TAD TD-4001
A two-inch-throat compression driver whose beryllium diaphragm became the reference against which high-frequency horns were judged.
Overview
The first thing you notice when you pick up a TD-4001 is the heft. It has a two-inch throat and a four-inch-diameter beryllium diaphragm, and every part of it feels machined rather than assembled. TAD (a division of U.S. Pioneer Electronics Corp.) built it as the flagship of its compression-driver line. The marketing copy of the period was direct about why: pure beryllium diaphragms and surrounds that are, in TAD's own words, "extremely lightweight and rigid." Beryllium is stiff enough to push the breakup modes far above the passband, and light enough that the moving mass barely resists the voice coil. The company advertised the resulting high frequency response and sensitivity as "unsurpassed."
The TD-4001 didn't arrive alone. TAD paired it with the TD-2001, which the company described as combining the TD-4001's features in a unit half the size. TAD claimed the smaller driver delivered high-frequency sensitivity that outperformed drivers of any size without sacrificing power handling. Whether that held up in every cabinet is another matter. Either way, the family layout was clear: the TD-4001 sat at the top, and the TD-2001 was the scaled-down sibling for places where a full-size two-inch driver wouldn't fit. Both ran under what TAD called "the evolution of sound," a phrase the company repeated across its advertising through 1981.
Sound-reinforcement designers who needed a compression driver to keep up with large-format horn systems adopted the TD-4001. The clearest example in the surviving documentation is the Sundown Sound SDS 2000 mid/high enclosure. It housed a pair of TAD TM1201 12-inch loudspeakers with polymer-graphite cones in a horn-loaded midrange section, then added a single TD-4001. The TD-4001 was described there as a two-inch throat, four-inch diameter beryllium driver. Its acoustic center was physically aligned with the midrange drivers, so no time delay occurred between them in the critical crossover region.
The driver also turned up in the SDS 400SF, a full-range cabinet intended as a side-fill monitor or auxiliary main-fill speaker. That box packed four TAD TL-1601A 15-inch woofers alongside a single TD-4001 mounted on an NWS-340A horn. It was rated at over 130 dB SPL at 40 Hz with a continuous program power handling of 2,000 watts. The documentation doesn't say whether the TD-4001 was crossed over high enough to stay comfortable at those levels.
TAD operated as a division of U.S. Pioneer Electronics Corp. out of Moonachie, New Jersey. The TD-4001 carried the kind of engineering budget a major Japanese electronics manufacturer could spend on a component most buyers would never see. The beryllium diaphragm wasn't a marketing flourish. It was the reason the driver existed. Beryllium is expensive, difficult to form, and hazardous to machine, which is why so few competitors bothered. TAD did, and the TD-4001 became the driver that other high-frequency compression designs were measured against in the pages of the trade press.
Period advertising for the TD-4001 ran in the recording-industry magazines through the spring of 1981. DB-Magazine (May 1981, p.19) carried TAD's copy describing the driver's pure beryllium diaphragms and surrounds. The same text appeared in Modern-Recording (May 1981, p.95) and Recording-Engineer (April 1981, p.101). These were advertisements rather than independent lab tests, so the "unsurpassed" language is the manufacturer's claim, not a measured verdict. The technical substance (beryllium diaphragm, beryllium surround, lightweight and rigid construction) is consistent across every appearance of the copy.
The driver's place in the historical record rests less on sales figures, which aren't documented here, than on its appearance in systems like the Sundown Sound rigs. There it was one component in a chain that engineers described as unusually smooth and clean.
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