Simaudio NHTs Xd User Manual

www.Stereophile.com, November 2005

N

HT’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.

NHT

Xd

ACTIVE LOUDSPEAKER SYSTEM

EQUIPMENT REPORT

Kalman Rubinson

NHT XdS loudspeaker with XdW subwoofer and dedicated XdA crossover-equalizer-amplifier.

DESCRIPTIONS Active 2.1-channel

speaker system with DSP crossover
and equalization. Consists of:

XdS Two-way acoustic-suspension,

magnetically shielded, stand-mounted
loudspeaker with molded composite
enclosure and fitted stand. Drive­units: 1" aluminum-dome tweeter
with neodymium magnet, 5.25"
magnesium-cone midrange unit.
Crossover frequency: 2.1kHz. Fre­quency response: 110Hz–30kHz,
±1dB. Peak acoustic output: 114dB.
Dimensions: 10.5" (270mm) H (not
including stand) by 6.5" (165mm)
W by 8.5" (220mm) D. Weight: 13
lbs (5.9kg). Serial numbers of units
reviewed: 100000049-38.

XdW Acoustic suspension powered

bass module. Drive-units: two side­mounted, 10" aluminum-cone
woofers. Input: balanced (XLR).
Controls: level only. Crossover
frequency: 110Hz. Frequency
response: 28–110Hz, ±1dB. Power:
500W RMS, 700W peak. Peak
acoustic output: 116dB at 60Hz.
Dimensions: 22.1" (565mm) H by

11.2" (285mm) W by 13" (330mm)
D. Weight: 57 lbs (25.9kg).
Serial number of unit reviewed:
02000001036-38.

XdA DEQX-based DSP

crossover/equalizer/4-channel class­D power amplifier. Maximum output
power: 150W RMS (21.8dBW),
400W peak. Controls: Boundary
compensation. Indicators: Power,
Standby, Fault. Input connectors:
XLR, RCA. Output connectors: XLR,
RCA, binding posts. Dimensions:
17" (435mm) W by 2.8" (70mm) H
by 15.7" (400mm) D. Weight: 12 lbs
(5.5kg). Serial number of unit
reviewed: 030000000025-39.

FINISHES As reviewed: maroon and

cream lacquer (speakers and
stands), high-gloss, Dark Chocolate
Red and Piano Black will be available
by the time this review appears in
print; black/brushed aluminum,
with blue LEDs (amplifier).

PRICE $6000/system. Approximate

number of dealers: 200.

MANUFACTURER NHT, 6400

Goodyear Road, Benicia, CA 94510.
Tel: [phone]. Web:
http://nhtxd.com.

ELECTRONICALLY REPRINTED FROM NOVEMBER 2005

NHT Xd

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 con-

figurations 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 tra-

ditional 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.

www.Stereophile.com, November 2005

MEASUREMENTS

T

heir active, digitally crossed-over and equalized
nature meant that I could examine the intrinsic
behaviors of the NHT XdS satellite and XdW sub-

woofer in some detail. However, it is important
when looking at the graphs that follow not to assume
that any problems and idiosyncrasies that are revealed
in the measurements of the raw drive-units are indica­tive of problems in the overall Xd system’s behavior.

With that warning out of the way, fig.1 shows the
impedance magnitude and phase for the XdS satellite’s
woofer and tweeter. With the speaker intended to be dri­ven exclusively by the system’s dedicated XdA amplifier,
these curves are really only of academic interest. Howev­er, the peak at 73Hz, reaching 26.3 ohms, indicates that
the woofer is tuned by the sealed box to a frequency
below the satellite’s intended passband, meaning that
the equalized drive-unit will not be asked to deliver large

cone excursions. There are some small discontinuities in
the woofer’s impedance traces in the treble that might
indicate the presence of resonances of some kind; simi­larly, the glitch at 27kHz in the tweeter’s traces is due to
the metal dome’s primary breakup mode. The XdS’s
small, rigid cabinet, however, seemed free from enclo­sure-wall resonances. Fig.2, a cumulative spectral-decay
plot calculated from the output of a plastic-tape acceler­ometer fastened to the center of the sidewall, reveals
only a couple of very-low-level modes. I found nothing
of note in the XdW’s enclosure, so I haven’t shown a
graph of its vibrational behavior.

Fig.3 is particularly interesting, in that it shows the
acoustic responses of the XdS’s unequalized drive-units.
The tweeter (blue trace) has quite a flat response above
2kHz and is significantly more sensitive than the woofer

Fig.2 NHT XdS, cumulative spectral-decay plot calculated from the output

of an accelerometer fastened to the center of the cabinet’s side
panel (MLS driving voltage to speaker, 7.55V; measurement
bandwidth, 2kHz).

Fig.4 NHT XdS, cumulative spectral-decay plot of unequalized woofer at

50” (0.15ms risetime).

