Amplifier Rotel RB-1092

The past few years have seen a quiet revolution in stereo amplifier design, as we've witnessed a migration from one way of designing power amplifiers to another. The move from Class AB to Class D operation is significant for a number of reasons - arguably almost on a par with the move from tube to transistor in the great scheme of amplifier things.

For many years, the natural choice for a do-it-all amplifier design was Class B, where the output transistors are switched on (and then off) if and when power is demanded from them. This was a sensible way of putting electrical energy into your speakers, being a lot more efficient that Class A which kept the output transistors permanently powered up. Thanks to the lack of 'switching distortion', the latter class sounded cleaner, but drank power like it was going out of fashion and generated massive heat which caused its own set of problems.

Then came Class D operation, using a completely new type of switching circuitry. The technology has been around for a while now, but early examples were certainly not very successful for audiophile applications, possessing an unpleasantly sterile sound. In the past two years though, we've seen some very impressive designs from the likes of Lyngdorf, Onkyo, Flying Mole and NuForce. Large reserves of clean power and the conspicuous lack of that metallic upper-midband mush that most Class AB designs possess has made them a qualified sonic success. However, Class D's detractors point to what is still a slightly matter-of-fact character and lack of tonal breadth.

Now, Class D operation is finding itself in more and more audiophile arenas, and Japanese manufacturers have been particularly swift to adopt the technology. Efficiency is the main advantage - power is only dissipated by the output transistors during the very short interval between their on and off states. Also, because this instantaneous power dissipated is so low, smaller heatsinks and less robust power supplies are needed, meaning lower weight (and lower build and shipping costs too). Love or loathe the Class D sound, there's no denying that it's cheap and efficient.

Rotel is the latest big name to go the Class D way, the RB-1092 using the ICE power technology first seen on its RMB-1077 amplifier. At $2600 it's not cheap, but promises a massive 500W per channel into 8 Ohms. There's also the RB-1091 monoblock design, retailing for just ?995. Both are available in either silver/black, or black finishes. Measuring 432x92x407mm, the RB-1092 is sizeable device and at 10kg is fairly heavy too, but much of the Rotel's bulk and weight is taken by the visually impressive matt black heatsinking.

The RB-1092 found itself driving both my Quad ESL989 and Yamaha NS-1000M loudspeakers, and turned in an extremely pleasant performance with both. Kicking off with Supertramp's 'Breakfast in America' CD, the Rotel showed itself as a quintessentially Class D sounding device. The phrase 'massive reserves of clean power' springs to mind - the RB-1092 goes very loud, just as its spec suggests. Furthermore, it sounds smooth and even right across the frequency band. Bass is dry and strong and well articulated, midband expansive, detailed and dimensional and treble was crisp and clean. This is quite similar to the NuForce Reference 9SEs in this respect - the sound proving so agreeable that on a superficial level that it's hard to criticise.

Bass was truly promising. It isn't overtly imposing - the amplifier doesn't keep reminding you of its bulk - but was never less than strong and insistent. As 'The Logical Song' builds to a crescendo the track becomes louder and louder, and ever more complex, but the Rotel resolutely refused to go light in the bass. Instead it remained firmly in control, shrugging off the increased demand on it with disdain. Bass lines were always firm and full of body. However, compared to the admittedly pricier pair of NuForces, the bass was a tad less tuneful, suggesting that the move to two RB-1091 monoblocks might yield returns in this area.

The midband was superb, in a Class D kind of way. I fear that valve aficionados - and I am one - shall never be convinced that digital amplifiers will achieve the same vocal presence as a good tube design. Kate Bush's 'The Sensual World' was a joy, with oodles of detail, fantastic control and composure, impressive dynamics and a strong stereo soundstage, but her icy voice sounded chilly - not 'cold' as such, just dispassionate, the Rotel lacking the intimacy you get from the similarly priced Unison Research S6 tube amplifier for example. Moving back to Supertramp, and Rodger Hodgson's voice (a tad shrill through the wrong equipment) was very clearly depicted, lacking little in detail and sounding only slightly thin compared to my reference World Audio Design K5881 valve amplifier - but then again, so it does through every other transistor amplifier I've heard!

Still, with its lovely taut and gutsy bass and smooth and expansive midband, the Rotel proved extremely satisfying to listen to. Even Dave Brubeck's 'Take Five' on vinyl was warm and engrossing experience, the fractionally dry character of the RB-1092 compensating for the euphonic tonal bloom of my vdH Frog cartridge here. It is a genuinely musically communicative bit of kit - forget those early tales of Class D amplifiers sounding like the aural equivalent of watching the grass grow. I was very impressed by its upper mid band, which compared to ye olde Class AB was most smooth - REM's 'Perfect Circle' showed none of the clang of my old Musical Fidelity XA-200s, for example. Its tonality isn't exactly dark and velvety, but is very neutral and most certainly not brightly lit across the midband. Same goes for the treble too, which was a wonderfully clean and spacious experience. True, tube treble is nicer, but whether it's more accurate is another story...

The Rotel was actually best suited to classical music. It is a taut, composed, tidy sounding design with massive reserves of power, and this shows itself at lower levels by a lack of strain on crescendos. Debussy's 'Submerged Cathedral' was a perfect example of this - the Rotel hanging on Trojan-like as the brooding piano cadences got ever louder. This is its forte - it keeps its cool when most other power amplifiers around are losing theirs. Again, that expansive midband, bristling with detail, was a pleasure to listen to - the RB-1092 being subtle enough to capture every nuance and inflection of the playing. A couple of years back, a power amplifier as competent as this across so many areas would have been a revelation, but we're now in an age where Lyngdorf's superb SDA-2175 can do a good deal of what the Rotel does at half the price. However, it isn't anywhere near as powerful or transparent. On the other hand, less than twice the outlay buys you a pair of NuForce Reference 9SE monoblocks, which have greater transparency and musical lucidity, plus dizzying speed too.

These rivals show the Rotel RB-1092 to be a brilliant bridge between the two. For many, the Lyngdorf might not be quite enough, whereas the NuForces may prove too pricey. Here the Rotel is a great buy - it has an extremely accomplished sound and massive reserves of power. I suspect the half a kilowatt is too much for any normal mortal, but it's the grace that the Rotel displays at lower levels that makes having this extra 'cubic capacity' under the hood worth having. It's a genuinely svelte sounding bit of kit, capable of making very nice noises with even the most demanding of loudspeakers. To do so much for $2600 means it represents excellent value and is an essential audition - but I'm still not so sure about those heatsinks...

Rotel RB-1092 Amplifier photo