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.
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.
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.
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.
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.
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