Fig.1 NHT XdS, electrical impedance (solid) and phase (dashed) of

woofer (top) and tweeter (bottom). (2 ohms/vertical div.)

Fig.3 NHT XdS, responses of unequalized woofer (red) and tweeter

(blue) on tweeter axis at 50", corrected for microphone response,
with nearfield response of woofer plotted below 300Hz.

I first came upon DEQX
(www.deqx.com) 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-and­joy speakers and void his warranty by
bypassing their stock crossovers.

www.Stereophile.com, November 2005

NHT Xd

(red). The latter rolls off at 12dB/octave below 100Hz due
to its sealed-box alignment, but features both a rising
response above 600Hz and some sharp spikes in its out­put at 8kHz and above. That these are due to resonances
in the small-diameter magnesium cone is revealed by the
raw woofer’s cumulative spectral-decay plot (fig.4). The
fact that these resonances are well above the driver’s
intended passband and should therefore have little or no
effect on sound quality will become evident in a moment.

First, fig.5 looks at the other end of the woofer’s fre­quency range. The black trace is the same unequalized
nearfield response shown in fig.3; the colored traces to
the right of this graph indicate the nearfield response of
the XdS’s woofer with the appropriate equalization
applied by the XdA amplifier. (I didn’t have any ground­loop problems with the XdA and XdW.) The red trace is
with the front-panel mode set to “1,” magenta is with it
set to “2,” green “3,” and blue “4.” Each mode applies a
slightly different amount of lower-midrange boost to
compensate for a different placement option. More
important, the XdA flattens the XdS woofer’s output in
the midrange and rolls it off steeply below 150Hz or so,
thus removing the need for the little cone to handle fre­quencies that require significant excursion.

The red trace to the left of fig.5 is the unequalized
nearfield response of the XdW powered subwoofer. It
can be seen to roll off below 60Hz and above 150Hz
with intrinsic 12dB/octave slopes. When driven by the
XdA (fig.6, left-hand trace), its output is increased for an
octave below 60Hz to give excellent bass extension, but
is rolled off rapidly below 28Hz to avoid overloading the

twin 10" drive-units. At the other end of its passband it is
rapidly rolled off to give a measured crossover point to
the XdS satellite of 110Hz. The equalized satellite’s
woofer and tweeter can be seen from this graph (middle
and right-hand traces) to have basically flat responses
within their passbands when driven by the XdA with the
crossover between them set to 2.5kHz, as specified.
Note the very steep filter slopes achieved by the XdA’s
digital-domain crossover: greater than 40dB/octave. The
woofer-cone breakup modes seen in figs.3 and 4 should
not be excited to any significant extent when the XdS is
driven by its partnering amplifier.

Fig.7 shows the overall response of the XdS and XdW,

Fig.6 NHT XdS & XdW driven by XdA, acoustic crossover on tweeter axis at

50", corrected for microphone response, with nearfield responses of
woofer and subwoofer plotted below 300Hz.

Fig.5 NHT XdS, nearfield response of unequalized woofer (black), and of

equalized woofer set to Mode 1 (red), Mode 2(magenta), Mode 3
(green), and Mode 4 (blue). Left-hand red trace is the nearfield
response of the unequalized XdW.

Fig.7 NHT XdS & XdW driven by XdA, anechoic response on tweeter axis

at 50", averaged across 30° horizontal window and corrected for
microphone response, with the complex sum of the nearfield woofer
and subwoofer responses, taking into account acoustic phase and
distance from the nominal farfield point, plotted below 300Hz.

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 (108dB/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 (www.powerphysics.com) to
biamplify the two XdS speakers. While

www.Stereophile.com, November 2005

NHT Xd

measurements, continued

averaged across a 30° horizontal window centered on
the satellite’s tweeter axis. Below 300Hz, this graph
shows the combination’s estimated farfield response,
calculated by adding the nearfield responses of the sub­woofer and woofer and taking into account acoustic
phase. The region covered by the subwoofer, below
110Hz, can be arbitrarily raised or lowered using the
XdW’s rear-panel volume control. However, it does look
as though the integration between the two units is not
quite as perfect as that between the woofer and tweeter.
Other than that very slight lack of lower-midrange ener­gy, and the slight shelving-down of the speaker’s output
above 12kHz due to the tweeter becoming more direc­tional in this region, the NHT speaker system offers
extraordinarily flat response!

