Electro-Voice 15TRX

Forty-four pounds of concentric three-way engineering that put a woofer, a midrange, and a tweeter through a single hole. A lot of two-way systems sounded like they were missing something next to it.

Overview

The 15TRX is a 15-inch triaxial reproducer. A woofer, a midrange, and a tweeter share one die-cast frame and one magnet assembly, with the smaller drivers mounted concentrically inside the big one. Electro-Voice announced it in 1954 as its "latest contribution to the advancement of high fidelity sound." The pitch was straightforward: you get the benefits of a separate three-way system, but the drivers radiate from a single point, so the response stays smooth and the sound appears to come from one source rather than three. The whole assembly weighs 44 pounds and carries a 5¼-pound magnet. It is not a bookshelf driver, and nobody at E-V pretended it was.

The headline number is the response: 30 cycles to 15,000 cycles, specified as ±5 dB in E-V's recommended enclosures. A ±5 dB window is honest rather than flattering. It tells you the 15TRX was designed to be installed in a proper cabinet, either a direct-radiator box or one of E-V's folded-horn enclosures, and not bolted into whatever was lying around. The rated impedance was 16 ohms, which suited the tube amplifiers of the day. Two crossovers divide the spectrum: a mechanical one at 2,000 cycles and an electrical network at 3,500 cycles. The network is built around a full half-section M-derived design that E-V claimed minimized distortion products.

E-V's engineering story leaned on the voice coil. The 15TRX used an edgewise-wound coil, which the company said afforded 18 percent more efficiency than a conventional winding. Free-air resonance sat at 38 cycles, low enough that the driver had real output down to the bottom of its rated range. The treble was adjustable, too. The 15TRX shipped complete with a brilliance control, an AT-37 unit that could be mounted remotely and used to match the speaker to room acoustics. Nobody had measurement software in 1954, so that was a useful thing to have.

Pricing was tiered in the way hi-fi was sold back then. The 15TRX listed at $225.00 with an "Audiophile Net" of $135.00. The list price was the fiction, and the net was what a dealer would actually charge. By 1957 the net had climbed to $145.00. The 15TRX was the bigger sibling of the 12TRX, a 12-inch version of the same concentric three-way idea that listed at $190.00 and netted $114.00. It sat above the simpler two-way Radax coaxials like the SP15 and SP12. E-V sold the 15TRX into its Regency enclosure, which gives a sense of the footprint the driver expected.

Contemporary coverage of the 15TRX specifically is thin but real. E-V's own advertising through the mid-1950s billed the Triaxial line as getting "the most from specialized driving media for each portion of the audio spectrum," promising augmented bass, a full-bodied midrange, and silky upper octaves without masking effects. That is marketing copy, not a verdict. The underlying geometry, three drivers on one axis, was a real engineering choice.

The 16-ohm impedance and modest sensitivity suit low-powered tube amps, and the concentric mounting means it images in a way that separate-box three-ways of the era generally did not. The brilliance control gives some tailoring to modern rooms. The ±5 dB envelope means the driver rewards a proper cabinet and punishes a hastily built one. The 15TRX turns up in period classifieds: a 1991 swap-sheet listing offered a pair of 15TRX MK II triaxials at $170. The original version is the one to look for if period correctness matters. The X36-1 crossover network and AT-37 brilliance control are part of the package, and a driver missing either is an incomplete example.