Rotel RMB-1085 Power Amplifier Review

Rotel RMB-1085 Introduction

There are few names that are both as well known and associated with high-end as Rotel. They have a reputation for quality and power and great aesthetics. We haven't done a Rotel review around here in quite some time so we were all excited (and lined up) when they expressed an interest in having their new Class D RMB-1085 amp reviewed. Somehow, through a combination of luck, bribery, and begging, I got the review sample, shipped overnight no less.

Build Quality

Opening up the RMB-1085 box, it is easy to see that they care about whether the amp arrives undamaged. Even though "digital" amps are notoriously lighter (because they are much more efficient that traditional Class A or A/B amps and don't require the massive heat sinks typically found in linear amp designs), it was still packed extremely well. There is both a top and bottom foam cap that completely surrounds the amp protecting it from all sides. The cables are tucked in the back between the foam and box (not touching the amp at all) and the amp is sheathed in a plastic bag to help protect it from the elements. The manual was on top of the amp and was held down tightly enough that is was still perfectly center on the top of the amp when I removed the top foam cap.

The RMB-1085 is a thin, fairly light (under 12 lbs.) unit that I had no problems palming as I was moving it around. The front face is a light grey with only a single power button and two lights. The blue light indicates power and the red a fault of some kind when the unit is powered on or standby mode when it is powered down. The only other adornment to the front of the unit is the Rotel logo and model number on the front and an embossed logo on the top. The unit stands less than 3 inches high.

The rear of the unit has five RCA style inputs and five sets of 5-way binding posts. There are no balanced inputs for this amplifier. The rear also sports a removable power cord, a 12 volt trigger input and output, and an on/off switch for the 12 volt trigger. The right main and surround were grouped as was the left main and surround. The center channel input and outputs were alone. The only point on concern here was that with the grouped inputs, the positive was always on the outside with the negative in the center forcing you to switch the order of the speaker cables. Rotel stays that they do this to reduce the chance of shorting positive to negative. The 12 volt trigger is used to turn the amp on and off automatically when another piece of gear is powered up. Setting the switch to "Off" ensures that the amp is only turned on manually while in the "On" position, the amp is looking for input from the trigger input.

Cracking open the amp, we can see just how compact the Class D amps are. No where to be found is the huge toroidal power supply we see in Class A/B amps. Instead, just three small circuit boards with minimal heat sinking. The power supply is an SMPS type with two switchers and the amp section consists of three two channel modules, one of which apparently isn’t being used in this design. While I understand that 6-channel amps tend to confuse people, the lack of use of this 6th channel seems wasteful to me. Of course, for $400 more you can pick up the RMB-1076 which activates this 6th channel, adds the input and output, and three level knobs on the front. The 1076 seems to be more tailored to audio distribution with the lower quality binding post and volume controls but we're willing to bet that the internals are nearly identical. The top of the RMB-1085 has vents for cooling but there is no fan. Since switching amps are much more efficient than conventional amp designs, this is to be expected.

About Class D or Switching Amps and SMPS Power Supplies
Class D or Switching amps are sometimes mislabeled "digital." This is perhaps in part due to the "D" in the name. Really, it isn't any less analogue than other amp types but it utilizes an "on/off" typology rather than "as needed" for A/B amps. This makes them much more efficient but can introduce more distortion than other amp typologies. Initially, switching amps weren't considered of much use for home theater other than for subwoofers because they were susceptible to non-linearity based on the impedance of the speaker. In recent years, Switching amps claim to have overcome this limitation and so far have received a somewhat positive welcome by the home theater community. For more information, please read our article The Truth about (Class D) Amplifiers.

SMPS (Switched Mode Power Supplies) are smaller and more efficient than traditional linear power supplies but typically are noisier and have less headroom. SMPS transformers run at higher frequency than linear power supplies which is why the transformer can be much smaller.  But, they are also more challening to produce clean power for audio applications which is why they are usually reserved for PC and car audio or any application where space, efficiency and cost are the three primary concerns.

The Rotel RMB-1085 is a good looking amp with a very clean face. The light grey of the front means that unless you have other Rotel gear it will probably stand out. Luckily, the lights on the front aren't of the "nightlight" variety that we see so often with some gear. There is no way to dim them but it wasn't really needed in my room. It was the one piece of gear that both of my sons and my wife noticed the first day I set it up. It just looks that different from everything else.

Rotel RMB-1085 Setup and Listening Tests

Setting up the Rotel isn't exactly rocket science. You'll need to connect RCA cables from your preamp/processor/receiver to the amp and speaker cables out to your speakers. I used Blue Jeans Cables. There is a 12 volt trigger cable supplied if you are planning on using that function. One thing that sort of irritated me was the configuration of the speaker terminals. Switching the order of the positive and negative just seemed like an unnecessary step that was just begging for someone to reverse polarity on their rear speakers. Plus, the grouping made everything a little tighter than it could have been. I'd much prefer a visual separation (like a line on the back panel) with as much room given to space out the inputs and outputs as possible. One of the things I hate about most receivers is how you need to have the hands of a 12 year old anorexic boy to get to the speaker terminals. I expect one of the bonuses for paying for external amplification is more room to move. The grouping of the left and right main and surrounds on the RMB-1085 felt nearly as cramped as some receivers I've used.

