JBL 2450J
A 2-inch exit compression driver that put neodymium on the map for JBL - and got bolted into everything from stage monitors to a cryogenic refrigerator.
Overview
The 2450J turns up in places you don't expect. It's a 2-inch exit compression driver built around a neodymium-iron-boron magnet with iron pole pieces on either side, a titanium diaphragm, and an aluminum voice coil. JBL's own advertising from the period called it the company's first use of neodymium magnet technology. Anyone who has hefted a ferrite-magnet compression driver gets the point right away. The point was weight. The 2450J is smaller and lighter than any comparable compression driver JBL had built to that point, and the company claimed it was a full 1 dB more efficient than its own 2445J, which used a conventional ferrite magnet structure.
The other half of the story is the Coherent Wave phasing plug. JBL's marketing billed it as providing in-phase combining of sound waves, which the company said translated to smoother response and a clearer, cleaner signal. Take the advertising with the usual grain of salt, but the phasing plug was the real engineering story here. Neodymium was a means to an end. The rare-earth compound showed up in JBL's R&D lab shortly after it was first formulated, but its cost and heat sensitivity had to be solved before it could be used at all. The arrival of a lightweight, highly magnetic material happened to line up with JBL's development of the Coherent Wave plug, and the 2450J is what came out the other end.
For anyone running a rig, the practical consequence was spacing. JBL's own promotional copy noted that the 2450's smaller size translates to tighter spacing. Closer driver spacing means more coherent summing between adjacent boxes and horns, and it means you can hang more cabinets in a given array before the whole thing gets unwieldy. That's the argument JBL was making, and it's a fair one: a lighter, smaller 2-inch driver opened up system designs that a 2445J made awkward.
The driver's second life is stranger. A modified 2450J - specifically the neodymium-iron-boron magnet version, with titanium diaphragm and aluminum voice coil - was chosen as the electrodynamic transducer for the STAR project, a NASA-adjacent effort to space-qualify a continuous-cycle cryogenic refrigeration system based on thermoacoustic heat pumping. The refrigerator had no sliding seals, ran at a cycle frequency of about 300 Hz, and used acoustic resonance to boost power density and efficiency. The thesis work behind it built a computer model of the driver/resonator system and produced a final driver design that used the JBL compression driver as its electro-acoustic power conversion stage, with leak-tight electrical feed-throughs, a microphone, an accelerometer, a pressure gauge, a pressure housing, and a resonator interface. That's not a normal retirement plan for a PA driver. In that build, the housing, pusher plate, pressure lid, and reducer cone were all machined from 6061-T6 aluminum, all openings in the driver housing were sealed with 1/16" or 1/32" diameter lead O-rings, and the electrical feed-throughs were potted with Emerson & Cuming's STYCAST 2350FT epoxy. Wire leads were copper, the steel bolts connecting the resonator to the driver were 1/4-20, and all other steel bolts were 10-32.
Where the 2450J earned its keep was in JBL's Concert Series. The updated Concert Series line paired Vented Gap Cooling low-frequency transducers with the 2450J, and the combination significantly reduced overall system weights. It's the same pitch as the standalone driver, applied to a whole box. The 4802A and 4805A both used the 2450J coupled to a new 2396 diffraction horn, whose wide 160° horizontal pattern was pitched at freedom of movement on large stages. The 4802A was a high-power 2-way monitor with two 2206H VGC 12-inch low-frequency transducers. The 4805A was a 2-way with a single 2226H VGC 15-inch. The Concert Series also added two Diffraction Stage Monitors, the 4832A 2-way dual 12-inch and the 4835A 2-way 15-inch. Both used Vented Gap Cooling transducers downstairs and the 2450J neodymium 2-inch exit compression driver coupled to a diffraction horn upstairs.
The period press treated the neodymium angle as the headline. Sound & Communication (1988, p.68) ran the driver under a headline insisting it takes more than a little neodymium to change the face of driver technology. The magazine's framing was that the material alone wasn't the story, and that JBL's Coherent Wave phasing plug was the other half of the equation. The same copy noted the 2450's smaller size translating to tighter spacing, which is the claim JBL repeated across the Concert Series announcements in Mix (October 1988, p.195) and again in Sound & Communication (July 1989, p.60). Recording Engineer (undated, p.58) summarized the pitch plainly: JBL's first use of neodymium, smaller and lighter than any comparable compression driver, with a Coherent Wave phasing plug for in-phase combining of sound waves, and 1 dB more efficient than the ferrite-magnet 2445J.
The record has gaps. There's no confirmed production run, no original list price, no published frequency response, no impedance figure, no power handling rating, and no throat impedance in any of the documentation on hand. Collectors chasing a 2450J today are typically buying a driver pulled from a Concert Series cabinet or a studio monitor. The usual concerns for a titanium-diaphragm compression driver apply: diaphragm condition, voice coil integrity, and whether the phasing plug and throat are clean. The neodymium magnet itself is not typically the failure point. Because the 2450J was used across multiple JBL systems, spare and replacement units surface with some regularity, but a matched pair with clean diaphragms is a different proposition from a single loose driver.
The 2450J is a component, not a product. It's not a collector's trophy in the way a 4343 or an L300 is. It gets bought by someone restoring a 4802A or a 4835A, or by someone building a horn system who wants a 2-inch exit driver that doesn't require a forklift. It's a lighter, more efficient 2-inch compression driver that let JBL pack boxes tighter, and it did that job in a lot of systems.
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