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Intermediate findings
Like Edgar who went through a few samples of Reference 8s before commencing his review with flawless units, I had an issue with one Reference 9. Powered up, no signal, it suddenly switched madly between on/off states. Eventually, it refused to come on altogether. Jason Lim responded to my descriptions with "this amp could have come from a batch of circuit boards that are not well built. Seems that something went wrong (got fried) with the short circuit protection circuit and it got stuck in that mode. Oh well, we have 3 year full warranty (just extended retroactively from 1 year full + 2 year parts) so customer will be protected from such incidents. It is possible that some cold solder joins eventually came lose after a short period of heating and contracting. Usually if the electronics survive the first 4 to 8 weeks, they're likely to last a long time."

To be completely honest? "Two reviewers, multiple incidents, same magazine" doesn't inspire complete confidence, does it? With the very positive scuttlebutt NuForce has going for its amps, putting a minor damper on the parade could seem a bit contrarious for its own sake. It's not. It's mere reportage. Where to start? In his review of the DNA-500, Chip Stern included excerpts of conversations he'd had with designer Steve McCormack about the genesis and refinement period of that amplifier. Steve stressed the importance of amplifier voicing once the basic circuit had stabilized and measurements confirmed it. Listening to the Reference 9s -- and recalling the measurement-driven claims of its design superiority over conventional so-called linear amplifier architectures -- I found myself wondering how much of their development process occurred on the test bench and how much in hardcore audiophile voicing sessions.

While these amps could indeed measure perfectly, my ears tell me that when it comes to voicing -- those "more art than science" sessions where parts swapping or subtle bias adjustments make no or very little difference on the bench but human listeners respond patently differently to the music nonetheless -- these amps still leave some room for improvements. And that's not merely compared to tube amps like the Fi 421A or the Canary Audio CA-308s. Include my chip amps as well. For all their ultra-low noise and ultra-low distortion brilliance, the NuForce monos lack the last word in harmonic body. Their treble is somewhat on the cold side, too.

For a hardcore engineer, digesting reviewer or listener comments about sonic humanity and suchlike must seem worse than kidney stones. Not only painful but hard to let go of. What are we talking about? Show me this humanity on my scope, mister reviewer. That's why we see amplifier ads like Halcro's touting "the world's lowest measurable distortion". It implies perfection which distortion analyzers may well prove. Does it translate in the sweet spot however? The Reference 9s without subtle assistance from a preceding tube pre are not as emotionally compelling, tonally developed, elegant in the treble or as convincing about vocal intimacy as the best solid-state or tube amps I've heard. They lack that mysterious, scope-defying humanity. Stop!

Perspective. Of course, the Reference 9s don't cost the fortunes most statement amps demand that will go the small remaining distance. Also, they're still relatively early products on their particular genetic tree, with the mentioned connector upgrade option already proof that its designers don't consider the book closed yet and are still tweaking and fine-tuning. Anyone who has followed some of the on-line exchanges between end users (of the earlier Reference 8s) and NuForce also knows that certain issues (such as susceptibility to RF interference) were only discovered post-launch, with certain fixes and refinements prompted by customer problems and feedback. This caused rapid modifications and numerous iterations of the 8s as documented by Jim Saxon in his review for the SoundStage! Network's OnHiFi pages.

From where I'm sitting, NuForce is in exactly the same boat as Tripath, ICEpower and Hypex. Class D has clear advantages - operational efficiencies, cool operation, small possible packaging and lower costs than equivalent power ratings in traditional linear implementation. Good amps can clearly be authored using the Class D concept with its various proprietary wrinkles. Statement amps, too. My first samples thereof were Henry Ho's H2O Audio monos which use an ICEpower module sans SMPS and instead rely on an absolutely massive traditional linear power supply. Those amps were full-bore no-compromise compete-with-the-best beasts priced a lot higher than the NuForce minis.

The Reference 9s very much fit the mold of the Class D amps I've heard thus far - ultra-transparent, fast, very resolved and revealing of extremely low-level information but not as harmonically developed and tonally full as their older (first-rate) competition. Over the ICEpower amps I'm familiar with (H2O excluded), the Ref 9s go an important steps further in how they handle bass. In short, they avoid the unnaturally chiseled, overdamped, monster LF-transient mien that struck me as out of proportion and texturally discontinuous in those ICE sessions. They also exceed the Tripath amps I've heard in body though they still fall short of Class A amps, especially and undoubtedly tubes.

However. There's a simple solution to get statement performance from the NuForce amps. Use a superior, financially matched tube preamp like the overachieving ModWright SWL 9.0SE. This nearly gives you the best of both worlds - spatial layering and holography as well as harmonic density from the preamp; no noise, speed, power and control from the amplifier/speaker interface. In fact, this particular combo does it all and only a magnifying glass attached to a super-finnicky audiophile would find minor nits - by comparison. Before I delve into those, I want to wait until the modified amps have returned from their beauty sleep in Milpitas: "Very soon we're going to standardize the Eichmann Cable Pod version since people have been requesting it and we're tired of replacing binding posts."

Since those low-mass connector-fitted amps will soon be standard, it's only fair to reserve final comments for that particular version. For now, my feelings on the Reference 9s mirror my findings for the passive transformer-attenuator preamp from AudioZone. I love that piece on tube amps but things get too lean and bare on solid-state. The 9s with a solid-state or passive preamp are likewise lean, cool and ever so slightly on the whitish side of things, asking for just a small blood infusion from valves. Do that and presto - a winning recipe for still affordable high-power high-current transistor amplification that's dead-quiet even on ultra-efficient loudspeakers and will be considered statement by all but the most well-off audiophiles who shop the truly rarefied leagues. So much for the initial performance sketch.

Perspective. The NuForce implementation of Class D in these Reference 9 monos eclipses the most popular Tripath amps and, for my tastes, also avoids the über-bass antics of what I've heard from ICEpower with switch mode supplies (I have not yet heard the Rowland monos with input transformers or the new Bel Canto e.One monos which John Potis will review). Viewed in this context, the Ref 9s make a very powerful statement indeed. In the venti scheme (Starbucks, you know), there's statements beyond theirs still. But the gap is shrinking steadily. My upcoming commentary on the RCA/binding post mod, with sonic comparisons to other amplifiers, might well show it to be closing further still...

PS. On the subject of damping factors: It's the technical term for an amplifier's ability to control the voice coil/s. An amplifier output impedance of 0.04 ohms will equate to a damping factor of 100 into a 4-ohm load. According to Avantgarde Acoustic, a 5-meter long speaker cable with a resistance of 0.36 ohms and a crossover coil resistance of 0.6 ohms will diminish the effective damping factor at the voice coil to 4. So much for number's magic.