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Equipment Review

July 2006

NHT Xd Loudspeaker System

by Doug Schneider

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Review Summary
Sound "I’ve never heard such a small speaker system sound so big." "Playing Johnny Cash’s American IV: The Man Comes Around [Universal 063339], for example, produced a sound that was so big, bold, and ballsy that anyone who listened couldn’t believe it was all coming from a speaker system the Xd's size." "The Xd’s system’s sound was also highly focused and precise. In fact, I’ve never heard a speaker system big or small that had this level of immediacy."
Features "A compact and stylish active satellite/subwoofer system that features DSP-based crossover technology from DEQX in Australia and digital-switching amplifiers from PowerPhysics in California." "The Xd system has three main parts: the XdS satellite speakers, the XdW subwoofer, and XdA amplifier/crossover."
Use "There were some technical glitches right from the start." "When the Xd system was simply on and idling, there was an easily detectable buzz from each XdS’s midrange/woofer (a ground loop was my first thought), quite a bit of hiss from each tweeter (hiss from the tubes in my BC3000 preamp crossed my mind), and a high-frequency whine from the XdA itself (no idea of the cause there)."
Value An innovative $6000 speaker system that "reaches for greatness, and achieves it in some specific ways," "but it doesn’t clearly better the best of the current 'traditional' competition."

NHT’s Xd speaker system is one of the most intriguing audio products to be released in some time. It’s a compact and stylish active satellite/subwoofer system that features DSP-based crossover technology from DEQX in Australia and digital-switching amplifiers from PowerPhysics in California. It’s priced at $6000 USD for the whole shebang (more if you want to have three additional speakers for home theater), complete with cabling.

NHT discovered the work of DEQX some time ago and was smitten by the possibilities that digital crossovers allowed; to make the system work, though, NHT needed amplification, and while the company probably had umpteen dozen choices, they selected the PowerPhysics technology. This makes the NHT Xd active speaker system a three-company technical tour de force, and all three contributors are bent on providing state-of-the-art performance in a compact, high-style package.


The Xd system has three main parts: the XdS satellite speakers, the XdW subwoofer, and XdA amplifier/crossover.

The XdS is a sealed design that uses a 1" dome tweeter and 5 1/4" midrange. Because the crossover technology is DSP-based and located in the XdA, no components other than the drivers and connectors reside in the speaker cabinet. This helps keep the speakers small and light -- each XdS measures 8 1/2"W x 11"H x 8 1/2"D and weighs 13 pounds.

At 11"W x 22"H x 13"D and with two stout 10" aluminum-cone woofers flanking each side, the XdW is the largest and heaviest component of the bunch. It weighs 60 pounds, but it’s not that bad to move around. As you’ll find out below, I didn’t have to fiddle with the XdW's placement at all to make the system sound great. Unlike with the XdSes, there is an amplifier inside the XdW that NHT rates at 500W.

The XdW and XdSes are styled identically and look sharp. Their curves and slopes one moment seem futuristic and at others a touch retro. I particularly liked the color scheme of the system sent to me. The XdW and both XdSes had a light-colored wood finish that contrasted with maroon on the XdS’s curvy baffle and the XdW’s rounded underside. I love the look; in fact, I think the Xd is one of the best-looking speaker systems on the market today. A bit of trivia: When I was at the NRC measuring the speakers, a Swingline stapler on a desk was the exact same maroon color. Coincidence? Perhaps. Or did NHT have one of those around their offices when colors were being chosen? The Xd speaker system is also available in a deep-red/piano-black combo.

Finally, there’s the silver-and-black XdA, which measures 17"W x 2 5/8"H x 16"D, and seems to weigh hardly anything at all (12 pounds I’m told). Although it’s not as slickly styled as the speakers -- you can only do so much with a black-metal chassis and a black-and-silver faceplate, even if it has blue LEDs -- it’s lightweight and easy to configure. Inside are four independent PowerPhysics amplifiers, with each amplifier powering one driver in each XdS. You need one XdA for two XdSes, but one can actually drive two XdW subwoofers.

