NHT XdS Build Quality & Design Overview

Figures 4 – 7. 4: XdS Unplugged: Cabinet, stuffing & grill cloth, drivers. (Note absence of any crossover coonected to the input jacks seen at lower left of picture), 12” ruler for scale; 5: XdS drivers; 6: Tweeter/heat sink close up (note metal tube attached to tweeter back); and 7: Cabinet backpanel. Allen wrench hanging from bolt that holds the distal end of the tweeter’s heatsink in place. Note color-coded jack panel at bottom of photo.

XdS Midrange Driver

Rather than simply packing the Xd system with cheap drivers and letting the XdA’s DSP functions pick up the slack, NHT have opted for fitting out the system with a collection of premium-grade drivers.

The XdS midrange driver is a 5.25” magnesium alloy, tapered cone unit sourced from SEAS, a Norwegian high-end manufacturer. The frame is cast aluminum/magnesium alloy. It features a rubber roll surround, exposed pole pieces and magnetic shielding. Sound reflection, air flow noise & cavity resonance are all minimized by the comparatively large basket frame openings. It has a free air resonance, fs ≈ 48 Hz. Visually speaking, it bears a family resemblance to the SEAS Excel W15CY001, a drive unit not unknown in DIY circles.

XdS Tweeter

The XdS tweeter (also sourced from SEAS) features a 26mm aluminum/magnesium dome, self-shielding neodymium motor structure, underhung voice coil, a magnetic fluid-filled gap and a comparatively wide roll surround. (With both drivers magnetically shielded, the XdS is a CRT display-friendly system). The tweeters free air resonance sits at about 1.6 kHz and its first break mode can be found at about 26 kHz.

XdS Cabinet

The XdS cabinet is a classic study in industrial art, rich in the features of an aesthetically successful merging of form & function. Here, basic design principles – driver offset, rounded faceplate edges, relatively inert, crossbraced cabinet panels and so forth - are all sculpted into a visually attractive presentation that has a way of positively growing on you the more you look at it. You won’t go hunting for an unobtrusive spot in your listening space’s terrain to hide them away. On the contrary: you’ll likely catch yourself looking for spots that highlight these latest additions to your art collection. They’d look right at home in the Guggenheim museum (5^th Avenue between 88^th & 89^th Streets in Manhattan).

The XdS cabinet is a comparatively small (10.25”H x 6.5”W x 8.5”D), totally enclosed (acoustic suspension) construct comprising panels of MDF and a faceplate made of a dense composite material referred to as “BMC”. Internally, it is cross-braced and heavily stuffed. The stands supplied with each XdS are, visually speaking, perfectly complementary, weighted for maximum stability and put the XdS at an ideal height for seated listening. The stands are made from pure MDF, except the base which has a ¼” thick steel plate in it for stability. They weigh in at 18.5 lbs. Clamps, thoughtfully located down the back of the each stand’s spine, help keep the system’s cabling neat and out of sight. Great idea and typical of the attention that has obviously been paid to the details of the system. Spikes are a provided option.

XdS Crossover

There isn’t one. At least not the usual inductor/capacitor/resistor passive crossover commonlyfound inside a more typical loudspeaker. In actuality, all crossover functions are handled upstream by the XdA DSP. (More about this momentarily). One particularly interesting feature that, to a degree, obsolescent-proofs the XdS is that the filter functions are implemented as firmware that’s updateable/changeable via the XdA’s USB port, all handled via a loader utility you install on your PC or laptop. Various filters along with other useful downloads are available at the NHT website.

XdW Bass Module

The XdW is a powered, acoustic suspension, LF system featuring two opposing 10” CNC-machined aluminum cone drivers, a 500W Class D amp and a notably sparse control panel bolted into the back of the unit. There you’ll find an on/off switch, trim (“Less” “Just Right” and “More, i.e., -10 dB, 0 dB, +10 dB), voltage selector and balanced input jack. Like the XdS, all processing occurs upstream, in the XdA’s DSP. On carpet, its surprisingly easy to maneuver around while, for example, you’re looking for the sonically ideal place for it in your listening room.

Remove the control panel and you’re looking at the power amp, seen at right. You’ll also see the space formerly occupied by the amp is sectioned off from the rest of the cabinet’s internal volume by a bulkhead through which the leads connecting the amp with the drivers are routed. The bulkhead prevents, in a simple, straightforward fashion, any wind noise emanating from openings in the control panel when the sub is active – a not an uncommon problem seen elsewhere.

