Simaudio N HT's Xd 5

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

N H T X d

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

m e a s u r e m e n t s , c o n t i n u e d

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 drive- unit’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.12 NHT XdS driven by XdA, equalized step response on tweeter axis at 50” (5ms time window, 30kHz bandwidth).

Fig.11 NHT XdS driven by XdA, step responses on tweeter axis at 50" of equalized tweeter (red) and woofer (blue). (5ms time window, 30kHz bandwidth.)

Fig.13 NHT XdS driven by XdA, cumulative spectral-decay plot at 50” (0.15ms risetime).