Loudspeaker
System:
The
loudspeaker is a Bi-amplified 4-way design featuring the following components:
Subwoofer(15
to 30 Hz):
AuraSound 1808, 18” long-excursion driver with underhung, neodymium motor
system. At the time of its
introduction (Mid 90’s), the 1808 was the lowest-distortion 18” driver
available. It was designed to
handle Dolby THX cinema sound program material in the 25-50 Hz range for
theaters seating up to 300 people (one driver).
Power handling = 800W continuous, 1100W peak.
The enclosure is 12 cubic feet with a 50 in2 shelf port tuned
to 13.5 Hz (the shelf is 45” long). The
back wall of the enclosure has a generously thick lining of fiberglass to
control the half-wave port resonance at ~150 Hz.
The vent itself, however, has no added resistance to air flow.
The alignment is an EBS (extended-bass shelf).
Although the full-range response is down 10 dB @ 20 Hz, the electronic
low-pass crossover is contoured to attenuate the driver’s output above 30 Hz
in such a way that the resultant crossover/driver response blends seamlessly
with the 10” midbass driver to give a system response that is –3 dB @ 16 Hz.
Due to the slow transition from passband to stopband response in the EBS,
the subwoofer gives usable response down to 8 Hz.
Electronic
Low-Pass Crossover:
In order to properly contour the midband response of the 1808 to blend
with the Midbass Driver, the crossover uses a quasi-third-order low pass
function. This features a 2nd-order
low pass Sallen-Key filter with an Fc of 33 Hz and a Q of 0.7071.
Additional shaping and phase compensation is performed via a lag-lead
(pole-zero) network that places an additional pole @ 7 Hz and a zero @88 Hz. The low pole frequency is largely responsible for leveling
the rising “shelf” response of the 1808’s acoustic output above 30 Hz.
The zero frequency is essential in matching the composite phase of the
subwoofer output to track the phase of the Midbass Driver.
Low-Frequency
Power Amp:
The Aragon 8008 BB is an extremely capable full-range amplifier, and is
therefore more than up to the task when used as a subwoofer amp.
Clean circuit design, twelve output devices per channel, dual-mono power
supplies, 140,000 uF of power supply filter capacitance, and large finned heat
sink are some of the features of this design.
The amplifier is rated to deliver 200 watts average continuous sine wave
power into 8 ohms (20 Hz to 20 KHz), and 400 W into 4-ohms, both channels
driven. The DCR of the 1808 is 5 ohms, which means the max average
power available from the 8008 is 320 WPC.
Midbass
Driver (40 Hz to 600 Hz):
The Focal 11K7511 (which is, unfortunately, no longer made) proved to be
the perfect complement for the big 1808. Its
combination of low Fs (29.5 Hz), medium Qts (0.36) and moderate Vas (94 liters)
and good Xmax (9.0 mm) made it the perfect candidate to function as a midbass
driver covering the most vital frequency range.
Housed in a 2 cubic-foot closed box with non-parallel walls, (affectinately
called the
) the LF
response is –3 dB @ 47 Hz and –6 dB @ 36 Hz (Qtc = 0.64 with 0.45-ohm DCR
series air-core inductor). The box
is ¾ stuffed with fiberglass and Dacron fiberfill.
The frequency response is smooth and uncolored.
This driver is the heart of the speaker system, as it covers most of the
music’s fundamental frequencies.
The
newer version of this driver, the 11K7512, has a higher Fs (38 Hz) and lower Vas
(52 liters) primarily due to a new spider design. The motor is identical to the 11K7511. This driver is more suited to high-impact vented designs,
whose greater phase shift below cutoff would present integration challenges with
a subwoofer.
Midrange
(800 Hz to 2.5 KHz):
Accuton C279. The
newest version of Accuton’s 3.5” midrange design, this driver features a new
suspension design that is a smaller, negative half-roll rubber surround (the C278
used a larger, positive half-roll suspension).
This driver, along with the tweeter, is featured in Avalon Acoustics’
$20,000 Eidelon loudspeaker system.
Tweeter
(3KHz to 20 KHz):
Accuton C212. Aside
from the new diamond tweeter made my Accuton, there may be no more transparent
tweeter available.
Passive
3-Way Crossover:
The 3-way crossover for the full-range speaker uses simple, basic slopes
with no tanks or traps anywhere. The
topology is paralleled filter networks for each driver.
Woofer and Midrange drivers have “Zobel” RC impedance compensation @
the voice-coil terminals. The
Woofer network is 2nd-order low pass @ 600 Hz.
The midrange band pass network is the combination of a 3rd-order
high pass and a 2nd-order low pass.
The Tweeter network is 3rd-order high pass.
Two
years were devoted to crossover optimization with the aid of RTA and pink noise
along with some MLS measurements taken with the LAUD suite. Each pass on the crossover was done in conjunction with
critical listening by the GOOB panel. The
midrange bandpass characteristic was the most challenging to integrate.
In this speaker system, the crossover points cannot be heard – only the
music.
The
crossover uses only the finest components available. Series capacitors on the midrange and tweeter drivers are
MultiCap PPFX-S separate film/tin foil polypropylene. Series inductors are Solen Litz-wound air core (even on the
Midbass driver). The Midbass
driver’s inductor has a DCR of ~0.45 ohms, which is taken into account in the
closed-box alignment. Resistors are
Link or Mills non-inductive wire wound.