It is not only a flat on-axis response that contributes to

a speaker having a neutral balance; its radiation pattern
also has an effect in all but anechoic rooms. Here the
NHT XdS also offered superb performance. Fig.8 shows
its lateral dispersion referred to the response of the
tweeter axis, which has been subtracted from all the
traces so that only the changes are apparent. The con­tour lines are evenly spaced, and there is only a trace of
the usual off-axis flare at the bottom of the tweeter’s
passband. Other than its restricted dispersion above
12kHz or so, this speaker’s radiation to its sides is not
significantly different from what it puts out in front. In
the vertical plane (fig.9), the use of noncoaxial drive­units means that suckouts in the crossover region
between the XdS’s tweeter and woofer appear at
extreme off-axis angles. But the use of very steep cross­over filter slopes means that the XdS is otherwise very
tolerant of listener ear height.

Performing a loudspeaker’s crossover and equalization
functions in the digital domain means that its acoustic
performance, in theory, can be optimized in both the fre­quency and time domains. So, when examining the
XdS’s time-domain performance, I first looked at the
step responses of the raw drive-units (fig.10). Without
the XdA equalization, the tweeter (red trace) leads the
woofer (blue) very slightly, with the tail of the latter’s
step overlaid by the high-frequency ringing of the cone
breakup modes noted earlier. Repeating these measure­ments with the XdS driven by the XdA gave the steps

Fig.8 NHT XdS driven by XdA, lateral response family at 50", normalized

to response on tweeter axis, from back to front: differences in
response 90–5° off axis, reference response, differences in response
5–90° off axis.

Fig.10NHT XdS, step responses on tweeter axis at 50" of unequalized

tweeter (red) and woofer (blue). (5ms time window, 30kHz
bandwidth.)

Fig.9 NHT XdS driven by XdA, vertical response family at 50", normalized

to response on tweeter axis, from back to front: differences in
response 45–5° above axis, reference response, differences in
response 5–45° below axis.

www.Stereophile.com, November 2005

NHT Xd

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’s 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

measurements, continued

shown in fig.11. The first thing to note is the different
time scales of these two graphs. Fig.10 shows that the
wavefronts from the unequalized drivers took 3.75 milli­seconds to reach the microphone, which was 50" away.
By contrast, the outputs of the equalized drivers in fig.11
took 7.5ms longer to reach the microphone. This is the
time taken by the XdA’s digital circuitry to perform its
filtering and equalization functions.

What can also be seen from fig.11 is that each drive­unit’s step is preceded by some low-frequency ringing.
But because the tweeter’s and woofer’s acoustic outputs
appear to have opposite polarities, this pre-ringing should
to a large extent cancel, at least on the tweeter axis. That
this does in fact happen is shown by the XdS’s overall step
response (fig.12), the tweeter’s positive-going step smooth­ly handing over to the woofer’s negative-going step, this
in turn correlating with the superb frequency-domain
integration between the two drive-units seen in fig.7.

Finally, the XdS’s waterfall plot on the tweeter axis
(fig.13) features an impressively clean initial decay
across the band. There are two ridges of ultrasonic
delayed energy apparent, one at 26kHz and one at
30kHz, at the rightmost edge of this graph. These are
high enough in frequency not to have any subjective
consequences. However, there is some low-level hash
evident in the mid-treble, perhaps resulting from the
woofer’s residual cone breakup modes.

The combination of the XdS, XdW, and XdA offers
superb measured performance in both the frequency
and time domains. Is the speaker perfect, therefore?

Unfortunately, I have no means of assessing dynamic
range, but it must be remembered that this superb
performance is obtained by shaping the output of what
is still a small drive-unit. Even though the crossover to
the XdW will eliminate the low frequencies that would
otherwise demand large excursions from the XdS’s
woofer cone, I felt in my own auditioning that the
system would not play quite as loud as I wanted it to
in my wilder moments. (I hope to report further on my
experience with the Xd in a “Follow-Up.”) But other than
that minor limitation, this system offers extraordinarily
good performance at what is really an affordable price.
I agree with Kal Rubinson—the Xd system is the best
thing to come down the pike in a long time, and, along
with the pioneering designs from Meridian, a harbinger
of speakers to come. —John Atkinson

Fig.11NHT XdS driven by XdA, step responses on tweeter axis at 50" of

equalized tweeter (red) and woofer (blue). (5ms time window,
30kHz bandwidth.)

Fig.12NHT XdS driven by XdA, equalized step response on tweeter axis

at 50” (5ms time window, 30kHz bandwidth).

Fig.13NHT XdS driven by XdA, cumulative spectral-decay plot at 50”

(0.15ms risetime).

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 signal­sensing 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 elliptical
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 maroon­and-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.

