Class D amplifiers convert analog input signals to pulse-widthmodulation signals that drive switching output stages. This extremely efficient use of energy eliminates the need for massive heat sinks and other bulky features normally associated with power amplifiers—such as oversized capacitivestorage power supplies. It also eliminates amps as room heaters and utility-bill boosters. The Class D category originally got its foothold in the autosound market, where small form factor and tremendous output capability made it a natural fit. Class D amplifiers have also been used for a long time in powered subwoofers, where absolute power and dynamics are more important than musical nuances.
Over the past decade there have been notable attempts to refine Class D designs sufficiently to bring them to the audiophile market. I remember seeing early examples at the Consumer Electronics Show in the mid-1990s, but the nasty tonal characteristics that seemed to plague most trial efforts weren’t overcome until the advent of “ICEpower” technology a few years ago. ICE designs have largely eliminated the nasties that were so prevalent among earlier Class D amps, to such an extent that many companies are now making small, high-efficiency (better than 90%) amplifiers that not only deliver oodles of power, but can do so in a decidedly musical manner. (See our extensive feature on Class D amplifiers in Issue 166, and follow the controversy on our Web forum at AVguide.com.)
ICE (“intelligent, compact, efficient,” a Bang & Olufsen intellectual property) has been licensed to several manufacturers, including Rotel. The $2499 RMB-1077 is Rotel’s first multichannel Class D amp, with seven channels rated at an identical 100-watts-per-channel at 8 ohms. With a simple silver faceplate and black chassis, the RMB-1077 has a low-slung look, measuring approximately 3" high by 17" wide. The back panel is loaded with heavyduty gold-plated five-way binding posts, enough for all seven channels, seven gold RCA jacks, plus a 12V-trigger switch and jacks. There’s no provision for balanced inputs. The ubiquitous detachable IEC power cord plugs in the back; all that’s found on the front panel is the power switch.
Technical refinements include two levels of internal feedback, said to make the amp more stable and to improve its musicality, and a protection circuit that kicks in if the amp is outputting too much current.
I used the RMB-1077 in two different setups—as a two-channel amp driving a pair of DALI IKON 6 loudspeakers, with a Linn Unidisk SC universal disc player as a source, feeding a PS Audio Digital Link III digital-to-analog converter, in turn feeding a PS Audio Trio P-200 preamplifier. I also used it as a substitute multichannel amp in my home-theater system, replacing a Parasound Halo A51.
In both cases the results were not only gratifying, but downright surprising. For sheer musicality, the Rotel easily beat out the PS Audio Trio A-100, a $995 twochannel Class D amp that itself was no slouch, but not up to the pace set by the more expensive Rotel. Despite the PS Audio’s higher per-channel power output spec (150W into 8 ohms, 300W into 4), the Rotel delivered deeper, more tightly controlled bass; a more three-dimensional soundstage; sweeter, more delicate high frequencies; and fuller, more rounded midrange tones. The PS Audio was dry and somewhat veiled by comparison.
The Rotel was not only musically enjoyable, but enjoyable to such an extent that it had me plowing through my library to dig out current favorites and long-lost treasures, such as the Sony Masterworks reissue of Bidu Sayao’s 1945 recording Bachiana Brasilera No. 5/ Opera Arias and Brazilian Folksongs. The “Brazilian songbird” came to life not only across the decades, but into the room in a completely unexpected way. So did favorite current soprano Renée Fleming’s By Request [Decca], her rich honey-tones and unearthly emotionality extracting new meaning from such timeworn favorites as Puccini’s “O mio babbino caro” and Gershwin’s “Summertime.”
Audio traditionalists may not be ready to accept this, but to my ears the Rotel RMB-1077 is one Class D amp that can hold its ground in a showdown against any tube or Class AB solid-state amp. It offers weight and authority that exceed most traditional capacitive-storage solidstate designs, with upper-register delicacy that competes against the harmonic sweetness of tubes. Furthermore, it has an immediacy that in my experience is completely its own. Pure speculation on my part, but perhaps this immediacy is attributable to what must be an off-thechart damping factor, derived from Class D’s high efficiency and ability to supply current on demand.