Stromberg-Carlson RF-475
A fifteen-inch coaxial that carries 168 ounces of Alnico V and treats its woofer and tweeter as two separate engineering problems.
Overview
A fifteen-inch coaxial weighing roughly forty and a quarter pounds net is not something you install casually. The magnet assembly explains why. Stromberg-Carlson built the motor around 168 ounces of Alnico V in a parallel-gap structure. High-Fidelity (January 1954, p.27) singled it out, describing the industry's biggest, heaviest magnet of the day at ten and a half pounds of Alnico V. All that metal buys flux density: 15,500 Gauss in the woofer gap and 11,000 Gauss in the tweeter gap. Higher gap flux means better low-frequency damping, and that is the first thing the design chases.
The driver splits its work cleanly. The woofer covers 30 to 1500 cps, and Stromberg-Carlson rated it for more than 100 watts of program power. Put it in the company's Acoustical Labyrinth enclosure and the bottom of that range drops to 20 cps. The tweeter is a pressure type feeding a parametric horn with an acoustic lens. It takes over from 1500 cps, runs to 20,000 cps, and handles more than 32 watts. Each half gets its own voice coil on an aluminum form - three inches for the woofer, one and a half inches for the tweeter. DC resistance is 7.8 ohms and 5.2 ohms respectively, feeding a nominal 16-ohm system impedance. Intermodulation distortion was specified at 1.4% for a 1:1 ratio between 200 cps and 2,000 cps at 5.5 volts, a test input corresponding to an instantaneous 6-watt power input. Power response was held linear within 3 dB.
Dispersion is where the coaxial layout pays off. The woofer is rated at 180 degrees and the tweeter at 100 degrees, the latter widened by the acoustic lens and its multiple-layer dispersion system.
The RF-475 was not a lone project. The engineers reportedly checked the line at frequent intervals with both laboratory measurement and comparative listening against live symphony concerts, FM broadcasts, and the best LPs and tapes they could get.
Living with one has trade-offs. The 16-ohm impedance and the efficiency implied by that magnet structure make the RF-475 friendly to the low-power tube amplifiers of its era, and the coaxial format means a single cabinet does the whole job without a separate tweeter box. But forty pounds of driver plus a magnet that size demands a cabinet built to take it, and the Acoustical Labyrinth is not a small piece of furniture. The tweeter's 1500-cps crossover point and horn loading are period-correct, but a modern listener expecting extension far above 20 kcps will not get it.
Collectors chasing the Custom Four Hundred era tend to note that the RF-475's Alnico V motor is both its glory and its maintenance concern; magnet assemblies of this vintage deserve a check before any serious listening session. Original Stromberg-Carlson documentation for the RF-475 is thin on the ground, so buyers should lean on what the period literature actually states rather than on later folklore. When one surfaces complete and unmessed-with, it is a rare chance to hear what a 1950s high-efficiency coaxial was really capable of.
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