Class D Amplifier Trims the Fat, not the Sound
By Jim Bray
Audiophile power amplifiers traditionally have been big, hulking beasts, at least comparatively speaking - heavy and generally ugly components whose primary function is taking audio signals from the preamplifier and injecting a healthy dose of steroids into them before passing them on to your speakers.
As with so much in our lives today, however, technological advances are making
themselves felt in the world of amplification, too, in this case helping take
the power amp to the gym and sweating off of it more than a few pounds and
But can a small and svelte amp pack the punch
of a more traditional powerhouse?
You bet, if Rotel's seven channel, $2500 U.S. RMB-1077 is indicative of the
nature of the beast!
I've been using Rotel components for several years now as my reference for comparing audio systems, and have been impressed consistently by their blend of overall sound and build quality for the dollar. Rotel equipment ain't cheap, but it's a lot better than the generic stuff you're likely to find in a big box store and it's
a lot more accessible than the more stratospheric equipment. In short,
Rotel offers an excellent compromise between the so-called mainstream
and the stuff that requires you to take out second mortgage on your
The heart of my reference home theater is Rotel's RMB-1095 amplifier. This "conventional" amp oozes 200 watts of oomph (at 8 ohms) into five channels, and needless to say it's a heckuva performer. But as is the nature of the beast, it also measures 17 1/8 x 9 3/8 x 15 inches and weighs 75 pounds, making it about the size and weight of a Mini Cooper.
Okay, maybe that's a bit of an exaggeration, but the point is: it's as big and heavy as an amplifier is expected to be and it requires a certain amount of strong shelf space and a few inches of air space around it for heat dissipation. It also consumes 1200 watts, so the power company loves it, too.
The "Class D" RMB-1077, on the other hand, rides to the rescue in a positively diminutive package that's just shy of three inches high and weighs a paltry 17.2 lb. And it only consumes 400 Watts, while hardly giving off much heat at all.
How do they do it? They exploit something called ICEpower technology, which appears to have originated with the seriously innovative wizards at Bang & Olufsen. According to Rotel, Controlled Oscillation Modulation (COM) and Multivariable Enhanced Cascade Control (MECC) form the "intelligent power processing (the brains) of the technology to ensure that the amplifier's power supply and output is rock solid even under complex speaker loads or power supply variations."
And if you understand that my hat's off to you!
Here's how Rotel's Michael Bartlett explained it to me: "The secret in the
performance is that the amplifier is very efficient and, because of the "power
on demand" link (MECC) back from the output stage to the power supply, when
current is needed the circuit can draw it immediately. Basically the power
supply is being turned on and off very quickly as the amplifier operates.
"The input analog signal is converted into a PWM (digital) signal using the patented Controlled Oscillation Modulation (COM) technology. The PWM signal after the power stage is fed back to the Multivariable Enhanced Cascade Control (MECC) system. The MECC compares the PWM signal with the original input signal and compensates for any differences, thus suppressing the distortion introduced in the power stage.
"The analog signal after the output filter is also fed back to the MECC, which compensates for the load-dependencies in the output filter."
I had to laugh when I first unpacked the RMB-1077. I mean, how can you take seriously an amp this size that calls itself audiophile and commands a price of $2500 U.S.? If it hadn't had the familiar Rotel name on it - and a classy silvery front panel that looks very rich (it's also available in black) - I'd have had trouble taking it seriously.
And it should be taken seriously: the 100 watts per channel (into 8 ohms)
this little critter cranks out do an amazing job of filling the listening room
clean and transparent sound.
For my listening tests, I tried the RMB-1077 in a couple of different locations and a few different configurations including stereo, 5.1 and 7.1 audio/video setups that would give it a good workout.
I first set it up in my reference home theater, hooking it in where the big RMB-1095 parks normally. The preamp/processor/switcher was Rotel's RSP-1098 and the speakers were Definitive Technologys all around. Circumstances soon led to me moving the amp to a friend's to complete our listening tests, but while it was set up in the home theater where I spend so much otherwise productive time I was very impressed.
