NHT Xd active loudspeaker system
By Kalman Rubinson   •   November, 2005 NHT's Xd system is what audiophiles have been saying they want: a matched loudspeaker system that optimizes the performance of its components for a real-world domestic listening environment. But with their dollars they've voted against just such systems for years. If we put our money where our mouths are, active speaker systems such as Meridian's DSP or those used in recording studios would dominate the High End.

Unlike studio active monitors, the NHT Xd is insidiously easy to use and to like. In configuration it seems to have more in common with 2.1-channel computer speakers, which are designed more for convenience than for performance. However, the Xd system uses sophisticated digital signal processing (DSP) for all its equalization and crossover functions and, since it has only analog inputs, forces all signals through additional A/D and D/A cycles. This may confound purists, but that's the tradeoff for what sets it apart. The Xd arrangement lets the DSP correct for the less-than-perfect performance of the drivers and cabinets while taking into account the abilities and limits of the built-in amplifiers. The result should be as close to acoustic perfection as the design budget and physical elements permit.

The basic Xd configuration tested here was: the XdA, a box containing four channels of power amplification and crossover/equalization for up to two satellites and two subwoofers; a pair of two-way XdS satellite speakers with stands; and the XdW, a powered bipole subwoofer module. Other configurations of electronics, speakers, and woofer can accommodate anything from 2.1 to 6.1 channels and beyond.

Brains in a box
The Xd system may be a significant conceptual advance in home audio, but it will be a tough pill for the traditional audiophile to swallow. Although each component was developed by a different specialist company—speakers by NHT, amplifiers by PowerPhysics, EQ/crossover by DEQX—it is an integrated active system that does not allow easy substitutions of other gear.

I first came upon DEQX at the Consumer Electronics Show several years back. The DEQX system measured the response of a speaker's individual drivers and synthesized both digital filters to linearize phase response, time alignment, and amplitude response in the frequency band where each performed best, and crossovers operating in the digital domain. By moving the test microphone farther from the speakers, the influence of the room could also be measured, compensated for, and included in the filters. As the designer, Kim Ryrie, switched from the passive factory crossover to the active DEQX crossover, I was amazed. The system sounded like an absolute winner.

Several things prevented DEQX's rampant popularity. One was that not everyone is ready to gut his pride-and-joy speakers and void his warranty by bypassing their stock crossovers. Another was that the basic DEQX unit cost about $3000, and the price rose with the addition of A/D, D/A, and preamp-type controls. Yet another was the need to have a PC or laptop to do the number crunching for the measurements, calculations, and setup. Still, the DEQX enjoys deserved popularity in recording and mastering studios and with DIY speaker builders.

The DEQX processing in the XdA does all the hard work for the user. Crossover and EQ are customized for the two-way XdS satellite and XdW woofer to endow the XdS with a flat response on-axis and optimal dispersion in the horizontal plane. Perfection in the vertical plane is thwarted by the physics of stacked drivers, but due to the very high (48dB/octave!) crossover slope, the suckouts above and below the listening axis are narrowly restricted. The steepness of the slope makes possible a relatively low crossover frequency for the tweeter (just above 2kHz) without compromising its power handling. This, and a smoothly shaped cabinet devoid of edges on its front surface, contribute to the smoothness of the XdS's off-axis response. In addition, some low-end boost is provided at around 26Hz, which makes the XdW's in-room response fairly flat into the 20Hz range before it begins to roll off.

The XdA also includes four PowerPhysics class-D (switching) amplifier modules to biamplify the two XdS speakers. While the general design of class-D amps is well known, the PowerPhysics designs are claimed to offer better fidelity, especially in the higher frequencies, because of a proprietary feedback design that provides output correction on each output cycle, even though the switching is at rates of 250kHz or more. Particular attention was paid to the design of matched switching power supplies, which virtually eliminate overheating at high levels but do not require large heatsinks or fans.

NHT has packaged the amps and DSP in an attractive black box that takes up no more space than a stereo power amp. Such integration means that all judgments about the Xd must apply to it as a complete system, without one component or another being singled out.