As I mentioned, the PowerPhysics amps are digital-switching amplifiers -- all the rage today, mostly because of their small size, low weight and low cost -- so they can deliver quite a bit of power yet help the XdA remain small in size and allow it to run relatively cool. NHT says that each amp can deliver 150W. However, while these digital amps are small and efficient, they’re also quite noisy -- more on this later.

Also inside the XdA is the DEQX processing engine, which handles the crossover tasks as well as other speaker manipulation. Now this is quite cool and, frankly, what most of the fuss is about with regard to this speaker system.

The XdA is fed line-level signals (it accepts singled-ended RCA or balanced XLR) that are digitized once they enter. Once the signals are in the digital domain, superior crossover manipulation can be done, which wouldn't be possible if the signals remained analog. For example, do you think 24dB-per-octave (fourth-order) crossovers are steep? NHT is claiming that DEQX’s engine can produce 300dB-per-octave slopes (the crossover slope between the midrange and tweeter in the XdS is 108dB per octave, with 48dB per octave between the XdS and XdW). These steep roll-offs at the crossover points (the XdS midrange crosses over to the tweeter at just above 2kHz, and the transition with the subwoofer appears to be at 120Hz) result in greatly reduced driver overlap, which can improve dispersion and increase power handling.

The designers say they can also do quite a bit of frequency-response correction to make the speakers’ output far more linear as well. Because this is all done digitally, phase errors won’t be introduced the way they are with passive crossovers. Basically, the designers don’t have to choose their compromises; they can have their cake and eat it too.

Well, almost. The one thing working against the NHT design team is time -- time to do all this processing. If they were just concerned with producing audio, it wouldn’t really matter how long it all takes. But when you need to synchronize with video, such as in a home-theater system, you can’t hold up the audio portion too long or what’s happening with the sound won’t match what’s happening on the screen. Therefore, because this system has been designed for use with audio and video systems, the designers had to put a limit on how long the XdA could take to do its thing. Their limit was about 9ms, which they say shouldn’t be noticeable when synching with video.

The XdA has a few user-definable features. First, it can be set to be on all the time, "triggered" on by other components, or turned on automatically when it senses an incoming signal; this is set with a switch in the back. (The XdW has a power switch, but then only an auto-on option for its electronics, something I’ll talk about below -- it created a bit of an issue for me.) The "Mode" switch on the front of the XdA alters the XdS’s frequency response. Which setting you use depends on whether you have the speakers placed out in the room and away from boundaries (my preferred setup, and what I used for this review), close to one boundary, close to two boundaries, or on the TV acting as a center-channel. Each channel can be adjusted independently.

Quite a bit more can be said about all of the Xd system's technology, but the best resources are the NHT and DEQX websites, where there are technology papers that go into all the technical detail in far greater depth. What’s safe to say, though, is with the way things are going today -- digital, digital, and more digital -- digital crossovers like the one used in the Xd system represent the future of high-end audio, and we’re bound to see similar products in the years to come.

Unpacking and setup

Although I winced at the number of boxes that arrived to make up the Xd system -- six in total -- I was impressed with how fast and easy it all was to connect. The Xd system was set up and playing with a mere 20 minutes of assembly.

The XdS speakers are the trickiest part of the setup because of their integral stands, but even though this part required the most labor, it’s obvious that NHT put a lot of thought into the assembly. You simply flip each XdS speaker on its top and screw a threaded rod into the bottom of the cabinet. After that, you slide the pedestal portion of the stand over the rod and a base plate over that. Then, with just a wing nut and washer for the end inserted through the base, you tighten it all up. Done. Threaded spikes are also supplied for each base to make the stands sturdier on a carpeted floor.

Setting up the XdW sub and XdA processor/amp was even easier, although the XdW is a fairly heavy beast in comparison to the rest. Simply unbox and place the components where needed. In my case, the XdW went between the speakers, and the XdA was placed on my equipment rack. As I mentioned, NHT supplies the cabling, and quite a bit of it (about 20 feet of each), giving lots of placement freedom.

With such a quick setup, I was able to experiment with ancillary components, which proved vital to getting the most out of this speaker system. From the get-go, my Blue Circle BC3000 preamp and Theta Data Basic/Stello DA220 digital front-end produced a sound that was big, bold, and incisive with the Xd system. In fact, I’ve never heard such a small speaker system sound so big. This setup could play at low levels with great detail, and it could play quite loud with good control. It sounded wonderful right off the bat, but there were some technical glitches right from the start too.