You notice the difference between 100 and 200 watts (into 8 ohms) of course
but, all else being equal, this amp sounded great whether I pumped music or
At the second location, we used it with a couple of different preamps including the aforementioned RSP-1098 and in place of a 100 watt x 7 Rotel receiver. Here, with a more "apples to apples" power output comparison and more time to do the listening, the RMB-1077 had a chance to really shine.
And shine it did. We reviewed it with a variety of audio and DVD material (and a bit of HDTV broadcasting, though its audio was suspect), and were blown away by its performance and overall oomph. It sounded great whether we played it at lower, more intimate volumes or cranked up to "Neighborhood Annoyance" levels.
The sound is open and spacious, making the speakers disappear from the listening
area and putting you right into the experience - just as a good amp should.
The stereo sound stage had me jumping up to make sure the center front speaker
wasn't on (it wasn't), and in surround mode we were enveloped completely by
music and/or movie action, with precise placements of instruments and/or sounds
in a full and sweet sound stage.
When you close you eyes and let the music take over, it's almost as if the music becomes all that matters. You can listen to this thing all day without it getting fatiguing. The only flaw is that the sound is so clean and natural that, as with other audiophile components, it'll show up flaws in the source material or other parts of your component chain.
The best musical performance came from DVD-Audio discs. Donald Fagen's "Nightfly" was mesmerizing, with synthesizer sounds emanating from all around the room. Vocals sounded very natural as well. And Medeski, Martin and Wood's "Uninvisible" sounded as if we were in the studio with them.
Likewise, on Elton John's "Goodbye Yellow Brick Road," which has analog origins but which sounds great on DVD-A (despite some analog hangover), it was easy to get lost in the 5.1 mix. Fortunately, we were listening using the buddy system and so could take turns slapping each other back to reality.
Listening to regular CDs played through the RMB-1077 is revealing and will quickly bring out the best - and worst - of the material. CD's we tried included Santana's Shaman album and the Rotel again created that space of being there, presenting a compelling sound stage. Santana's intricate guitar work sounded smooth as silk, with fast transients as he shifted from chord to chord, and the horns sounded nice and brassy without being harsh.
Then we went to the movies, with such titles as Star Wars Episode III and, a better choice, LOTR Return of the King in dts ES. Wow! Again, the RMB-1077 brought out the best and the worst, making us wish for a better quality sound track on Star Wars and absolutely blowing us away on superior soundtracks such as ROTK's. The amp had great "punch," very good dynamics, and those battle scenes almost made us want to armor up and pitch in to help defeat the forces of Sauron. Of particular note were the unearthly sounds of the Nazgul swooping around the room on their dragon-like mounts, hollering their deathly screams. Yikes!
We played stuff at "more than acceptable" listening levels for the most part, and the room positively rocked. It was easy to forget that this amount and this quality of sound was coming from that dinky little box.
Bundle the RMB 1077 with a good preamp and some fine speakers and you'll have a system you could grow old with, without it growing old on you. And with a claimed frequency response of 10-80,000 Hz + or - 3 dB, your dog will be impressed as well!
Thanks to its size and low heat output, the RMB-1077 will fit into places a traditional amp won't. This not only makes it ideal for a home theater installation, but for "architectural audio" applications in which you want an efficient, compact multi-channel amp to pump sound from one central location to seven speakers wherever they may be. And its 12-volt triggers will make it perfect for integrating into automated systems.
Bottom line: the Rotel RMB-1077 is a wonderful little amp. It offers excellent sound quality in a package small enough to fit just about anywhere. For what more could anyone ask?
Maybe a 200 watt version?
Well, actually, if you're running 4 ohm speakers the RMB-1077 does put out
200 watts. And if your needs (heck, even if it's just your wants!) go toward
even more heavy duty output, Rotel has also introduced the RB
1092 stereo amp and the RB 1091 mono amp, which also use ICEpower technology
and crank out 500 watts per channel into 8 ohms - or A THOUSAND watts into
Now that's power! Imagine putting two or three RB 1092's and one
RB 1091 together and building your 5.1 or 7.1 channel home theater that way!
The grin would never leave your face.
Jim Bray's columns are available
through the TechnoFile
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