This was resolved in two ways. First,
I measured an in-room response that
dipped several dB in the upper-bass
range centered on 130Hz. This detract­ed from the general impression of
weight, warmth, and richness of sound.
While the crossover slope between the
XdS and XdW may be a very steep
48dB/octave at 110Hz, there is still sig­nificant signal overlap between the
woofer and satellites; the positioning of
the woofer is important. A lower fre­quency, of course, would compromise
the power-handling limits of the XdS’s

5.25" driver. I found that moving the
XdW forward so that it was the same
distance from the listener as the satel­lites filled in the integrated response, as
confirmed by instrument and ear.

www.Stereophile.com, November 2005

ASSOCIATED EQUIPMENT

DIGITAL SOURCES Simaudio Moon Orbiter, Denon DV-5900 universal players.
PREAMPLIFIER McCormack MAP-1.
POWER AMPLIFIER Bryston 9B-SST.
LOUDSPEAKERS Paradigm Reference Studio/60 v.3, Studio/20, Servo-15

subwoofer; Revel Ultima Studio; B&W N802D.

CABLES Interconnect: Harmonic Technology Harmony Rainbow, Crystal Cable

Cinemax, Kubala-Sosna Fascination. Speaker: Kubala-Sosna Fascination.
AC: Kubala-Sosna Emotion.

ACCESSORIES Echo Buster room treatments, RealTraps. —Kalman Rubinson

NHT Xd

BECAUSE ALL CABLES AND PARTS ARE INCLUDED
AND ALL THE NECESSARY EQ IS ALREADY
PROGRAMMED, THE XD SYSTEM WAS UP AND

RUNNING WITHIN HALF AN HOUR OF MY
OPENING THE FIRST CARTON.

www.Stereophile.com, November 2005

NHT Xd

But I was the real culprit: I was sim­ply too timid with the volume control.
Perhaps my long-term bias toward big
speakers was making me treat this little
system with condescension, but when I
stopped babying the Xd, almost all of
my reservations about its performance
went out the window.

Let’s start with the basics: the
human voice. The two-way XdS, act­ing nearly as a point source and with
virtually the entire range of the human
voice within its compass, reproduced
voices naturally, without added bloom
or coloration. Well-recorded, closely
miked singing, such as Lorraine Hunt
Lieberson’s on her Handel Arias
(SACD/CD, Avie AV0030), or the
soloists on La Tarantella (SACD, Alpha
ALPHASACD503), stood out in relief,
their accompaniments defining the
space behind them. Larger vocal forces,
from Polyphony to the Mormon
Tabernacle Choir, were rendered as
ensembles of individuals, maintaining
resolution and image granularity
regardless of the dynamic scale.

Instruments, from individual soloists
to large orchestras, were just as well
defined in tonality and space through
the Xd. I was particularly taken with
Julia Fischer’s new set of J.S. Bach’s
Sonatas and Partitas for Unaccompa­nied Violin (2 SACDs, Pentatone PTC
5186 072), recorded with less ambi­ence than is usual for Pentatone—I felt
as if I were no more than 5' from her
violin. The clarity of the illusion of the
central image was so good that even its
vertical dimension was convincing.

And it was the Xd system that first
allowed me to appreciate the new Water
Lily Acoustic SACDs of Yuri
Temirkanov’s performance of Mahler’s
Symphony 5 (WLA-WS-76-SACD) and
Alexander Dmitriev’s of Shostakovich’s
Symphony 7 (WLA-WS-77-SACD),
both with the St. Petersburg Philharmon­ic. The two-channel sound, somewhat
congealed through my big-city system,
was clarified enough by the Xd that I
could discern the unique perspective and
the individual instrumental choirs, even
though producer-engineer Kavi Alexan­der’s classic Blumlein miking technique
means there’s no instrumental highlight­ing. (See Art Dudley’s “Listening” col­umn in the October issue for more about
these releases and their creation.) These
recordings also revealed the Xd’s sheer
potency: Normal credulity made it hard
to believe that such large-scale events,
spread wide and deep, were coming from

such small speakers. This is doubtless a
result of good matching of the amplifiers
to the demands of the speakers.

Not only was the Xd system capable
of big, spacious, wide-range sound, it
was nearly immune to conventional
room problems. You know the princi­ple: Absorb or diffuse the first reflec­tions from the sidewalls so that those
early reflections don’t screw up the
imaging. This is partly because most
speakers’ off-axis responses only
vaguely resemble their on-axis
responses and both contribute to the
perceived balance. The Xd’s accurately
customized, extremely steep crossovers
are supposed to linearize the XdS’s on­and off-axis responses, at least in the
horizontal plane. I found that I could
remove my freestanding Echo Buster
wall treatments and still get a great
soundstage from the Xd system with­out corrupting the imaging specificity.

