The KEF B110

The bass/mid drive unit used in legacy LS3/5As and the current Falcon model is the KEF B110 version SP1003.

Legacy 15 ohm versions of the LS3/5A made between the start of production and 1987 use the B110 version SP1003. The drive unit design and crossover was revised by KEF in 1987 and the newer 11 ohm LS3/5A uses a B110 version SP1228.

NB. The BBC considered the new 11 ohm LS3/5A to be a close enough match to earlier units that they could be mixed in a stereo pair. Had this not been the case they would have insisted that the new version be given a new designation- probably it would have been the LS3/5B to prevent mixed pairs being used. As a result Rogers continued to use up their stock of rear lables and approximately 5000 pairs were sold which although labelled 15 ohm, were actually newer 11 ohm units. The serial numbers of these fall in the range between about the early 31,000s and early 36,000s.

Falcon Acoustics are able to supply new B110 SP1003s manufactured to the original KEF specification and which meet the LS3/5A selection criteria.

It is possible to distinguish a SP1003 from a SP1228 by the appearance of the cone and surround. The SP1003 had a PVA coating damping material, Plastiflex, brush applied to the front of the cone by Rogers and the other licencees. The damping on the SP1228 was, I believe, done by KEF before they supplied units and was applied to the rear of the cone.

B110 SP1003
B110 SP1228

B110 SP1003 from 1977. Later units had a black chassis

B110 SP1228 used in 11 ohm LS3/5A

A few B110 SP1003 and SP1228 units have a TS1003 or TS1228 label. KEF have described this as a label printing issue, SP1003 is identical to a TS1003 and a SP1228 is the same as a TS1228.

Older B110 SP1003 often show a white ring around the central dome and these have become known as White Belly LS3/5As. Many enthusiasts believe these White Belly LS3/5As are desireable but it’s one of many urban myths about the LS3/5A. The white ring is indicative of a problem, and is due to the glue delaminating. So it is a fault. As old B110s age the LS3/5A develops a peak in its frequency response at around 1.5kHz. If this peak is only slight it can sound nice and gives the speaker an airy transparent mid-range sound. This may account for why some enthusiasts like older LS3/5As so much. I’ve certainly heard many older LS3/5As sounding lovely, but I have also seen & heard nasty sounding examples where the peak is extreme, sometimes as high as 10dB!

The 15 ohm BBC design used B110 SP1003 drive units right at one end of their spread of specification. As a result many otherwise perfectly good units were rejected for use in the LS3/5A. So there are many examples of 15 ohm LS3/5As where the drivers have been replaced with B110 SP1003s bought secondhand and pulled from old KEF speakers. The balance will be affected and the LS3/5A will fall outside original specification.

B110 A6362 4-8 ohm 1967-early 70’s. Unpainted frame, doping on front of Bextrene cone. Neoprene rubber surround. The A6362 was used in the original LS3/5, KEF Cresta and early Concerto.

B110(A) SP1003 8 ohm early 70’s-early 90’s. Used in the 15 ohm LS3/5A but selected because the BBC design used units right at one end of their design spread. Some have unpainted chassis but most are black. doped front Bextrene cone and Neoprene rubber surround. Also used by IMF.

B110C SP1228 6 ohm 1987-late 90’s. Used in the 11 ohm LS3/5A. Black chassis, higher power voice coil materials, Bextrene cone doped on back, PVC surround, production restarted 2002 in China but no longer made. Also used in the Rogers AB1 woofer.

B110 SP1380 is a SP1228 out of spec above about 250 hz. – can be used for AB1

B110B SP1057 8 ohm Mid 70’s-early 90’s. Black chassis, higher power voice coil materials, Bextrene cone doped on back, PVC surround. Used in the KEF Cantata, R101, R105/107s.

B110 SP1190 used in the KEF 104/2 speaker.

Early KEF B110s had a date label in the form of a letter and 4 numbers. A = January, B = February through to L = December. The first two numbers give the day of the month and the second two numbers the year of manufacture. So this B110 SP1003 with the date code B0777 was manufactured on 07 February 1977.