When the Xd system was simply on and idling, there was an easily detectable buzz from each XdS’s midrange/woofer (a ground loop was my first thought), quite a bit of hiss from each tweeter (hiss from the tubes in my BC3000 preamp crossed my mind), and a high-frequency whine from the XdA itself (no idea of the cause there). I talked to NHT’s Jack Hidley, and he also felt the buzz was a ground-loop problem, and that if I could switch to balanced connection (my BC3000 only has single-ended outs) that would help. He thought the high-frequency whine was likely a component in the XdA that NHT pots at the factory to eliminate the whine, but the review sample still exhibited some of the problem. We didn’t really discuss the hiss.

So out went my regular source and in came the Aurum Acoustics all-in-one Integris CDP CD player/preamplifier, which has single-ended and balanced outputs. It’s so quiet itself that you’d swear it’s not even turned on. The Integris CDP cured the buzzing completely, but it wasn’t necessarily a single-end/balanced thing. The buzz was gone regardless of the connections I used, which left me a tad puzzled.

The high-frequency whine from the XdA remained, however; nothing was done about that. Admittedly, it was very slight and completely drowned out when the music began, even at low levels. Still, it was there. And the tweeter hiss -- well, it remained too. It wasn’t diminished a bit, so the tubes in the BC3000 weren't causing it, as I first suspected. I turned off the CDP, left only the XdA running, and heard the same level of hiss through my speakers.

My conclusion is that the XdA -- at least the XdA I received -- is just a noisy fella, something that does happen with digital-switching amplification. The hiss gets drowned out when listening, mind you, but if you’re just in the room and the system is on, there’s a steady sssshhhh that’s hard to miss.

Other than these anomalies, the Xd system sounded very good -- and even moved up a notch in overall sound quality when I inserted the Aurum Acoustics Integris CDP, because it’s as near-perfect a CD player and preamp as I’ve had at my place. In fact, it might be one of the best components of any kind out there.


Playing Johnny Cash’s American IV: The Man Comes Around [Universal 063339], for example, produced a sound that was so big, bold, and ballsy that anyone who listened couldn’t believe it was all coming from a speaker system the Xd's size. I was surprised too. I’d never before heard a subwoofer-satellite system this small produce a sound so big and with so much guts. And as much to my surprise, and pleasure, I didn’t have to do a thing to make the XdS satellites and XdW sub sound "as one." I simply fiddled with the XdW’s back-panel level control a bit, finally settling at the mid position that NHT calls "Just Right," and it was perfect. In my very large room, it's hard to say whether I was getting truly full-range sound (i.e., down to 20Hz) or not, but it was close enough. In short, the "scale" of the presentation impressed me given the speaker system’s size and price.

The Xd’s system’s sound was also highly focused and precise. In fact, I’ve never heard a speaker system big or small that had this level of immediacy. When the music plays, it’s just there in a way that you can’t miss. Voices, instruments, and what have you pop into space with starkness that’s spooky. Frankly, some may find the presentation a tad Technicolor-ish, bordering on being a little "too there," but few will deny that it’s impressive. There’s also good clarity and "evenness" across the system's bandwidth that indicate the Xd system has been voiced to sound relatively neutral.

I chose the word "relatively" on purpose. Although the Xd is pretty flat, I could hear that the lower midrange sounded a touch "up" in comparison to the top end, which, obviously, came across as a touch "down." This made things like Cash’s voice sound a little more forward than normal -- not much, just a wee bit.

As for the bass level, well, that’s a different story because it can be adjusted with the XdW’s level control. But regardless of where I set that control, the XdA satellites seemed to emphasize the lower mids and nothing with the Mode switch changed it. (Mode seems to affect response lower than the region I’m talking about.) However, "up" or "down" just a touch is not necessarily a good or bad thing; it’s just the way the Xd sounds -- likely the way the system has been voiced -- and many people do like a speaker with a bit of added presence, which the NHT Xd system certainly has.