But the beauty of NHT’s approach
went beyond freeing me from concerns
about wall treatments. The Xd is also
less critical of other domestic issues
because its DSP options can compensate
for boundary proximity. Of the XdA’s
four settings, I was able to test only two.
The default setting is for a freestanding
location several feet from any floor, wall,

or ceiling, and this was how I enjoyed
the Xd most. I also tried the Xd with all
three speakers flat against my room’s
front wall. At normal listening levels, the
appropriate EQ compensation reduced
the system’s tendency, due to its near­ness to the wall, to “shout.” These prob­lems could return at really high levels,
but this was of little consequence—those
levels were incompatible with serious
listening or even sanity. I couldn’t try the
EQ setting for corner placement
because my room’s only unoccupied
corner is behind a door. The last EQ set­ting is for a single XdS placed on a large
object, such as a TV. Again, I could not
assess that, but I remember that a high­light for me of the 2005 Consumer
Electronics Show was a brief demo of a

6.2-channel Xd system—and if I recall
correctly, the center XdS speaker was
inside an entertainment center.

Given a well-recorded ensemble of

the size that might rationally fit into my
listening room, the Xd system virtually
disappeared. This uncanny effect was
accomplished by its balanced and open
sound, as well as by its complementary
room interactions and appearance. On
the Chris Lomheim Trio’s The Bridge
(SACD, Artegra ART2004), the piano,
bass, and drums were immediate, tight,
and focused, with no noticeable confu­sion from the sidewalls or, indeed, any
awareness of the listening room itself—
just the music. Because the Xd speakers
are so small and so visually unobtrusive,
they encouraged the illusion of trans­port; other speakers demand that I
close my eyes to achieve the same level
of enjoyment.

Comparisons

While the XdS satellites are a bit small­er than my warmer-sounding Para­digm Studio/20s, the Xd system easily
outperformed them in dynamics and
bass. The Xd was also more smoothly
integrated across the audible spectrum
than either the Studio/20s or the Stu­dio/60s. As nicely as the Paradigms
work in my room, there are moments
when I can hear that each has more
than one driver. With the XdSes and
XdW the same distance from my lis-

tening seat, I never heard anything
other than a unified sound source; in
that regard, the Xd system approached
the performance of the B&W N802Ds
that I will be reviewing next month.
The presentation, however, did seem
more light-weight than the big B&W
at low levels, much like small planar
speakers such as my old Stax ELS-F81s
or Magnepan’s MGMC1. This may
simply be due to the Xd’s radiation
patterns not exciting all room modes,
or a lack of coupling to them at low
levels. Whichever, this reduced interac­tion with the room contributed to my
observation that the Xd system seemed
less limited by the room when pushed
to very high dynamic levels.

The Xd’s bass was nearly as full and
extended as that from a pair of Paradigm
Studio/60s, but couldn’t compete with
the output of the B&W N802Ds or the
Revel Ultima Studios. Nonetheless, the

WHEN I STOPPED BABYING THE Xd, ALMOST ALL
OF MY RESERVATIONS ABOUT ITS PERFORMANCE

WENT OUT THE WINDOW.

quality of the XdW’s bass was competi­tive with any of them. When I piped the
stereo signal through the Outlaw ICBM
bass-management system, all frequencies
below 40Hz were sent to my Paradigm
Servo-15 subwoofer, which added a wal­lop to the bottom end that the Xd, as
configured here, couldn’t duplicate. Of
course, the XdA provides drive for a sec­ond XdW, in stereo or dual-mono, and
that might be a better solution if you’re as
much of a bass addict as I am. But the
unassisted Xd did measure nicely to
below 25Hz in my room; normal folks
need not apply for these extras.

The other area of distinction was the
midrange, where the Xd’s presentation
of voices never seemed lacking but rarely
gave me the shiver of eerie recognition
that I get from the B&Ws or the Revels.

On balance, the Xd could go toe to
toe with any of these speakers. Given
its design and physical configuration,
it’s likely to outperform them in real
domestic listening rooms without
acoustic treatment or major spousal
accommodations.

Conclusions

NHT’s Xd is a remarkable system

whose small size and flexibility in
placement should make it welcome in
most homes. Its small drivers do place a
limit on ultimate volume levels, and
adding more speakers on the same
channels might compromise the Xd’s
marvelous in-room behavior and imag­ing. That limit, however, is well beyond
what even most audiophiles might
need. Add in its clarity and a neutral
harmonic balance unfettered by output
level, and you have a truly outstanding

system that works in the average home.

However, the Xd is more than just a
speaker. All of its electronics contribute
to the result, particularly the digital
equalization and crossover, which make
a felicitous marriage between amplifiers
and speakers that the average audiophile
can only dream of accomplishing by the
usual mixing and matching of compo-

nents. Moreover, NHT has designed the
Xd as a modular, expandable system that
can be configured as necessary for two­channel and multichannel applications.

The NHT Xd is the best thing to
come down the pike in a long time. I
hope it is a harbinger of designs to come.
The current Xd system, however, is com­pletely satisfying in itself—its presence in
my listening room is an enhancement
both visual and aural. Because the Xd’s
performance is competitive at the highest

levels, anyone interested in high-quality
music reproduction must hear it. It might
convince you to reconsider a lot of your
accumulated equipment, but beware—if
you bring along your nonaudiophile
spouse, you may have a lot of explain­ing to do, about why your current sys­tem is both more complicated and less
attractive-looking.

