Simaudio N HT's Xd 3

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 align- ment, and amplitude response in the

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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.

(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 ground- loop 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.