As a reviewer, I often do things that others wouldn't. I needed to test how the amp worked with and without the 12 volt trigger. So I connected the 12 volt trigger cable and just flipped the switch back and forth from "Off" and "On". What I found was that while in the "Off" position, with the cable attached, the amp was still affected by the component connected via 12 volt trigger (in this case it was a Denon AVR-2307CI). For a while this made me think there was something wrong with the amp. While this isn't likely to be a real world issue since most people will not connect the trigger cable unless they are planning to use that feature, it is worth mentioning. Rotel states that the amp shouldn't do this but I had two units do the same thing. There was never any audible sound as the amp switched back and forth from standby mode and it was suitably responsive to the 12 volt trigger. The RMB-1085 was always powered up before the receiver could finish its power up cycle. The amp went from cold to ready in about 2 seconds if the lights on the front are any indication.

The RMB-1085 is 100 watts by five channels - fairly modest power from an external amp standpoint. What is important, of course, is how "clean" the power is. If you are getting an honest 100 watts - that is much better than some of these other receivers whose claims are dubious at best. One thing I noticed was that even if I ran my speakers full range at outrageous volumes, the power consumption on my APC H15 never got higher than 20% (which isn't that much higher than with the amp off). That's pretty impressive!

With only five channels, it is unlikely that someone would want to bridge the amp but it is possible. Unfortunately, the RMB-1085 is not bridgeable. Add to that the lack of balanced inputs and some of the more serious audiophiles might think twice about this amp. Rotel claims that the RMB-1085 is perfect for medium and large sized rooms. I would argue that this claim would be dependant on the speakers you own. Speakers with low sensitivity are going to require more power. The manual for the RMB-1085 suggests not pairing them with speakers with impedances that dip lower than 4 ohms which might be a concern for some. All you electrostat owners will have to look elsewhere for your amplifier needs.

One thing the Rotel RMB-1085 really does have going for it is size. You don't know how many potential amplifier reviews I've turned down simply because there wasn't anywhere in my theater that I could locate another receiver-sized box. This, of course, is the joy of Switching amps - their usually physically smaller than their linear amp counterparts. At less than three inches high, you can pretty much put it anywhere. Rotel suggests a minimum of four inches of clearance and proper ventilation for the amp and I agree. If you are thinking that Switching amps run cool so you can shove them under your couch you're going to be disappointed. While I wouldn't say the Rotel ran hot, it definitely wasn't cool. I had it located in a cabinet with a fan blowing directly on it and after a few hours it was warm to the touch. So warm that I thought I'd burn my hand? No. But definitely hot enough that I thought it was too hot for a Class D amp. I wouldn't mind seeing Rotel put some of that unused real estate in the RMB-1085 to use with a few heat sinks to help cool it off.

Listening Tests

As mentioned above, the Rotel RMB-1085 was fed by a Denon AVR-2307CI which was in turn fed by either a Motorola DCT3416I for HDTV, a Toshiba HD-A2 for HD DVD, or a Denon DVD-3910 for DVD-A. It was all powered and protected by an APC AV H15. All connections were accomplished via Impact Acoustics HDMI with the Oppo HM-31 providing switching duties. Since the AVR-2307CI sports 100 watts per channel, it was fairly easy to do a comparison. Unfortunately, this required manually switching the speaker cables which pretty much guarantees that only the most obvious of differences could be noticed. For speakers I used the Emotiva ER-series speakers consisting of three ERM-1s across the front and two ERD-1s set in dipole configuration for surround duties.

HDTV: Terminator: The Sarah Conner Chronicles
Before I really got into the more serious tests of the Rotel, I wanted to finally finish watching the season final of the Sarah Conner Chronicles which had been sitting on my DVR for months. One of the joys of HDTV (for me at least) is the surround sound. With some programs it isn't much of a bonus but with an action oriented show like Terminator, it can make a big difference. Plenty of bass, pans and enveloping music elevate this series past most of the audio drivel on TV these days (especially with the canceling of this years 24).

While most of the vocals are centered in the front, what makes a big difference is the music. Whenever a terminator (good or bad) shows up, the soundtrack kicks in full force in all speakers. The Rotel RMB-1085 kept me involved and never ran out of power even when I cranked the volume. As we all know, advertisers love to bump up the volume on their ads compared to the shows. The Rotel didn't hiccup as the local car dealer started screaming at me and I went diving for the remote. Thank goodness for the DVRs and the fast forward button. This wasn't the most strenuous of tests but it did give the Rotel a chance to warm up for some of the other tests I had planned.

