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K2 is the name of the second highest mountain in the
world after Everest. It is also the name chosen for JBL's fourth Project
speaker. However, the name is somewhat misleading. While K2, the mountain,
will always remain in Everest's shadow, K2 the loudspeaker, would stand
above all of its predecessors.
Project K2-S7500
© Harman International, Courtesy
Greg Timbers
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Design work on the K2 began in 1988 as a follow-on
product after the success of the DD55000 Everest. At the time, it was
planned to produce a "Statement" speaker for the Japanese market every four
or five years. Whereas the previous Project speakers were single systems, K2
was designed as a series of speakers based on a common design principle. The
basic premise was a two-way design that would be unrestricted in dynamic
range and ultimate response. At first, two speakers were envisaged. The
top-of-the-line would be the K2-S9500 utilizing two 14" drivers above and
below a high-frequency horn. The K2-S7500 would utilize only a single 14"
driver with a horn mounted above.
As with the Everest, the concept for K2 was defined by
Bruce Scrogin, the President of JBL International, and a team was assembled
to execute the design. Greg Timbers was responsible for the system
engineering, cross-over, horn and enclosure geometry. Doug Button was
responsible for developing the bass transducers and Francher Murray was
responsible for the high-frequency compression driver. Brian Lusty, Product
Development Manager for JBL International, worked with Dan Ashcraft on the
industrial design.
1400nd Bass Driver
© Harman International, Courtesy
Mark Cerasuolo
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Unlike the Everest, the K2 was designed from a clean
sheet of paper. All of the drivers, crossovers and enclosures would be
developed from the ground up. Most unique was the 1400nd bass driver. This
would be the world's first low-frequency transducer designed with a
neodymium magnet structure. The starting point was the LE14A and the new
driver would use the same cone geometry. However, the cone construction and
suspension were specific to this design. Doug Button developed two driver
variants using underhung and overhung voice coil topologies. The overhung
design measured better, but the underhung design sounded better and was
adopted. Doug designed the driver around rugged, pro use parameters.
However, it was discovered that this compromised the reproduction of subtle
detail. Therefore, some degree of ultimate output was sacrificed to restore
this detail. Nonetheless, the 1400nd driver has inordinate output for a home
speaker.
475
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Compression Driver
© Harman International, Courtesy
Mark Cerasuolo
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The high-frequency compression driver was also a
dedicated design for the K2. The driver would be named the 475 in homage to
JBL's famous 375 transducer. As with the 375, it would use a 4" voice coil
and diaphragm and two-inch throat exit. However, the 475 would use a
titanium diaphragm and a unique phase plug (Coherent Wave™). The phase plug
used a curvilinear path in the annular slits to provide an equal length for
each slit from the diaphragm to to the horn throat. This ensured that the
output from each slit was combined in phase in the throat. Francher Murray
developed this driver in parallel with the professional 2450, but they were
significantly different. The 475 used an internal neodymium magnet structure
so that it is intrinsically shielded. The 2450 used an external neodymium
magnet. The 475 also had a copper shorting ring and an aquaplas coated
diaphragm. The aquaplas coating would later be applied to other JBL
compression driver diaphragms. However, the 475 diaphragm remained unique in
that it had a pure dome geometry. The 2450 diaphragm superimposed a rib
structure on the dome to increase output without breakup, This resulted in a
slight penalty in accuracy.
Project K2-S9500
© Harman International, Courtesy Mark
Cerasuolo
The above photo and one
the at the top of this article were part of a press kit that was
developed by Mark Cerasuolo, JBL International's' Director of
Communications. Mark states, "The photo was taken in either late 1989.
The speakers were functional prototypes, not production versions, and
the photo was taken late at night by local photographer Randy Fugate. We
set the speakers up in the JBL International listening room and borrowed
the faux-granite "JBL" wall hanging slab from the main lobby for effect.
We were trying to make a deadline for Harman Asia's marketing
introduction of that product."
