KEF B-139 (active in period sources, 1964–1979)

A flat, rectangular bass driver that looked wrong next to every round cone of its day. It ended up at the heart of some of Britain's most respected speaker systems.

Overview

Thirteen inches by nine, and flat as a board. The KEF B-139 is a bass drive unit, not a complete loudspeaker. Its most obvious feature made it an outlier in the mid-1960s: a rectangular, solid, flat diaphragm at a time when almost every woofer was round and conical. Collectors who have pulled one from a cabinet know the look - a wide, shallow "racetrack" rectangle, which some sellers still advertise under exactly that word.

The driver was made in England, and Falcon Acoustics credits its design to Malcolm Jones, the man behind both the original B139 and the B110. It carried the catalogue reference B139 SP1044 by the time it appeared in KEF's late-1970s literature, and variant names like B139B and B139 A6171 turn up in the record as well. The documented activity window runs from 1964, when it was already installed in a production KEF system, to 1979, when KEF was still offering it to DIY builders. Owners report the design belongs to the expanded-foam class of bass units, of which the B139 is the most famous example. That class came with real trade-offs: piston-band operation through the midbass, but very low efficiency, limited power handling, and severe high-Q resonances in the midband.

Impedance is 8 ohm, per the B139B SP1044 documentation and Falcon's own reproduction listings. Beyond the face dimensions and the diaphragm description, the bare-driver specification sheet has not survived in the archive's material. There is no sensitivity, no Thiele/Small values, no original price. Sources note that T/S parameters exist in old KEF spec sheets, but those numbers aren't reproduced anywhere in the research on hand. Anyone quoting a full parameter set for this driver is working from a document the Archive hasn't seen.

Key Features

Rectangular 13 × 9 Inch Flat Diaphragm

This is the defining feature, and the reason the driver is still recognizable at a glance. A solid, flat diaphragm in a 13 × 9 inch rectangular frame is a different object from a round cone, and in the mid-1960s that difference was conspicuous. Whether the flat face actually delivered an acoustic advantage is harder to establish from the surviving record. What's documented is the shape and the diaphragm construction, not measured performance.

Expanded-Foam Bass Construction

The B139 sits at the head of KEF's expanded-foam bass units. Piston-band operation through the midbass was the payoff. Very low efficiency, limited power handling, and severe high-Q midband resonances were the cost. That means the driver wants a generous amplifier and doesn't reward being pushed hard. It also means the midband needs help from the crossover and enclosure rather than being left to run wide open.

A Natural Fit for Transmission Lines

Builders who have lived with the B139 for decades tend to run it in transmission-line enclosures. One long-time designer reports still playing TL speakers loaded with B139s. The driver also turns up in published triangular transmission-line designs and in dual-driver "coffin" subwoofer plans. The rectangular face and the flat diaphragm suit cabinet work that round cones make awkward.

Historical Context

The B139's first documented appearance is inside a finished product, not as a loose component. The KEF Duette, a compact two-way built around the B139 rectangular bass unit and a 1¼-inch T15 spherical dome tweeter, was documented in 1964. The Duette measured 24 x 15 x 9¼ inches, weighed 48 lb, was rated at 15 watts maximum, and sold for £39.19.0.

Fifteen years later it was still in the catalogue. A KEF advertisement in Audio (April 1979, p.31), built around the company's "Total Design Concept" of computer-aided integration of cabinet, drive units, and filter network, listed the B139 SP1044 as a 13 x 9 inch bass driver with a solid flat diaphragm and pitched it squarely at DIY builders. Alongside the Duette, the driver is documented in the KEF Cantata and the KEF Concerto, in the Linn Isobarik, and, in its B139 A6171 variant, in the Tangent TM2. KEF's own DIY ecosystem grouped it with the B110 and T27, a trio that turns up together in period do-it-yourself speaker projects.

Collectibility & Value

The original is out of production. One used pair was listed at £45 for the pair in "very good condition for age." That's the only concrete asking price in the record, so treat it as a data point and not a benchmark. A truncated note on Cantata woofers hints at surround condition as the thing to check, though the full claim isn't present in the sources. Buyers should inspect the surrounds themselves.

For anyone who needs a working driver rather than a collector's piece, Falcon Acoustics has been remaking KEF drive units, and its B139 reproduction is described as the only genuine equivalent available. The reproduction is not identical to the original. It carries greater power handling, now 150W, and an extra 25mm of chassis depth, helped by a bump-stop bottom plate and a vented pole. Owners of the reproduction report it plays much louder thanks to a longer Xmax and a larger motor structure.