Many boxes, one system
The Xd system arrived in six separate cartons, but unpacking and assembling it was pretty simple. There' a Setup Guide, but most of the work consists of screwing a long, threaded steel rod into the bottom of each XdS satellite, slipping the sculpted stand shaft over it, and affixing the heavy base to it with a wing nut. Spikes or pads are user options. After the XdS is inverted onto its stand, the cover that protects the drivers can be swapped for a fitted grille. Each XdS connects to the XdA with a color-coded cable terminated, on each end, with a clearly labeled fitting bearing four banana plugs.

The 57-lb XdW comes fully assembled, requiring only the attachment of its IEC power cord and XLR signal cable (both provided). It also has its own power switch and level control; I found the latter correctly calibrated to match the XdS in my room.

The XdA's rear panel has left and right RCA and XLR inputs, L/R RCA and XLR woofer outputs, two sets of four binding posts, one for each L/R amplified output, and AC power and switching facilities. Although that's all that's needed for general use, there are also: a mike input for the future addition of room EQ functions, a USB port for software updates such as modified EQ and crossover curves, and options for remote switching and/or signal-sensing for turning the unit on. The XdA's front panel, stylish in black and silver, has indicators and pushbuttons for each channel that adjust the system EQ for the proximity of the XdS satellites to walls, corners, and other large objects, such as video monitors.

Because all cables and parts are included and all the necessary EQ is already programmed, the system was up and running within half an hour of my opening the first carton. In fact, most of my effort was expended in removing my resident floorstanding speakers so that the Xd could have room to breathe.

Beautiful looks
My oh my, but this is one beautiful speaker system. Even before turning it on, I could appreciate that NHT has put a lot of effort into making the Xd welcome in the home. Each XdS becomes one with its base, and the ovoid XdW is a refreshing relief from the usual cubes and cylinders. While the two-tone scheme may not appeal to everyone, the Xd is spectacularly finished and definitely furniture-grade. Other finishes are promised; if they're executed as nicely as this one, the WAF bugaboo should be gone forever.

At first, I put the XdS's about 7' apart and about 2' from the front wall. The unported XdW was dead center between them but closer to the wall. Damn, it looked nice, and the maroon-and-cream color scheme worked well with our bubblegum-pink walls.

Fed by the L/R outputs of the McCormack MAP-1 preamplifier, the Xd system was plagued with a low, buzzy hum. Because the hum came only through the XdS satellites and not the XdW, I didn't conclude that it was a ground loop until after lots of cable-swapping, AC rerouting, and fooling with ferrites. NHT's Jack Hidley thought the ground loop came from cable TV or a satellite receiver, but neither was the case because I'd disconnected both. However, he did say that "the PowerPhysics switching amplifiers have an H bridge output. The consequence of this is that there will be half of the rail voltage (70VDC) on the plus and minus speaker outputs of the XdA all the time." I suppose that might make the Xd more sensitive than other systems to the leakage of rail-borne noise. Nonetheless, the simple solution suggested by NHT was a ground cheater on the XdA's AC cord. It worked.

When I set the XdA to turn itself on only when it sensed an audio signal, it responded in 3–5 seconds; the XdW, which is always in this mode, took about 3 seconds more. This is not a problem, but you'll become aware of it when you put on your first CD and don't hear its first notes. I was more bothered by the XdA shutting down after several minutes without signal, which occurred during testing when I took 10 minutes to analyze the results of one signal sweep before taking another. All of this can be avoided by setting the XdA to be permanently On.

Beautiful is as beautiful does
My first impression of the Xd system's sound was that it was clean and well-balanced but somehow lacked great projection and weight. Imaging was stable and discrete, with a wide, deep soundstage into which the XdS speakers themselves almost "disappeared." To get more satisfying performance, I tried fiddling with the XdA's speaker position control, which equalizes the system to compensate for the XdSes' distances from room boundaries, but the differences, though easily discernible, were not effective. Later, when I got the graphs showing the boundary-compensation EQ, it was clear why these settings would not affect this range: All of those adjustments affect only frequencies above 200Hz.