Bigger is better. That’s probably the dominant argument in favor of buying a separate multichannel amplifier and surround processor instead of an A/V receiver. It’s also the wrong argument. There are three good reasons for you to choose separates: to scale up your system to a larger room, to power more-demanding speakers, or to achieve higher performance than you can get with an average AVR.
A big room, of course, demands more power—all other things being equal. And a dedicated power amp can often deliver more oomph. The designer doesn’t need to worry as much about shielding delicate preamp and surround processing circuits from interference. Thermal management is also easier when power-hungry output stages are located in a separate box.
With more robust power supplies and output stages, you can drive more demanding speakers. Most A/V receivers strain at louder volume when they power speakers that have low sensitivity ratings or a nominal impedance of fewer than 6 ohms. That’s a job better suited for a muscular dedicated amp. If you use an outboard amp, you’ll have more freedom to choose the exact speakers you want, regardless of their power requirements. You’re also free to run them full range, or at a lower subwoofer crossover point, when the amp bears more of the bass-producing burden.
Separates may also have an advantage in performance, especially in reducing distortion and noise. A surround processor that lives in its own box—far from hot, power-sucking amps—has a better shot at producing a pristine signal. Separates generally offer a qualitative edge as well as a quantitative one.
But separates aren’t for everyone. Today’s A/V receivers are no slouches, as long as you mate them with speakers of appropriate sensitivity. In fact, you could argue that a more cost-effective way to fill a large room is to buy more sensitive speakers and use them with an AVR. Another argument for AVRs is that most of them have some modern niceties often missing from separate high-end surround processors, such as room correction, low-volume listening modes, satellite radio, Internet radio, PC networking, iPod docking, and Bluetooth compatibility. Still, the larger-scale, unlimited speaker selection and that final increment of performance are the tipping points for those who demand separates. Click here for more on separates vs. receivers.
Power Amps: Types and Specs
Power amps fit into surround systems in a variety of configurations. You can buy a one-piece amp with five or seven channels. For more power, you might prefer a rack full of stereo amps or even single-channel monoblocks. There are also a few three-channel amps that you can mate with a beloved old stereo amp to bring a system up to five channels.
There are different classes of power amps. They differ most obviously in how much power they waste while idling. That aspect determines their energy efficiency, since the idling power dissipates in the form of heat, not sound production.
Class A amps run at virtually full power constantly, regardless of signal demands. This is the least efficient type of amp. Some audiophiles prize them for their warm, dynamic sound, but they are shameless power pigs. Also, they usually run hot to the touch when they operate at any volume. Most multichannel amps and AVRs are Class AB. They’re more efficient, and they run cooler at lower volumes and hotter when you play them louder.
Class G and Class H amps are more efficient than Class AB, although they’re less efficient than Class D. They use a combination of low- and high-voltage rails, switching among them in sort of the same way a car shifts gears.
The potentially most efficient type of amplifier is Class D, often mistakenly called digital amplification. These switching amps consume virtually no power when idling. They switch power to the loudspeakers at extremely high speeds, so they modulate the output and draw power only when the signal demands it. There is now a good selection of Class D multichannel amps. Class D is also being used in A/V receivers, two-channel integrated amps, and HTIBs, though we at Home Theater have found mixed performance on sound quality so far among the new offerings; read the reviews before you buy. With energy conservation increasingly on the national agenda—and thrifty consumers seeking to trim their power bills—Class D will continue to grow in the years to come.
When you read power amp specs, be skeptical. Providing a spec for one or two channels of a multichannel amplifier is just a dodge, but this is the industry norm. The specs should ideally indicate the amp’s ultimate capabilities if all channels are driven at the same time.HomeTheater provides this all-channels-driven measurement in our reviews. Power output is generally best expressed in numbers using RMS values. The input may be a continuous tone (sine wave) at any available frequency, even the extremes of the range (a tough challenge indeed). Or it could be a 1-kilohertz continuous tone from the middle of the range (much easier). It could even be a burst of signal that lasts a tiny fraction of a second (about as easy as power tests get). Each of these methods produces different results, often radically so. As such, they should be stated in the conditions of the test. The number of watts specified is usually higher into 4 ohms than into 8 ohms, but this is a practical advantage only with the appropriate speakers and an amplifier that is comfortable driving them for long periods of time without overheating or shutting down. Peak or dynamic power ratings are often meaningless because they aren’t measured to a consistent standard from brand to brand.
On our test bench, we perform all power measurements using a continuous, in-phase 1-kHz sine wave on both the left and right channels, then we move up to five, and ultimately seven channels simultaneously. This is a particularly brutal test and more stringent than almost any real-world program material would ever demand of an amplifier. But it indicates how substantial the unit’s power supply is or isn’t and gives you a sense of how much clean power the amplifier can deliver when playing complex, multichannel soundtracks at high volume. Small differences in total harmonic distortion (THD), signal-to-noise ratio, crosstalk, and other measurements may not be especially audible on program material, although large ones may be. Our best advice is to read HT’s reviews to see how much power a product really delivers under clearly stated, consistent testing conditions.