The soundstage was as big as I’ve heard in my room -- an illusion free of the speakers, left to right and front to back -- and the imaging was razor-sharp. The opening track of Blue Rodeo’s classic Five Days in July [Discovery 77013], "5 Days in May," was projected with an enormous soundfield and imaging so precise that the musicians seemed almost holographic -- almost real.

Finally, as I've mentioned, this rather small speaker system could play impressively loud or quiet, but I did run into limitations both ways. The Xd system can certainly crank out, and I didn’t hesitate to throw any type of music at it. The Xd played it all with ease, even hard rock, but there were limits as to how loud it could ultimately go. At ear-splitting levels playing hard rock or even Cash’s The Man Comes Around, the lower mids start to turn hard and gritty. Obviously, the little 5 1/4" driver is being taxed to its limits at this point. On the other end of the spectrum -- i.e., playing quietly -- the XdSes sounded as good as they do at normal listening levels -- something that good-quality speakers should do, but don’t always. But I hit an unexpected snag here too.

I have a habit of listening to "light" music in the mornings while I’m working, with the music often getting progressively louder and more raucous as the day wears on. My "light" choice these days is Bruce Cockburn’s all-acoustic Speechless [Rounder 3250]. The album is not only light on my ears, but light in the bass as well -- there just isn’t much of it. But it’s also not bass-devoid; there is some. The XdW simply didn’t seem to like this low-level, bass-light music. Although it has an on-off switch, the XdW puts it into standby mode when it has idled too long. The low listening levels and the absence of consistent bass made the XdW turn on and off throughout listening, which was irritating. The turn-on is heard as a very light "thump," but it’s enough of a "thump" to be distracting -- to the point where I stopped using the speakers for background listening.

Once again, it was the electronics portion of the Xd system that seemed to be the Achilles heel of this otherwise outstanding product. Having an always-on switch on the XdW would fix this problem.

Comparison quandary

Comparing the Xd system to any speaker system is difficult, mostly because the Xd is active and there just aren’t many of these on the market. Although active speakers are a mainstay in the world of pro audio, they’re few and far between in home audio.

For example, the last active speaker system I reviewed was Paradigm’s Reference Active/40 -- that was in July of 2001. I loved that speaker system, finding its sound quality comparable to speakers two or three times its price. I still own a pair. The problem, though, is that the Active/40 is a five-year-old design and has been discontinued. Furthermore, although the NHT Xd system does better the Active/40 in terms of overall sound quality and styling, it's priced a whole lot higher. Even factoring in five years of inflation to the Active/40's 2001 price of $2000 per pair still doesn’t even bring the price close.

Then there’s the Aurum Acoustics Integris 300B active speaker system ($30,000) that I also have for review. The Integris 300B is a whole lot larger than the NHT Xd and sounds better across the board. The NHT Xd sounds great for the price, but the Integris 300B is truly state of the art. However, it’s five times the price of the Xd system!

So, with comparisons few and far between, the most logical thing to do is figure out what you can buy for $6000, speakers and amplifier included. When you think that way, the door’s wide open. One of the best small speakers on the market is PSB’s Platinum M2, which is $2000 per pair ($2500 with dedicated stands) and is about twice the size of the XdS. I’ve used a pair of M2s for well over a year, and it just so happens that the M2s sound quite similar to the Xd system in terms of providing a rather even-keeled presentation, although I find the M2s to be closer to the "flat" ideal. However, the M2s alone don’t have the bass that the XdS/XdW combination can provide. But even without a sub, they can still play louder and cleaner than the entire Xd system.

And once you add a sub, the M2s and Xd are even more competitive. The SubSonic 10 ($2500), PSB’s best subwoofer, is self-powered and matches the M2s visually. It is at least twice the size of the XdW and almost double the weight. It is said to have a -10dB point at 24Hz. The M2/SubSonic 10 together, dressed with stands, will set you back $5000. The two can be considered a full-range speaker system that delivers reference-grade sound. However, to equal the Xd, you'd still need an amp to drive the M2s, and cables as well.

