. They need to be understood and carefully auditioned without prejudice. Some audiophile might "click in" immediately (and fall in love all of a sudden), some other might take months to understand. Those audiophiles who judge components by their weight or price tag will simply refuse to understand, I'm afraid: it is hard to let your world to be turned upside down.
It may sound like a paradox, but one of the best qualities of these amps can quickly become one of the worst complaints: their irreverent and extreme transparency might mercilessly expose other system faults. For example, if your system is the result of a complicated and delicate balancing acts (a warm cable for a bright preamp, an aggressive source for lazy speakers and so on) the REF9's will unveil all the tricks...and every component will sound "naked" with all its pros and cons. Finally, if you build your system with the aim of getting a sort of "my-fi" (instead of hi-fi) steer away from these amplifiers: if you aren't ready for virtual reality you might be shocked.

Some advice

Being extremely revealing one should pay extra care in choosing the right partners. Poor sources or preamps are definitely a no-no, the REF9's show no mercy. As for preamps, NuForce makes one that should be a good partner (we'll review it anytime soon) but even other preamps, provided the quality is high enough, could match well. It doesn't matter the technology used, be it vacuum tubes or solid state (or even passive!), the golden rule is to choose a preamp with a good signal/noise ratio.
As for speakers...you can choose everything you prefer: the NuForce REF9's will drive ANYTHING with ease and authority. They might help you discovering how good your speakers are...
Cables don't seem to be relevant, provided a comparable level of quality is given.
These amps don't run hot, even when driven hard. They are small and lightweight so you can place them anywhere. A good set of antivibration feet is strongly recommended, though. For example, I've obtained very good results with the BrightStar Isonodes.
At the mains end, I would avoid power conditioners or voltage stabilizers, the switching power supply might not like these devices. Use good mains cables, instead...and RFI filters eventually. The mains cords supplied with the amps aren't bad, either: good cross section and a ferrite ring to suppress interferences are a good way to start.
Please don't forget these amps are capable of extremely high peaks of dynamic power: for this reason you should avoid plugging and unplugging cables (be it interconnects or speakers cables) while the amps are on. A violent transient might kill your speakers.
Finally, two words about break-in and burn-in. I'd say 100 hours of break-in are necessary in order to get the best results. They sound good even right out of the boxes, but please be patient, they'll get better and better.
As for burn-in...forget the annoying procedure of turning on the system days before any critical listening. These beasts are "ready to go": just power on and let's go party! Of course, they do sound better after a couple of minutes...but that's all. Relieving, I'd say :-)


The NuForce REF9's are the most evident proof that Class D technology is mature and can perform extremely well, so much to be able to replace "traditional" technology in terms of quality/price ratio, to say the least. Personally I believe these amplifiers represent a "point of no return" in terms of absolute transparency, dynamics, bass control and articulation. They simply re-write the whole concept of quality/price ratio. Actually, they introduce this concept in a niche (high-end) where it was missing. High-end amplifiers weren't used to be judged in terms of quality/price ratio!
It is hard to go on ignoring one can get this extraordinary level of performance at this price.

I'm quite sure die-hard traditionalists won't accept this sign of the times. I'm not optimist. Still, I hope clever audiophiles will - at least - try to understand WHY this technology and these amplifiers in particular are a true revolution, the first one in decades of solid-state technology.
It is NOT an easy step, I do understand, it hasn't been easy for me, either. The NuForce REF9's represent a modern era Rubicon.

In the end, let me quote the lyrics of one of my favourite songs of all times, Rubicon by the band Killing Joke. Why? Because the words below seem to be written having these amplifiers in mind. The song, from the album "Brigther than 10.000 suns", is a small, dark, decadent and obscure masterpiece.

(1). For the correct historical meaning of "Crossing the Rubicon" see a page on Wikipedia

Copyright © 2006 Lucio Cadeddu - www.tnt-audio.com

The age of miracles, assent of man in strident tones
Realms of fantastic have been forged by folly speed and steel
Out of the cones stream forth, creation is not yet complete
New species come, old species fall to nature once again.

Now that I've found God on every side - and in every legion

Points of no return
We cross the Rubicon

The shipyards blaze, vibrant arsenals wait their turn
Idols of rational worlds to worship power, to worship strength
Great crowds excited by riot, pleasure, work
Insane crusades, destructive gesture of the freedom bringers
And all the bells shall toll, as holy banners fly
And all with talk of freedom


Let rage and hate of races run from Adam down
The magic of our science shines brighter than a thousand suns
Liberty in new dimensions ruthless and spectacular
Obliteration shall be poetry of 'Golden Dwans'
And as the people thrill, I stand and comprehend upon the threshold.


© Killing Joke

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