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Greg Timbers specified a Bessel transformation tuning for
the low frequency drivers. This was in deference to the sonic preferences of
the Japanese market. In general, the Japanese market places priority on fast
transient response over deep bass extension. The Bessel tuning minimized
phase shift for unparalleled low-frequency transient speed. However, the
tradeoff is a falling low-frequency response compared to the flatter, and
more traditional, Butterworth tuning. A ten-sided enclosure was also
specified for each bass driver to minimize standing waves and panel
radiation. Further, the enclosure was made of MDF bonded to a shell of
Reaction Molded Foam to result in an solid, nonresonant cabinet.
Greg Timbers developed the horn as a constant
directivity, Bi-Radial® design. This was a significant departure from the
previous Everest. The Everest used an asymmetrical horn to provide a wide
soundfield of constant volume. The tradeoff in this design was that the high
frequencies could not be made phase coherent due to the unequal horn path
lengths. The K2 would revert to a symmetrical horn, but would use a constant
directivity design that provided wide, controlled dispersion that did not
change with frequency.
The K2-S9500 and K2-S7500 were introduced to the press
and reviewers in 1989. That year, the K2-S9500 received Stereo Sound
magazine's Component of the Year (COTY) award. It went into regular
production in 1990 and was a marketing success even with a price tag of over
$30,000/pr. The K2-S7500 did not receive the same market acceptance. In
hindsight, its price point was too close to the K2-S9500, and those that
could afford the K2-S7500 could likely afford the marginally more expensive
flagship speaker. Therefore, few opted for the K2-S7500 and it was
discontinued after a couple of years.
Project K2-S5500
© Harman International, Courtesy
Ed Lacinski
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Nonetheless, JBL recognized that there remained a market
for a lower priced system based on the same principle. This led to the
development of the K2-S5500 in 1993. It would use two 12" bass drivers and a
small format compression driver in the same configuration as the original
K2-S9500. JBL's product brochure describes this system's background and
design in detail, so it isn't repeated here. However, there are a couple of
features worth highlighting. That system pioneered an original cross-over
design developed by Greg Timbers. It is referred to as "Charge-Coupled
Linear Definition Dividing Network". It utilized a biasing voltage in the
crossover network to keep the music signal from crossing the dielectric
zero-point of the capacitors. The result is more linear operation of the
network with a significant improvement in sonic performance.
The second feature was the replacement of the Bessel
tuning with a new configuration referred to as "Imaginary Equivalent Tuning"
(IET). The intent was to combine the fast transient response of Bessel
tuning with the more extended response of Butterworth tuning. Greg Timbers
developed this design that is based on twin bass enclosures of different
sizes above and below the high-frequency horn. The drivers and ports of each
enclosure are tuned to different frequencies. The combined energy from all
four sources provides deeper extension than the Bessel tuning while
maintaining transient speed.
M9500
© Stereo Sound Publishing Inc. (Japan),
Courtesy Koji Onodera
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These two technologies were later applied to the original
K2-S9500 design to result in the M9500 of 1993. The M9500 used the same
components in a larger, industrial enclosure. The IET tuning allows deeper
and more linear bass output than the K2-S9500, with room response flat to
25hz. This system was intended as a professional monitor and did find its
way into a number of studios.. It was a very competent reference speaker but
was too big for soffit mounting and this limited its application. As a
result, it was much more successful in the consumer market and received
Stereo Sound's 1993 COTY award.
The K2-S9500 and K2-S5500 remain in production and are
produced in limited runs as demand permits. The M9500 has not been produced
since 1997. Nonetheless, this series remains the pinnacle of loudspeaker
technology yet developed by JBL Innovations associated with this series have
been incorporated into professional products such as the DMS-1 monitor and
Array series of tour sound systems.
© 2000 Don McRitchie
based on information provided
by Greg Timbers, Mark Cerasuolo
and Koji Onodera
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