The approach NHT have taken in using the opposing drivers effectively minimizes cabinet panel vibration. So, even though the XdW can be easily maneuvered around your listening space’s carpeting come set up time, it won’t vibrate itself around the floor or impart much mechanically-induced noise to neighboring rooms. A real boon to apartment dwellers or to those otherwise surrounded by noise-sensitive neighbors.

Fit & finish were superb all the way around. In terms of the finish, not a flaw or blemish were apparent anywhere – anywhere - on either XdS, their respective stands or the XdW. Even after going over each with a chamois cloth was there anything found that looked out of place. Outstanding! Construction was of an equally high caliber. These cabinets, the stands – all of it - are solid. The faceplate of the XdS – made of a composite material and molded into a smooth, flowing construct – is a particularly good example of the abundance of quality apparent in the architecture that is this system. I really wouldn’t be too terribly surprised if an Xd system ended up on display in a museum of modern art somewhere. Would that more of the things we surround ourselves with looked this good!

XdA Processor/Power Amp

The XdA is a hybrid of advanced technologies, melded into a product with a decidedly uncluttered, yet high-tech appearance. That self-same outward simplicity grandly belies the complexity that lurks just beneath the cover.

On the back panel you’ll find the On/Off switch, speaker-level outputs for both XdS, trigger jack with mode switch (In, Ext, Audio), analog (balanced & unbalanced) stereo pair inputs, balanced microphone input & one pair of balanced & unbalanced line-level outputs for use with up to 2 XdWs. The front features mode (boundary compensation filter preset) control buttons, their attendant indicator lights and nothing else.

The XdA’s brawn resides with 4 individual 150W Class D power amps. They’re designed and built by PowerPhysics, a relative newcomer to the audio scene, based in Newport Beach, California. The XdA’s amps sport an eye-popping efficiency factor of ~ 95%, which simply means more of the electrical energy fed in to the XdA gets fed out to the XdS speakers; less gets wasted as heat. (Compare the XdA’s ~95% amplifier efficiency with the ~20%, of a typical Class A or the ~50% Class AB linear amplifiers). Running at such a high rate of efficiency also means the various semiconductor’s junction temperatures are kept comparatively low which in turn means the XdA does not require any heavy, complex or costly cooling hardware to deliver the (electrical) goods.

Figure 8: Simplified Circuit Diagram, Class D power amp.

At the heart of the extraordinary efficiency performance demonstrated by D class power amps such as those found in the XdA, is the use of semiconductors as switches, seen at upper left in the circuit diagram showing above. When “On” high current at a comparatively low voltage flows, hence little energy is lost. When the switch is “off”, there’s no current flow and no energy loss. Essentially, the incoming audio signal is used to modulate the PWM carrier that drives the output devices.

The resulting (now amplified) signal is then fed through a lowpass filter stage (seen at upper right) to remove the PWM carrier component of the output. PowerPhysics has taken this semiconductor–as-switch design approach one step further and have been awarded a patent (US, #6084450) for their efforts. At risk of oversimplification, PowerPhysics have devised an elegant non-linear control method that further increases (!) efficiency, lowers power source regulation requirements and forces errors between the reference & switching variable to zero at each cycle.

The XdA’s considerable processing power resides, of course, in its DSP processor sections. Designed by DEQX (Brookvale, Australia), it presents with quite a substantial collection of noteworthy specs. The XdA’s processor section controls everything the 6 output sections (4 for the Xds’ and 2 for up to two XdWs) from crossover frequency point & resolution, crossover type & order (capable of up to 300 dB/octave!), amplitude, phase, & group delay correction to time alignment and so forth. The DSP board in the XdA was done by DEQX and NHT. The amplifiers and power supply were done by Power Physics. All other aspects of the XdA were designed by NHT And the firmware at its core is entirely updateable. In short, the DEQX DSP controls the performance envelope of the system to an all-encompassing degree and with a flexibility that isn’t found in more ordinary speakers. In addition, NHT are planning for the XdA’s future a mic-based end-user system setup/calibration feature. The necessary mic jack is already in place on the XdA’s back panel. A pre-amp/AVR is needed to drive the XdA as it does not provide for source selection, level adjustment and so forth; all preamp functions have been kept out of the processor.