Why you need one and what features to look for.

An A/V receiver is a nice all-in-one package for managing and powering a home theater system. But like anything in consumer electronics, there's a high-end approach that lets you push the envelope of performance. Just like the old adage in auto racing, "How fast do you want to go?" An A/V preamp should offer at least Dolby Digital and Prologic decoding. Choosing separate components for A/V processing and amplification can result in a more flexible and more powerful system. More power means you can play your system louder without hearing distortion.

Preamplifiers are sometimes called control amplifiers, because they control the flow of the signal from the source components to the power amplifiers. In a basic stereo preamp, you'll find controls for source selection, volume and balance, and tone. Home theater or A/V preamplifiers add video switching and surround processing to the mix.

Important Inputs and Outputs

When choosing an A/V preamp, make sure it has all the inputs you need-and the right types. You should expect at least some and usually most of the audio inputs to have video inputs associated with them. Normally both composite and S-Video connections will be supplied; high-end models typically include component video jacks as well. You'll also want digital audio inputs for most sources, especially for a DVD player that delivers Dolby Digital or DTS signals.

Digital audio connections may be either coaxial (electrical) or Toslink (optical). Both work fine, but coaxial outputs must go to coaxial inputs, and optical outputs to optical inputs. Be sure you have enough of each type of digital input to match the digital outputs on your source components. In fact, given that new digital sources seem to be arriving daily, you really can't have too many digital inputs to allow for future system expansion.

The latest features you'll find in state-of-the-art A/V preamps are DVI and FireWire inputs. Look for more sources to adopt this new standard, which streams multichannel digital audio and video signals on a single cable.

Decoding Capabilities

An A/V preamp should also offer at least Dolby Digital and Pro Logic decoding; most also include DTS decoding. If your home theater can accommodate one or two surround speakers behind the listening area (in addition to the usual side surrounds), you'll want THX Surround EX decoding for 6.1-channel Dolby Digital Surround EX soundtracks that provide this back surround channel. Many models with Surround EX capability support the 6.1-channel DTS-ES format as well (few people consider this essential, however).

The latest generation of A/V preamp incorporates the new Dolby Pro Logic II decoding technology. This advanced matrix surround processor not only does a remarkable job of decoding Dolby Surround soundtracks as if they were Dolby Digital, but also creates natural-sounding multichannel music from ordinary stereo sources, such as CDs. Dolby Pro Logic II is well worth the money. Most manufacturers incorporate additional surround modes designed to enhance standard surround decoding or to generate multichannel ambience from stereo sources.

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Tweaks That Matter

A very useful feature in an A/V preamp is the ability to calibrate your system from the front-panel display as well as from the TV or projector screen. Especially useful is an equalization circuit that cuts down the excessive brightness in some movie soundtracks. Lucasfilm originated this in THX processing, where it still resides. It is available in non-THX products such as Lucasfilm Cinema ReEQ, not to mention the knockoffs under different names. If you are interested in the new DVD-Audio and Super Audio CD formats, which currently provide multichannel output largely in analog form, you'll want to be sure your A/V preamp has multichannel analog audio inputs to mate with these players.

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