KEF B200

A woofer that designers reached for when they wanted real bass out of a box that would still fit in the lounge.

Overview

The B200 is a KEF drive unit, not a finished loudspeaker. Keep that in mind when you go looking for one. It shows up in two very different places: as the bass driver inside KEF's own systems, and as a part chosen by hobbyists and small manufacturers building their own designs.

The most concrete technical picture of the B200 comes from a construction project in a hobbyist electronics magazine. The author picked the unit for a compact powered subwoofer and documented how it behaved. According to that account, the B200 has a free-air resonance of 25Hz, and in a sealed 2.4 cu.ft. enclosure that resonance rises to 45Hz. The same source calculated the driver's acoustic volume (Vas) at 5.38 cu.ft. - the figure that tells you how large a cabinet the driver wants. These are the numbers a builder needs. They also explain why the B200 turns up in reflex designs and not only sealed ones: there's enough Vas to work with that a ported box can be tuned sensibly.

A builder will like how willingly the B200 takes modification. The project's author found that a 3.4 cu.ft. cabinet - the size needed for a smooth 30Hz response - was "a little large to be accommodated in the average lounge," so he added mass to the cone itself. He used two ¾-inch squares of bitumised felt, spaced opposite each other around the center, cut from self-adhesive car damping panels. That dropped the free-air resonance from 25Hz to 21Hz and allowed a smaller 2.4 cu.ft. box at a 40Hz in-cabinet resonance, reflexed down to 27Hz once damping was added.

The same project noted that the B200 needed only 20 watts RMS of amplifier power in that application. At 30Hz the port and cone together effectively doubled the radiating area, for a calculated 90dB SPL at one metre. This is not a power-hungry driver. The author observed that the bass unit still had "plenty of headroom" even when the main system was producing 96dB, with peaks handled comfortably beyond the average figures.

The B200 was also the bass driver in KEF's Cadenza system. There it was the low-frequency foundation of a two-way system, crossed over in the region where the midrange unit took over.

Identification is where buyers get tripped up. The B200 appears under at least one specific part number in the wild - a classified advertisement in audioXpress sought "two KEF B200 SP1075 woofers (triangular 8" units used in the KEF 103.2 from the early 1980s)." That one line tells you the B200 is an 8-inch unit, that at least one variant (SP1075) has a triangular chassis, and that it was fitted to the KEF 103.2. Confirm the SP number before buying, because a B200 from one system is not automatically a drop-in for another. The triangular frame is also a mounting constraint - it won't fit a round cutout without modification.

Condition is the usual story with drivers of this age. The surround and spider are the parts that age, and a B200 that's sat in a damp cabinet for decades may need attention before it works properly again. The motor and cone assembly is straightforward and well documented, which makes it serviceable. Its value sits less in collector premium than in utility: it's a working driver, sought by people repairing a KEF system or finishing a DIY project, not by people putting it behind glass. Prices reflect that, and buyers should expect to pay for function rather than rarity.

The B200 is a component and not a landmark product. It did its job in the Cadenza, it did its job in the 103.2, and it did its job for the hobbyists who bought it bare and built around it.