So, depending on the amplifier you use -- a reasonably good solid-state amp can be had for a grand, although, of course, you can spend a whole lot more if you want to -- the final sonic result may vary. I’ve heard the M2/SubSonic 10 system in my room and driven by a variety of amps -- the cheapest one being the DK Design VS.1 Reference Mk II, an integrated amps that sold in its day for $3000. While I can’t say that the PSB system betters the NHT system, or vice versa, I can say that they’re comparable in terms of the overall quality of their sound. Still, there are differences. The PSB system, on the one hand, is physically larger and sounds like it, despite what the NHT system can do at a fraction of the size. It can also play louder, go lower in the bass thanks to the SubSonic 10, and has comparable refinement and detail. The NHT Xd system comes close to the scale of presentation but in a much smaller package, which is darn impressive. Its sounds more immediate and precise, it looks even better, and it is easier to set up, partially due to its size and partially due to the fact that the XdS and XdW work perfectly together.

The point of all this is that for $6000, the NHT Xd system certainly has strengths, but it doesn’t clearly better the best of the current "traditional" competition. If you’re going to shop around, you’ll want to look at the NHT system, perhaps the PSB system, and similar systems from a variety of competitors. What the Xd system can claim, however, is that it can stand up with the best of like-priced products, and when you factor in the Xd's size, styling, and ease of use, you can understand why the people at NHT are justifiably proud of what they have created.


The NHT Xd speaker system reaches for greatness, and achieves it in some specific ways. Its big, bold, large-as-life sound comes from a gorgeous compact speaker system. In fact, it’s probably the best-sounding and best-looking small speaker system that I’ve had in my listening room. But the audible hiss through the XdSes, the mechanical whine from the XdA, and the auto-sensing glitch of the XdW mean that, despite greatness in some areas, there are bugs in the electronics that preclude it from being the breakthrough product that it purports to be -- and potentially will be if these bugs are fixed in the future. Please address these issues, NHT!

...Doug Schneider

NHT Xd Loudspeaker System
$6000 USD.
Warranty: Five years on speakers; one year on electronics.

6400 Goodyear Road
Benicia, California 94510
Phone: (800) NHT-9993

E-mail: customerservice@nhthifi.com
Website: www.nhtxd.com

NHT responds:

Thank you for taking the time to review NHT’s DSP-corrected speaker system, the Xd. We appreciate the attention and commentary; however, there are a few points we’d like to comment on.

As you may remember, when you first mentioned the buzz from the Xd system, I suggested that you try using the balanced inputs on the XdA processor/amplifier because the problem was probably rooted in the high harmonics from a capacitively coupled ground loop. This is a common problem with unbalanced high-end-audio components, and we were in fact able to defeat this problem by switching preamplifiers. After this episode, I never heard from you again regarding this or any other problem.

The signal-to-noise ratio of the Power Physics amplifiers used in the XdA are over 125dB. The hiss that you heard emanating from the tweeters is due to the signal-to-noise ratio of the A/D and D/A converters, which is about 103dB. In a system with an A/D or D/A converter before the volume control, the noise contribution of the A/D or D/A will drop as the volume is reduced from full level.

The volume control in your preamplifier is before the A/D conversion in the XdA. This means that the signal-to-noise ratio of the system is limited to the device with the lowest signal-to-noise ratio after the volume control. Since the signal-to-noise ratio of the A/D converter is 103dB and the maximum acoustic output of the Xd system is 114dB SPL at one meter, the result is a residual acoustic background noise level of 11dB SPL at one meter. This is very quiet.

I assume that the whine you heard from the XdA was due to an inductor in the power supply that was not fully potted. We were never given an opportunity to fix it, so I’m not 100% sure of the exact cause.

It is quite simple to move a jumper on the XdW power amplifier to defeat the Auto Power circuit that turns the XdW on and off with the audio signal. This would have fixed your problem with the XdW turning off during quiet passages, had we known about the problem.

The NHT Xd DSP-corrected speaker system represents the most advanced speaker technology on the market, as well as the latest achievement in the category. As such, we expect there to be a learning curve among end users, which is why we employ a customer service team that is second to none. Had you called and pursued all of these questions and issues with the company, I’m confident that we would have cleared up any and all problems, just as we do for every end user who calls.

Jack Hidley
Director of Engineering

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