What's Inside?

The following pictures are of a 15 ohm Chartwell Project 50 LS3/5a. Other manufacturers' versions (including ones made in the Chartwell factory) are similar but somewhat tidier than this example. These pictures can also be viewed with reference to BBC Research Department Technical Report RD 1976/29 and A Little Legend, The BBC LS3/5A. Which was written by Trevor Butler and originally published in HiFi News and Record Review in January 1989.

Project 50

A Chartwell "Project 50" LS3/5a. The design of the LS3/5a became very popular with BBC staff and many of them wanted pairs for use at home. The speaker was never cheap, tight manufacturing tolerences demanding that all the components be selected. Arrangements were made with Chartwell to supply cabinets, crossovers and drive units in kit form allowing staff to assemble their own speakers. Project 50 kits were never intended to be available to the general public although many pairs were bought by staff on behalf of their friends. The "50" refers to the price of a single unit, £100 plus purchase tax per pair.

Provided that they were built carefully Project 50s should sound the same as their factory made counterparts.

Project 50

The original Chartwell instructions for building the Project 50. Click on the image for a larger view. (With thanks to Dave Plowman).

The Drive Units

T27B110

The drive units chosen for the LS3/5a were the KEF T27 tweeter and KEF B110 bass/mid-range driver. The T27 nominal impedance is 8 ohms with a free-air resonance of 1200Hz. The B110 SP1003 (the redesigned LS3/5a, usually known as the 11 ohm version, used a revised model B110), had a free air resonance of 35Hz and nomial impedance of 8 ohms. All drive units were pre-selected selected at the KEF factory.

Variation in manufacturing saw a change in crossover design in the 1980s. LS3/5as are balanced for use with the Tygan grill in place. The T27 has a square felt surround to prevent interference effects associated with the cabinet edges. The tweeter dome is covered with a metal grill for protection from a Celestion HF2000. The grill has a small effect on the frequency response which was considered beneficial as it raised the output a high frequencies.

The Cabinet

Project 50

The cabinet external dimensions are 31 x 19 x 16 cms giving a internal volume of close to 5 litres. The cabinet walls are made from 12mm (0.5 inch) selected birch plywood with internal fillets from beech. Because of the critical nature of the design it was found that the substitution of other wood for the walls or fillets caused clearly audible changes. The use of any hardwood for the beech fillets produced a audible colouration caused by the resonance of the B110 on its chassis. The inside of the cabinet walls are lined with car body damping panels, a single layer is used on the side walls and two layers on the top and bottom. All internal surfaces except the front panel are lined with polyurethane foam; 25mm thick on the top & bottom and 16mm thick on the sides. The cabinet, including the screw holes, is carefully sealed.

The Crossover

CrossoverFL6/23

Because the LS3/5a is a passive design, bass extension can only be achieved by "throwing away" sensitivity at mid and high frequencies. This accounts for the very poor sensitivity of the design.

The crossover shown is model FL6/23 which was in use in 15 ohm models until 1987. A circuit diagram and photographs of the 11 ohm version used after 1987 can be found at Eric Chu's web page

The response of the B110 is equalised by the 159mH inductor and 82R resistor. The 10uF capacitor, 2.57mH inductor and 22R resistor (note that these values have been documented elsewhere as 6.2uF, 2.6mH and 33R), compensate for the hump in this characteristic. The crossover frequency is 3kHz. L3 operates as a shunt inductor for the T27 tweeter and is adjustable to allow matching of different sensitivity T27 and B110 units. C2 is selected on test (SOT) to keep the crossover frequency constant.

The crossover PCB is mounted behind the tweeter with a piece of felt between them to damp board resonances. Note the crossover tap (L3) which was adjusted on test.


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