HD DVD: Stardust
There are so few movies that are both available in the now defunct HD DVD format AND I want to see, that I'm quickly running out of titles to order from Netflix. I wasn't really excited about this movie even though Neil Gaiman, one of my favorite comic book writers, wrote the novel it was based on. Honestly, if it wasn't for the HD DVD version, I probably would have skipped this movie. All in all, it was about what I expected - moments of brilliance complete obscured by poor casting choices (Robert De Nero as a gay pirate?) and a ridiculous premise (a guy promises a falling star to this girl and when he finds out the star is really a different girl, he still wants to give her to the first girl). This movie would probably be enjoyable by young children but for everyone else, it just sort of doesn't make sense.

Of course, the reason I was using this movie in the review was because of the enhanced audio. While it didn't have Dolby TrueHD, the Dolby Digital Plus was a nice step up from the usual audio I experience when watching a DVD. The first thing you'll notice when you step up to the higher definition audio formats is an increased dynamic range. The Rotel didn't disappoint providing all the power needed for the quick volume changes. Surrounds weren't used a lot during this movie though when they were it was to good effect. Each of the effects was well placed in the soundstage and separation was excellent. I was especially impressed with the lightning effects and how realistic they sounded. Even during the peaks which were a good 10dB louder than anything else on the soundtrack the Rotel never seemed strained.

Rotel RMB-1085 Listening Test Continued and Conclusion

Next I decided to move on to a few DVD-As from my collection

DVD-A: Lowen & Navarro - carry on together
When testing a mulitchannel amp or receiver, you need some high quality multichannel recordings. Well, when I think of high quality multichannel music recordings, I look no further than AIX records. While their selection of music is a bit limited, for much of their music they offer not only a CD and DVD-A versions, but also "stage" and "audience" mixes. For this review, I chose the "stage" 96 kHz/24-bit mix. The stage mixes generally put you in the middle of the music which I find to be more enveloping and involving than the audience mixes which basically put the music out front striving to give you that "best seat in the house" experience.

The Rotel RMB-1085 really did this recording justice by pushing a very clean and noise-free signal. The silent passages (of which there are many) were truly silent. The responsiveness of the amp was evident as sudden volume changes were both convincing and forceful. One of the characteristics of AIX recordings is the faithfulness to the performance even when the performers make mistakes (not everyone uses Pro Tools to loop every track you know). With the RMB-1085, there was plenty of depth to the soundstage that I had no problems "drilling in" to the music and hearing these little flubs from time to time. The instruments all had a very lifelike and realistic quality with the proper decay and hold over (or not) intrinsic to its nature. Bongos sounded like bongos, guitars sounded like guitars, and I felt like I was sitting in the middle of a jam session. Quite an experience.

DVD-A: Porcupine Tree - Deadwing
Lastly, I really wanted to push the amp so I decided to put in Porcupine Tree's Deadwing. While it isn't all at ear-bleed levels, it definitely sneaks up on you. Tracks like Arriving Somewhere But Not Here start off fairly mellow but eventually crank up to a more metal sound. This allowed my ears to adjust to a loudish volume before the eventual reach for a remote to turn it down. Never did I think that the amp was approaching clipping nor did it seem like the speakers were being overly taxed. As we always say, too much power doesn't kill speakers - clipping does. And the Rotel seemed fairly resistant to clipping. Now, I have a smaller room which Rotel thinks their amp can more than handle. I agree. I'm not so sure you're going to be able to put it in a cavern of a room (like some of these "great" rooms that are more and more common) without risking clipping, but in a small to medium room with reasonably efficient speakers, you're going to be fine.

Conclusion

While I can think of one or two five channel A/B amps on the market with more power and lower price points, the Rotel RMB-1085 seems to be about on par with most of the rest of the competition. While some of the other Switching amps on the market (as well as other Rotel offerings) sport Bang & Olufsen's ICEPower amplifier modules, the RMB-1085's proprietary Switching amplifiers seem more than up to the task of keeping up with the demands of today's modern home theater. At only 100 watts, I couldn't say that it was a huge improvement over the internal amps of my receiver with my 8 ohm speakers though with a harder speaker load, such differences may become more apparent. It does have an undeniable cool factor and plenty of power if your current amplification needs are inadequate or your speakers have lower impedance and are giving your current amp/receiver trouble. If you are looking to beef up your home theater or just have a touch of the upgrade bug, the RMB-1085 is a little firecracker of an amp that might be what you've been looking for.

Rotel RMB-1085 Power Amplifier

1,199

Rotel Electronics
54 Concord St.
North Reading, MA 01864-2699
978 664 3820

www.rotel.com

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About Rotel
Rotel audio products are known worldwide for their quality, reliability, value, and, above all, exceptional sound reproduction. Since 1961, Rotel has utilized the resources of its formidable international design team and has manufactured its products in its own sophisticated production facilities. Rotel proudly offers a complete range of electronics that have consistently received critical acclaim and coveted industry awards. With substantial investment in new technologies and manufacturing, Rotel is prepared to meet the challenges of an evolving specialty electronics marketplace.

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Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.

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