Power Amplifiers

Rotel RMB-1085 Class D (Switching) Five-Channel Power Amplifier

ARTICLE INDEX
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Introduction to the Rotel RMB-1085

Class D switching amplifiers are here to stay. My first real interaction and extended listening session was with Rotel's two channel powerhouse, the RB-1092. More information about Rotel's Class D implementation can be found in that review. To summarize, the Rotel RB-1092 amplifier provides exceptional sound in a tiny package. The RMB-1085, reviewed here, is a five channel home theater amplifier that uses similar ICE power modules as the rest of the Rotel Class D amplifiers.


Rotel RMB-1085 Design

The overall package is very slim and discrete; the back panel has RCA inputs only, standard five-way binding posts and 12volt input/outputs. The front is very clean with only a power button and protection light, and comes with either a silver or black faceplate.

The big advantage to using class D is efficiency. The potential downside is sound quality and dynamics. Fortunately the Rotel design team has created a small five channel amplifier that does not sacrifice either, to achieve great efficiency. The Rotel Class D amplifiers utilize Controlled Oscillation Modulation which is an analog circuit that drives the switching output stage. Other Class D designs use a digitally generated control that switches the output stage which can sometimes lead to increased noise and distortion.

Specifications

  • Design: Class D - Switching
  • Power Output: 5 x 100 Watts RMS into 8 Ohms, 160 Watts into 4 Ohms
  • Dimensions: 2.8" H x 17.1" W x 15" D
  • Weight: 15 Pounds
  • MSRP: $1199 USA
  • Rotel

Another feature found in the Rotel amplifiers is the Enhanced Cascade Control. This circuit gives the amplifiers very low output impedance (high damping factor) across the entire operating range. All this translates to an amplifier that has flat frequency response no matter what sort of speaker impedance is presented.

Rotel RMB-1085 Setup

Setup of the unit is rather simple. Connect five analog cables from the processor to the amplifier and run five cables to the speakers. I then connected a 12v trigger from the processor and finally the detachable power cord. All connectors on the Rotel are solid and tighten easily. After that I ran a speaker calibration setup using the internal test tones in the RSP-1069 processor and my Radio Shack SPL meter.

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For critical listening I used a Rotel RSP-1069 processor (review to follow), being fed by my Oppo 981HD DVD player. For speakers I am using Raw Acoustics OB3 open baffle speakers (in the front) and Paradigm ADP surround/center speakers. To put this amplifier through its paces I used Pink Floyd's Dark Side of the Moon (SACD), Frank Zappa's Halloween (DVD-Audio), The Melvin's A Senile Animal (Redbook CD) and Kung Fu Hustle (SD-DVD).


The Rotel RMB-1085's Sound

Testing a five channel amplifier with two channel music does not paint the whole picture. The real test of how a multi channel amplifier works is when a heavy load is placed on all channels, producing a big demand on the power supply. The ability of the power supply to cope with this type of stress is what sets the low end amps apart from the high end. SACD and DVD-Audio discs place a heavy load on all channels and are effective in highlighting an amplifier's strengths and weakness.

For my first listening session I used the SACD version of Dark Side of the Moon. This album really needs no introduction and is one of the finest examples of the capability of the SACD format. If this amplifier has any potential weakness it will be with high frequency reproduction. Older class D designs usually failed as most were built for subwoofer duty only. The moment the clocks began to chime in Time, I was convinced that this amplifier is more than up to full range duty: flawless reproduction of all the individual clocks.

My next listening session was Great Gig In The Sky – C0laire Torry's vocal solo raised the hair on the back of my neck. Normally I flip discs and move onto my next target. This time I didn't move until the final heartbeat faded out on the last track. Quite a feat considering just how many times I have listened to this album.

Velodyne Microvee Subwoofer

Another session was with the Frank Zappa live compilation Halloween. This album takes five days of live recordings done around Halloween, 1978 in New York City. The songs have been reconstructed and released on DVD-A and DTS by Dweezil Zappa and Joe Chiccarelli. The way the mix is done puts you right in the audience and the sound is astounding. The recording is quite clean considering it was done live in 1978. While most of the surround sounds are audience noises it still puts some stress on the amplifier. There is great separation of instruments and sounds and the amplifier never fails to provide clean sound even at live SPL levels.

Most of my collection is standard redbook cd, so I chose one of the more dynamic and demanding discs I have, The Melvin's A Senile Animal.

Velodyne Microvee Subwoofer

This album features Dale and Buzz – but now the addition of the Big Business duo Cody and Jared to fill out the sound. It is the inclusion of the second drummer to the line-up that makes this an ideal heavy test disc. For the first time in recorded Melvin's history we have three part harmonies elegantly layed over excessive amount of percussion provided by two of the hardest hitting drummers in rock history. Percussion separation is what I am listening for with this album, there are layers of heavy distorted guitar, up-front bass playing and too many drums to count. Too often the cymbals, high-hats and gongs (yes gongs) get lost with the rest of the noise.

Lower quality amplifiers just cannot resolve the detail present in this album when pushed too hard. The Rotel had no problem presenting all the intricacies contained therein, including Jared's backup vocals/yelling. Every song was easily reproduced with no shortage of dynamics. Of course this is an easier test than the previous albums as the amplifier only has to power two of the five channels. Nonetheless the RMB-1085 delivered, and, effortlessly, I should add as there was very little heat coming from the chassis even after an extended listening session.

rotel-rmb-1085-multichannel-amplifier-fig-3-small.jpgAs this is a mutichannel amplifier it will see more movies than music for most people. For this test I chose Stephen Chow's epic Kung Fu Hustle. This amazingly funny and intense movie excels both in visual and auditory beauty. There are many scenes that test the systems acoustic balance with flying axes, swift punches and unreal bass. There is plenty here to tell you if the amplifiers are cut out for movie duty. An abundance of immersive special effects and slow motion fight scenes put you right in the middle.

This movie does not go easy on the surround speakers. I ran my OB3 full range without a sub which puts a greater strain on the amplifier. At no point during the movie did the amp get too hot to touch; in fact the processor ran hotter. There was no audible distortion produced by the amplifier throughout the entire movie.


Conclusions About the Rotel RMB-1085

Looking at the Rotel product line you will find six class D amplifiers. This shows the company is committed to utilizing this amplifier technology and has invested serious time and effort into producing high quality components. If you are still afraid of what class D can (and cannot) do, you should audition the Rotel version of Class D to hear for yourself what this topology can do.

This amplifier does what every well designed amplifier should do, that is - cleanly amplify the sound. The RMB-1085 is able to disappear and simply amplify without coloring the sound. The fact that it provides 500 watts in a 3" high enclosure is astounding. No audible distortion, ability to drive heavy loads and a relatively inexpensive price make the RMB-1085 an easy recommendation.