LS3/5a Information & Archive

LS3/5-LS3/5A CROSSOVERS

 Crossover Design Type FL6/16, used in original LS3/5

The LS3/5 used the B110 A6362, the original 4–8 ohm version produced by KEF, and the T27 A6340 with an injection-moulded black plastic case, the tooling for which eventually failed, resulting in the T27 being re-designed to the SP1032 version for the rest of its production. The original network was a standard 4th order configuration for the L.F. section., but this was not right and the changes to the topology were made on 9th October 1970, where the circuit topology changed to similar to that on subsequent deployed in the FL 6/23 found in the LS3/5a. The values were of course different as the impedance and the early B110 unit parameters were different. There were only a few of these made, it is believed about 20 units in total, and it never entered production.

Crossover Design Type FL6/23, used in the LS3/5a from 1974 – 1987/8 (15 ohms)

Following changes in both the B110 and T27 in the early 1970’s, the LS3/5a was introduced in 1974 with a re-worked crossover now designated Type FL6/23.

This design went through various modifications over the years, due to manufacturing variations in the KEF B110 units.  These were caused by various things but mainly changes in the Bextrene supplied to KEF. This plastic was bought by KEF in small quantities of ½ a ton at a time and was not supplied as a consistent product.  When KEF protested about the variations - usually not notified - they were told they were such a small user and therefore of small consequence compared to the car industry which used in those days used tons per week.

The problem with B110’s over the years was not helped by the original selection of the reference unit by the BBC.  About 24 units were initially offered and tested, and instead of an ‘average’ unit being selected, one of the units with the flattest response was chosen as the Reference.  As a result, the numbers selected of “LS3/5a Grade” units was limited and rejections were not uncommon.  Subsequent changes in the network were made on two further occasions to accommodate the variations in production materials etc.

The basic changes notified by the BBC to the Specification were: -

C5 from 6.2mFd to 8.3mFd to 10mFd.
R2 from 33 Ohm to 22 Ohm.

The tweeter input capacitors C2 were increased by about 10% to compensate for a tendency of the T27 to have a rising response. This was spotted in the early 80’s when Goodmans took on one of the licences (from RAM, who had gone bankrupt). The capacitor changes increased the output in the 5-8kHz. range. This was notified in April 1984.

The changes in the FL6/23 to compensate for changes in the B110 response are tabled below.

Crossover Design Type FL6/38, used in the LS3/5a from 1988-99 (11 ohms)

Following increasing problems in achieving adequate production of consistent “LS3/5a Grade” B110’s, KEF was asked by the BBC to re-design the B110 to provide a more consistent product and make it more compatible with contemporary amplifiers.  This resulted in the introduction of the B110 6-ohm SP1228 version with a moulded plasticised PVC progressive surround (n.b the B110B SP1057 version was never used at any time in the LS3/5a).

The opportunity was taken to also re-design the crossover, dispensing with the famous ( and expensive) BBC auto-transformers, and the resulting Crossover Design Type FL6/38 instead used standard inductors with a resistive ladder for attenuation of the T27, which was retained.

Crossovers and B110’s were supplied to manufacturers pre-matched up by KEF, and nominal system impedance was lowered to 11 ohms.

© Malcolm Jones 2009, Falcon Acoustics 2021