When you watch a movie like Spider-Man 2 in an Imax theater, as I did recently, you're likely to be struck not only by the sheer size and detail of the picture, but also by the seemingly effortless ability of the Imax system to put out immense amounts of sound. Much of this ability comes from having lots of amplifier power. The Imax theater in Manhattan claims to have 12 kilowatts of power on tap!
While even the most elaborate home theaters don't need that much power, it's daunting how much wattage you do need for a big space. For an Imax-like experience in a room bigger than, say, 3,000 cubic feet, you might need to double or even quadruple (!) the wattage you can get from the most powerful A/V receivers.
You can only get oomph like that from separate large power amplifiers driven by a preamp with the latest surround sound technology, such as Anthem's AVM 30 ($3,000), Krell's Showcase ($4,000), and Parasound's Halo C1 ($6,000). Like Cleopatra's barge floating down the Nile , these preamps are awash in luxury trappings. All, for example, are THX Ultra-certified, which means not only that their basic audio performance meets stringent requirements (especially for playing movie soundtracks) but also that they're armed with THX postprocessing functions (click to read see "Features" PDF).
The Anthem and Parasound are actually THX Ultra 2 -certified, which means they're optimized for a full 7.1-channel setup, incorporating three speakers across the front as well as four surround speakers - two on the sides and two behind the listener. The Ultra2 processing creates a stereo effect between the two back surround speakers for greater sonic envelopment and smoother pans between the surrounds. THX certification also ensures proper bass management and other refinements, like a peak-level limiter to prevent subawoofer overload and, in the Ultra2 preamps, boundary-gain compensation to help prevent excessively boomy bass.
A high-end feature that is not a THX requirement but immediately obvious from a glance at the preamps' rear panels is the extensive use of "professional" XLR (a.k.a. Cannon) connectors. These provide a "balanced" connection in which two signal-carrying conductors are surrounded by a shield. Balanced connections can reject most external interference that gets past the shield and are useful in big-room installations where the cable runs between the preamp and the power amps are very long or where the audio cables must run close to other wiring that might emit interference.
But even balanced lines won't avoid the hum-inducing ground loops often caused by connecting things like TVs and satellite or cable boxes. And to reject the most interference, the connectors' wiring should be slightly different on the sending and receiving ends. But this is stuff a professional installer can help you with. Let's get to the preamps in alphabetical order.
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