■■

NHT Xd

NHT’S Xd IS A REMARKABLE SYSTEM WHOSE

SMALL SIZE AND FLEXIBILITY IN PLACEMENT

SHOULD MAKE IT WELCOME IN MOST HOMES.

Posted with permission from the November 2005 issue of

Stereophile

® www.stereophile.com. Copyright 2005 PRIMEDIA Inc. All rights reserved.

For more information about reprints from

Stereophile

, contact Wright’s Reprints at 877-652-5295

ELECTRONICALLY REPRINTED FROM JANUARY 2006

NHT

Xd

POWERED LOUDSPEAKER SYSTEM

W

hen Kalman
Rubinson
reviewed NHT’s
$6000 satel­lites+subwoofer

loudspeaker sys­tem in the November 2005 issue
(pp.105–115), he was very impressed,
concluding that “The NHT Xd is
the best thing to come down the
pike in a long time…Because the

Xd’s performance is competitive at
the highest levels, anyone interested
in high-quality music reproduction
must hear it.” I was also impressed
enough by the Xd’s measured per­formance that I decided I would do a
“Follow-Up.”

5

A “Follow-Up” would also allow
me to experiment with different
crossover filters that NHT’s Jay
Doherty had e-mailed me after the

November issue had gone to press,
and which were claimed to endow
the system with greater dynamic
range.

“E-mailed…?”

The heart of the Xd system is the
XdA, a four-channel power amplifi­er that uses PowerPhysics class-D
amplifier modules and includes a
digital-domain crossover from Aus­tralian company DEQX for up to

FOLLOW -UP

two XdS satellites and two XdW
powered bipole subwoofers. The
XdA’s equalization and filter para­meters are held in nonvolatile mem­ory, and can be updated via a USB
link using a Windows XP program
(provided you have Microsoft’s
.NET extension installed).

The three filters NHT sent me

were:

1) “flatterhighend.flt”—similar to the
original filter set, but, according to
Jay Doherty’s enclosed notes, “Flat­ter above 10kHz; crossover point
between tweeter and midwoofer
moved up a hair (to 2.4kHz from

2.3kHz) to increase power handling;
reversed sub phase to try for better
sub/satellite integration”;

2) “150Hzcrossover.flt”—intended
for dual subwoofer use; keeps the
inverted subwoofer phase but moves
the crossover between the satellite
and subs from 110Hz to 150Hz to
enable the system to play louder; and

3) “150Hzxovereqto20Hz.flt”—
equalizes the subwoofers to be –3dB
at 20Hz instead of 26Hz.

There will be more filters to
come. According to Doherty, “We
are working on a linear-phase filter
to use between the satellite and sub­woofer. LP filters require firmware
changes that we should be able to
implement by the time the first
room EQ software package is com­plete. However, this will increase
total system processing time to about
18ms (from 7ms), so it might not be
usable with video.”

Kal had used only a single XdW
subwoofer, but as I intended to try
the filter sets with the higher cross­over frequency, I asked NHT to send
me a second subwoofer. This increas­es the system price to $7200.

Sound: As KR had described, set­ting up the Xd system was quick and
easy. Unlike Kal, I had no ground­loop problems, but this may have
been due to the fact that I used bal­anced connections.

Preamp was first the Mark Levin­son No.326S, connected to the
NHT XdA power amplifier with 15'
runs of Madrigal interconnects; then
an NHT Passive Volume Control,
connected with 6' runs of Canare
interconnects (the only balanced
cables I had with the necessary TRS
connectors). Digital source was
either a Mark Levinson No.31.5 CD
transport or a Technics DVD-A10

DVD player, hooked up to my ML
No.30.6 D/A processor via Kimber
Kable Orchid AES/EBU or Audio­Quest SVD-4 S/PDIF datalinks,
respectively. The No.30.6 was con­nected to the preamp with balanced
1m lengths of AudioQuest Cheetah.
I also used an Olive Symphony
media server to play back uncom­pressed 16-bit AIF files via a TosLink
connection to the Levinson DAC,
and an Ayre C-5xe universal player
connected to the preamps with 15'
lengths of balanced Crystal intercon­nects.

When he’d first set up the Xd, Kal
had remarked on a lack of integra­tion between the satellites and sub­woofer: “I measured an in-room
response that dipped several dB in
the upper-bass range centered on
130Hz. This detracted from the gen­eral impression of weight, warmth,
and richness of sound. While the
crossover slope between the XdS and
XdW may be a very steep
48dB/octave at 110Hz, there is still
significant signal overlap between
the woofer and satellites; the posi­tioning of the woofer is important. A
lower frequency, of course, would
compromise the power-handling
limits of the XdS’s 5.25" driver. I
found that moving the XdW for­ward so that it was the same distance
from the listener as the satellites
filled in the integrated response, as
confirmed by instrument and ear.”

