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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 dri-
vers and synthesized both digital filters
to linearize phase response, time align-
ment, 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-and-
joy 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 out-
put 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 fre-
quency 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 fre-
quencies 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.