Bookshelf Speaker Reviews

Most people would agree that the real goal of any audio system is an illusion of transport - the musicians to the listening room, the listener to the recording space, or both to another place entirely. I'll tell you right now that NHT's long-awaited Xd speaker system, though not without its flaws, is one of those rare products that lives up to this promise. In successfully applying advanced digital technology to an otherwise common bookshelf speaker, NHT and its partners have cooked up a magic brew that will quicken your heart and send your emotions soaring. Simply put, this system takes you inside the music and ultimately lets you forget the gear.

The product of more than four years of development, the basic Xd package includes a pair of 10-inch-tall bookshelf speakers called the XdS, a powered subwoofer designated the XdW, and an electronic component not much bigger than an old VCR. This box, the XdA, contains a digital signal processor developed by the Australian company DEQX (pronounced "decks") and a multichannel power amplifier contributed by PowerPhysics, a California firm that specializes in digital switching amps. Digital amplifiers are extremely efficient - they pump out gobs of power from a small chassis while staying remarkably cool. Despite the XdA's modest size, it houses four 150-watt amps, each powering just one of the drivers in the compact satellites. The subwoofer contains its own 500-watt digital amp, which is fed by the XdA's line-level sub output.

NHT has used first-class drivers throughout, including a 5.25-inch magnesium-cone midrange and 1-inch aluminum-dome tweeter (with a powerful neodymium magnet) in the satellite and two 10-inch aluminum-cone drivers in the subwoofer. Both speakers have sealed cabinets that offer solid build quality, attractive sculpted styling, and an unusual blond and plum finish in furniture-grade lacquer and veneer that NHT calls Classic. A pair of matching, weighted stands come with the system.

Given the Xd's unassuming presence, its $6,000 price tag could take your breath away, even after conceding that the power amp is included. And the real palpitations start when you move into home theater or multichannel music. NHT doesn't offer a 5.1-channel Xd system or even a horizontal center-channel satellite. Instead, the company forces you to buy three stereo Xd setups for a 6.2-channel system priced at $16,500, the $1,500 discount reflecting the omission of one subwoofer on the thinking that a pair of these oughta do for most rooms (trust me, it will). Considering what's not there - the 6-foot floor-standing towers in grain-matched Brazilian rosewood, the massive power amplifiers and all the metal they entail - it seems that most of what you're paying for is the digital processing.

WHAT'S ON THE EDGE? All multidriver speaker systems have "crossovers" that separate the music frequencies into bands so the drivers will reproduce only the sounds they're best suited for. Typically, these circuits slightly overlap the sounds coming from each driver. So, for example, in a simple two-way speaker with a woofer and tweeter, the woofer delivers a small portion of music that should ideally come only from the tweeter, and vice versa.

This overlap, though a necessary evil, creates distortions. Some parts of the musical spectrum may play at different volumes in the overlap area, causing frequency-response errors that can affect tonal balance. And since some notes are being reproduced simultaneously by different drivers, they arrive at your ears at slightly different times and with different audible characteristics. The individual drivers also introduce their own timing errors, as some frequencies will naturally lag behind others as the sound emerges from the speaker. Collectively, these errors undermine sonic clarity and the ability to accurately place instruments within a virtual soundstage.

Here's where DEQX technology comes in. It's applied in both the design phase and during playback. During design, the output of each driver in the XdS was measured to determine, among other things, its ideal frequency limits and which frequencies within its range were out of sync with the others. That information is later used in the XdA to correct the performance of the drivers and program the system's crossovers.

The beauty here is that fixing these frequency and time errors in advance allows the designers to use digital crossovers that virtually eliminate driver overlapping and the bad stuff that goes with it. In technospeak, the crossover slope between the Xd satellite's midrange and tweeter driver is set at 100 dB per octave instead of the typical 12 dB per octave for a conventional speaker. Fellow geeks can go to Deqx.com to learn more, but what's important is the end result. There's an improvement in the power handling and dynamic capability of each driver, allowing it to play louder without distortion. And the total system disperses the sound across all frequencies more evenly, which largely eliminates the "sweet spot" effect that favors listeners sitting precisely between the speakers. With the Xd, anyone in the room hears pretty much the same sonic image.

Best of all, the time corrections bring instruments and voices into sharper focus - you can follow them individually no matter how complex the music and hear the kind of detail that's uncommon on all but the very best (and even more costly) high-end playback systems I've heard. That this is possible from a pair of small bookshelf speakers is nothing short of astounding - and precisely the point. In time, NHT hopes to use DEQX processing to improve the performance of relatively inexpensive speakers.