With first one subwoofer, then
two, and using the same filter set Kal
had used, I couldn’t eliminate the
upper-bass discontinuity no matter
how I experimented with subwoofer
positioning. The bass region was rich
and deep, but didn’t integrate suffi­ciently well with the satellites. In
addition, while midrange tonalities
were reproduced with a delightful
lack of coloration, there was a some­what reticent quality to the highest
frequencies. Whether or not these
problems bothered me was very
dependent on the music played.

I have written before about how
the choice of playback equipment
can change the choice of music
played. The discontinuity between
the satellites and subwoofers was
more audible with rock music, with
its ubiquitous four-in-the-bar kick
drum, than it was with classical
recordings. For the three weeks I
used the Xd system with the original

crossover filters, I found myself play­ing a lot more symphonies and con­certos than I had done with other
speakers that have recently occupied
my listening room.

The Xd’s extended low frequen­cies were a much-appreciated benefit
with this kind of music, and the sys­tem’s somewhat veiled highs were
much less of an impediment to musi­cal enjoyment. I dug out discs that I
hadn’t played much, such as Michael
Tilson Thomas’ reading of Mahler’s
Symphony 3 (SACD, San Francisco
Symphony 821936-0003-2) and Seiji
Ozawa’s of Tchaikovsky’s Symphony
6 (SACD, Pentatone PTC 5186 107).
The double basses in the former’s
final movement had glorious weight
through the Xd system, though I
found the system’s midrange resolu­tion very revealing of the wayward
intonation of the occasional sus­tained brass chord in the latter. But
when I played Eric Johnson’s “Desert
Rose,” from Live from Austin TX (CD,
New West NW6084, recorded for
the Austin City Limits TV program),
the low bass just grumbled along
with the higher frequencies.

There was also something I
noticed with the toneburst track on
my Editor’s Choice compilation (CD,
Stereophile STPH016-2). This track
comprises equal-length tonebursts
that move from 32Hz up to 3.2kHz
in half steps, then back down again. I
created this signal to investigate
room and speaker-cabinet resonant
problems, but when I played it over
the Xd system, I was puzzled to hear
what sounded like very faint
“ghosts” accompanying the sinewave
bursts, almost but not entirely like
modulation noise. A puzzle, though
I did wonder if this phenomenon
had something to do with the veiling
I had noticed on music.

Time to install the new filter set. I
began with “flatterhighend.flt,” but
then changed to
“150Hzcrossover.flt” because the sys­tem was still having difficulty han­dling music with extreme amounts
of upper-bass energy. Stanley
Clarke’s double-bass solo on Airto
Moreira’s “Nevermind,” from our
Test CD 3 (Stereophile STPH006-2),
managed to shut down the left-chan­nel satellite-woofer amplifier at the
volume I was finding appropriate for
the music. (The image lurched to the
right and a red light came illuminat-

www.Stereophile.com, January 2006

FOLLOW-UP

ed on the XdA amplifier’s front
panel. Turning off the amplifier and
turning it on again fixed the prob­lem.)

Repeating this torture test with
the 150Hz crossover filters didn’t
result in any shutdowns with the
Clarke solo, and the chord that
begins “In the Flesh,” from Pink
Floyd’s Is There Anybody Out There?
(The Wall Live) (CD, Columbia C2K

62058), sounded positively cata­clysmic. The low-frequency crash of
the wall being demolished on this
album gave the XdW subwoofers
one heck of a workout. However, the
drum-and-unvoiced-guitar intro of
Eric Johnson’s “East Wes,” from his
live Austin City Limits CD, still man­aged to shut down the left XdS
woofer amplifier if I wasn’t careful
with the volume control. (Setting the
NHT PVC’s control to –6dB, equiv­alent to an in-room SPL of around
108dB, could trigger an amplifier
shutdown with this track.)

I listened again to the half­step–spaced tonebursts on Editor’s
Choice. Despite the revised filters, I
could still hear the slight modula­tion noise accompanying the
tonebursts. Probably not too much
should be made of this phenome­non; I mention it only because I
had never heard it before with con­ventional loudspeakers.

What about the high frequen­cies? The veiling that had bothered
me was much reduced, the top
octaves having more air apparent.
Perhaps more significant was the
minimizing of a slight mid-treble
“shoutiness” that had limited maxi­mum volume with the original fil­ters. The presentation was first-rate
in its lack of coloration, superbly
stable and detailed imaging, excel­lent retrieval of subtle reverbera­tion tails, and overall top-to-bot­tom integration. Wow!