To ensure a good match between your amplifier and speakers, check out the speaker specs for the recommended amp power and nominal impedance. Tougher amps deliver more watts and more power into lower impedances. For instance, two models may both deliver 100 watts into 8 ohms. But one of those may deliver 125 watts into 4 ohms, and the other may deliver 150 watts into 4 ohms. The latter is obviously the more substantial amp. However, as stated above, this potential is only useful with compatible speakers.
Speakers with nominal impedances of 6 ohms or less present a more challenging load than 8-ohm speakers. They also require more current. Be warned, too, that a loudspeaker’s impedance varies continuously with the signal. It’s a moving target, so nominal impedance may not tell the whole story. Exercise common sense and ask an A/V specialty retailer or custom installer for suggestions on speaker/amp mating.
THX-certified amps have enough power to drive THX-certified speakers to at least the reference level (105 decibels peak) in rooms of specified sizes (up to 3,000 cubic feet for THX Ultra2 Plus, up to 2,000 cubic feet for THX Select2 Plus).
Power amps connect to processors through line-level analog connections, either XLR/balanced or RCA/unbalanced. XLR is more suitable for long runs as it provides added immunity to induced noise and hum. Note that if you want to use an A/V receiver as a dedicated surround processor and mate it with a separate amplifier, it will need multichannel analog preamp outputs. Only better models typically provide these connections.
Surround Processors, HDMI, and Lossless Surround
Surround processors do much of what an A/V receiver does, including surround processing, preamplification, and A/V switching. But they don’t do power amplification.
Paradoxically, standalone surround processors tend to lag behind AVRs in terms of features, despite their high-end brand identities. The companies that produce them are often smaller and less agile at getting new features into the product pipeline. To avoid buying an obsolete product, you must be skeptical and do your homework. Don’t assume that a high price tag or prestigious brand automatically means you’re getting the latest and greatest features, though for many enthusiasts, the perceived gain in audio performance is worth the sacrifice.
HDMI is one area in which many surround processors have just recently caught up to AVRs (so beware of older products still in the pipeline). Those with HDMI 1.4a provide 3D compatibility and other benefits of the latest HDMI version. A few may still go no further than HDMI 1.3, although even that allows onboard decoding for the latest lossless and other surround codecs, namely Dolby TrueHD and DTS-HD Master Audio. Most offer high-resolution PCM processing over HDMI as well. Many can convert analog video signals to digital and output them over HDMI, which increases switching convenience and allows a single cable connection to the display regardless of the multiplicity of connected sources. Some have advanced video processing.
High-resolution PCM can be helpful because older Blu-ray players convert Dolby TrueHD and other newer surround codecs to this uncompressed digital audio format for output through HDMI. SACD and DVD-Audio players can also convert to high-resolution PCM. The result is still a high-quality signal. In addition, converting to PCM in a Blu-ray player and sending that signal type to the surround processor allows you to enjoy secondary audio from picture-in-picture extras and other interactive features that are lost when sending raw bitstream data from a Dolby TrueHD or DTS-HD Master Audio soundtrack.
Dolby TrueHD and DTS-HD Master Audio are currently the highest-quality surround codecs. They are lossless formats, which means they restore the original digital audio signals bit for bit with no degradation, so you hear precisely what the filmmaker intended. They also store data more efficiently in terms of space and require more modest data rates than uncompressed PCM.
In addition, there are two improved lossy codecs, Dolby Digital Plus and DTS-HD High Resolution Audio. They’re called lossy because they don’t completely reconstruct the original signal. Though of high quality, they are rarely used on discs but are finding acceptance in streaming media. It’s safe to assume that any surround processor will come equipped to decode Dolby Digital and DTS, the old-school lossy codecs used on DVD, as well as the Dolby Pro Logic II, IIx, or IIz listening modes. You can learn more about these in our article on A/V receivers.
Video Processing
At the very least, you’ll want your surround processor to convert all incoming sources to the HDMI output. This way, it can feed the display with just one HDMI cable. Some surround processors have advanced video processing solutions. Our surround processor (and AVR) Video Test Bench reports always include results for a number of video processing tests. HQV, Faroudja, Qdeo, Anchor Bay, and Gennum are among the better-known purveyors of high-end video processing. Surround processors certified by the Imaging Science Foundation (ISF) allow video to be fine-tuned by a custom installer.
Trickle-Up Features From A/V Receivers
Here are some features that appear in most better AVRs today. Some can be relatively rare in surround processors, though it depends on the manufacturer. As is the case with nearly all AVRs, some surround processors include advanced automatic-setup and room EQ features. These sophisticated features use a microphone (usually included) and internally generated test signals. They take a lot of the mystery out of setup by determining the size of the speakers in the system and the frequencies at which a speaker’s bass should cross over to a subwoofer. They also match the levels of the speakers in the system and tailor the system’s in-room frequency response for better performance. While many manufacturers use their own proprietary auto-setup and room EQ systems, Audyssey remains the best-known licensed solution. It comes in four variations. In descending order of quality, they are: MultEQ XT32, MultEQ XT, MultEQ, and 2EQ. The higher up, the greater the filter resolution and (in theory) the better the final result will sound.