Measurements: Fig.1 shows the
response of the Xd satellite (black
trace) and the XdW subwoofer
(green), as well as the individual
responses of the Xd tweeter (red)
and woofer (blue), with the XdA
crossover running the original digi­tal filters. The crossover points lie
at 110Hz and 2.3kHz, and the
Linkwitz-Riley filter slopes are a
very steep 48dB/octave. A touch of
excess energy can be seen in the
mid-treble, and the top octave is a

little shelved off both on axis (red)
and in the 30° averaged response
(black). It also looks as if the tweet­er’s ultrasonic dome resonance has
been notched out.

By contrast, fig.2 shows a similar
set of response curves taken with the
revised filters (NHT’s “150Hzcross­over.flt” file). (Because of continued
wind and torrential rain during
October, the responses above 300Hz
were taken indoors at 40" instead of
outdoors at 50"; this affects the accu­racy of the measurements in the
midrange but is otherwise inconse­quential.) The filter slopes are still
48dB/octave, and the crossover
between the XdW subwoofer and
the XdS satellite can be seen to have
been shifted up to 150Hz, as speci­fied. While the upper crossover
point looks very similar to that in
fig.1, the XdS drivers look better
integrated overall, with a flatter mid-

treble. There is also now slightly
more energy apparent between
7kHz and 15kHz, and the tweeter’s
ultrasonic resonance is unfettered by
a notch. The response on-axis (red
trace) now extends flat to the tweet­er’s resonance at 27kHz, though the
HF unit’s limited dispersion above
15kHz results in less energy in this
region in the 30°-averaged curve
(black).

But it is the response in the room
that matters most. To investigate
how the Xd system behaved in my
listening room, I ran my usual test of
averaging 120 1⁄3-octave power spec­tra taken individually for the left and
right speakers in a window centered
on the position of my ears. The blue
trace in fig.3 shows the in-room
response of the Xd system with dual
subwoofers and the XdA’s original
digital filters. The lack of energy in
the 50Hz and 63Hz bands is endem-

ic to my room and
seating position.
But note the exten­sion to 20Hz and
the very flat mid­range, meeting
±1dB limits from
250Hz to 16kHz—
superb perfor­mance for an in­room, listening­position measure­ment. There is a
slight lack of
upper-bass energy,

though not nearly
as much as I was
expecting from my
auditioning.

The red trace in
fig.3 is the in-room
response taken in
an identical man­ner with the 150Hz
crossover/EQ.
Overall it looks
very similar, but
there are detail dif­ferences that corre­late with the listen­ing impressions.
There is more
energy in the
80Hz, 100Hz, and
125Hz bands, indi­cating better inte­gration between
the subwoofers and
satellites. That

www.Stereophile.com, January 2006

FOLLOW-UP

Fig.1 NHT Xd system, original XdA crossover filters, anechoic response on

axis at 50", averaged across 30° horizontal window and corrected for
microphone response (black), with the nearfield XdS woofer (black)
and XdW subwoofer (green) responses plotted below 300Hz, and the
individual XdS woofer (blue) and tweeter (red) responses.

Fig.2 NHT Xd system, revised XdA crossover filters, anechoic response on

axis at 50", averaged across 30° horizontal window and corrected for
microphone response (black), with the nearfield XdS woofer (black)
and XdW subwoofer (green) responses plotted below 300Hz, and the
individual XdS woofer (blue) and tweeter (red) responses.

slight bump at 2kHz is gone, and the
top two octaves are between 1–3dB

higher in level.
The response
now falls between
±1.25dB limits all
the way from
80Hz to 16kHz,
which is simply
extraordinary in­room perfor­mance. Extraordi­nary!

Summing

Up: Art Dudley

recently wrote
that the true test
of a component
under review is

how you feel
when it comes time to return it to
the manufacturer. In the case of

NHT’s Xd, I felt bad about having to
send it back. It did almost all of what
I want an amp and speakers to do,
with very little downside. However, I
have five pairs of speakers on deck
for review and I have to move on.

In the meantime, the revised
crossover software, in conjunction
with stereo subwoofers, has taken a
relatively affordable, excellent­sounding loudspeaker system and
made it almost beyond criticism. It’s
true that the NHT Xd system still
lacks ultimate dynamic range, but
there’s only so much you can ask in
terms of ultimate loudness from a
pair of drive-units with a radiating
diameter of just 3". Highly recom­mended.

—John Atkinson

FOLLOW-UP

Fig.3 NHT Xd system, 1⁄3-octave, spatially averaged response in JA’s listening

room with original crossover filters (blue) and revised crossover filters
(red).

Posted with permission from the January 2006 issue of

Stereophile

® www.stereophile.com. Copyright 2006 PRIMEDIA Inc. All rights reserved.

For more information about reprints from

Stereophile

, contact Wright’s Reprints at 877-652-5295