Low-volume listening modes serve two functions. As the real-world volume drops below the reference level, human hearing changes its ability to perceive different frequencies. As a result, dialogue becomes harder to catch, backgrounds disappear, and the soundfield collapses. In addition, some content may vary widely in volume, which requires constant (and annoying) manual adjustments. There are several new technologies that combat these issues. Dolby Volume, THX Loudness Plus (which is now part of THX Ultra2 Plus certification), and Audyssey Dynamic EQ all aim to maintain consistent tonal balance, impact, and surround envelopment at lower volume levels. Dolby Volume and Audyssey Dynamic Volume can also even out varying volume levels from one source to another—or between TV shows and commercials. Any of them will make movie viewing more civilized to embattled ears. We hope all surround processors will have at least one of them someday.
Expanded listening modes add extra channels to surround sound’s basic 5.1 configuration. Height channels are available in Dolby Pro Logic IIz, Audyssey DSX, and DTS Neo:X. Width channels are also available in the latter two. If you’re using a seven-channel amp, you may choose height enhancement, width enhancement, back-surround decoding or enhancement, or (and this is a perfectly legitimate choice) none of the above.
Surround processors with an Ethernet connection may include networking features. For instance, they may receive Internet radio, a subscription music service, or pull music off the hard drive of a networked PC. If you’re addicted to your iPhone, iPad, or iPod, an iOS-compatible USB jack is a plus. Wireless AirPlay or Bluetooth connectivity are relatively rare in surround processors, but we suspect the more feature-conscious manufacturers—specifically those who also make A/V receivers—will start including both wired and wireless network audio features to accommodate generational changes in the way people listen to music.
Some surround processors and multichannel amps are certified by THX to produce sound levels of up to 105 dB peak—that’s loud!—in rooms up to 2,000 cubic feet (THX Select2 Plus) or 3,000 cubic feet (THX Ultra2 Plus) when mated with THX-certified speakers, ensuring that the entire system will measure up to stiff THX specs.
Legacy Connections
Component video, which most often uses color-coded red, green, and blue RCA connectors to transfer the signal, had been an HDTV-capable connection for many years, but that is now changing. Unlike HDMI digital video, component video is analog and is often limited to passing only 480p standard definition resolution on newer source components. You’ll find component video connections mostly on older HDTVs and DVD players.
S-video is an analog video connection that separates the brightness and color signals on a single connector. This round, multi-pin jack is not HDTV worthy. Nor is composite video, which uses a yellow color-coded RCA-style jack. Antediluvian signal sources like VCRs and analog cable boxes use this type of connection. Many surround processors now convert all these incoming “legacy” signals to digital for output via HDMI, so you can feed your display with one cable.
Although HDMI is usually the best way to carry audio signals, older signal sources may require other kinds of audio connection. The next best choice is a digital connection using a coaxial or optical cable. There is some controversy over which is better, but they’re roughly equivalent. You can find coaxial or optical digital outputs on Blu-ray, DVD, and CD players and various set-top boxes. Note that on Blu-ray players, coaxial and optical jacks do not support the newer lossless surround codecs.
Analog audio connections are still relevant. Sources that may need 5.1- to 7.1-channel analog jacks include older Blu-ray and SACD/DVD-Audio players. Audiocassette decks and other analog sources require stereo analog jacks. A turntable requires a special, dedicated phono input, or you’ll need to add a separate phono preamp.
Other connectivity options benefit custom and advanced installations. These include multizone outputs, which usually feed a stereo line-level signal to a second room, and infrared remote jacks that help the AVR accept commands when trapped in a gear closet. They may also include a 12-volt trigger, which activates other products like projectors, screens, curtains, etc., plus RS-232 for servicing or control systems.
You and Your Installer
Your surround processor’s user interface affects how you use it and how you feel about it. Some surround processors are designed for professional installation. It’s best to know in advance how much complexity you can handle and how much of it you’re willing to outsource.
Consumers often use separates with premium remotes or high-end control systems. Apple’s iPhone, iPod touch, and iPad are increasingly popular as touchscreen controllers, and smarter manufacturers are supporting them with apps. The remote that comes with the surround processor may be too rudimentary for elaborate home theater control. If you do end up using a conventional remote control, you’ll want something with buttons that are differentiated by size, shape, color, and layout. Many remotes include either learning capability, preprogrammed command-code libraries, or both. Therefore, they also operate other components like your HDTV and disc player.
Buying separates isn’t rocket science, but separates often end up in dedicated home theaters that would benefit from careful setup and other things a good custom installer can provide. You may want to factor that in as part of the cost.
