LS3/5a DIY info.

The LS3/5a is a legend, as a grade 2 monitor for OB vans for the BBC it has, since the 70's become a cult speaker. Why, well its small, images well, fits in many places in the home, is quite uncolored and plays loud enough for most home listening. It ís a beautifully smooth sounding speaker, the only shortcoming is no real deep bass but still acceptable.

Building an LS3/5a is not a piece of cake it needs care in materials, quality of drive units most will be second hand so beware, accuracy of construction and care in assembly is important. At various stages the B110 SP1003 drifted out of spec. i.e. the peak that occurs at 1100hz was rather too prominent, my samples (6) are good in this respect so be aware choose carefully listen to them if at all possible, also some T27ís tended to have a hot top end and a higher dome resonance than spec up from 1100Hz to as high as 1300Hz once again listen and measure.

Hopefully this document will help centralize sources of information and also give some useful inside info, I am only covering the B110 SP1003 / T27 drivers other newer B110s need different crossovers, Some links in here will be helpful if you have newer B110. Check Falcon Acoustics web site. (I have no connection with Falcon btw.)

Most people buy second hand LS3/5aís on Ebay, being the place of choice.

Some links a must read

It ís wise to download and print what you need for DIY from any sites given here, sites do come and go.

A link to my site with info on how to build your own version of the BBC LS3/5a

Other resource sites... (very complete general LS3/5a site) (various links) ready built crossovers for the LS3/5a.

Cabinet link for the exact BBC box dimensions in Birch Ply

Drive units.

These drive units are out of production btw. So look for Ebay sales or older KEF speakers systems going back to the 70s e.g. Concerto or Coda others too.

B110 SP1003 bass mid unit the original version, 8ohm neoprene surround (SP1057 or SP1022 version vinyl surrounds not compatible)

Hand Damped Bextrene cone using PVA coating each side, very large magnet and 6mm Xmax cone excursion, it has a peak around 4.5Khz other wise smooth.

(SPXXX no is a KEF design variant)

T27 tweeter SP1032 version. 19mm Melinex vacuum formed Dome Aluminized and painted black the magnet has a small rear chamber behind the dome with inserted damping material in the cavity, it shows very good pulse response is clean but some think a little tizzy sounding compared to modern units, its still pretty good never tiring though pretty advanced for its time, A tribute to KEF's Ray Cooke ... founder of KEF. he was an early adopter of the dome principle Ed Villchur (AR USA) was the other.

Who built the first LS3/5a licensed systems?

Rogers built a good many BBC licensed Ls3/5a's with the above drivers and many are still used by home users.

The Crossover.

Crossover is basically a 3rd order Butterworth, with a series trap circuit to the bass unit to null a surround / cone peak at ~1100hz of a few db, there is also 6db boost to the bass unit for diffraction compensation from ~1khz leveling of at 100hz there is also a shallow trough centered at 500Hz this gives neutral flat response when the speaker is kept away from boundary walls on a shelf flat against a wall bass will be a bit thick so open stands and away from walls is best but I have one pair on a shelf, amplifier bass cut solves the lumpy bass.

To design this crossover is complex but if built to specification (KEF CS1A) gives a wonderful smooth response. You must match the B110 spec no to the correct crossover other newer B110 spec numbers need a diff. crossover.

Crossover for DIY. Either buy complete or do your own. Built up from here

I favor the KEF LS3/5a CAD designed crossover (CS1A) if you want to do it all yourself, you will need to verify component accuracy with necessary equipment, ready built is far easier (see Falcon Link). The CAD design that KEF designed for the B110 SP1003 / T27 SP1032, has an improved High pass filter to compensate for T27 shortcomings i.e. Undamped 1100Hz dome resonance and smoothing of the top octave 10-20Khz.

The Cabinet

See this link.

Stirling Broadcast can sell completed finished cabinets with baffle and grille

Or for DIY get panels cut locally by a cabinet shop or cut them yourself, the baffle is going to need patience in machining for the DIY selfer, the T27 recess and baffle screw holes need precise work, you will also need a router and some skill this is a very small baffle. If the walls are precut butt joints work ok corner fillets are a must. Veneer over later.

The BBC specified cabinet material is 12.7mm birch multi-ply with bitumen pads to control cabinet wall resonance. My method is thick ultra braced Plywood or MDF panels. In practice my cabinet has NO vibration at high sound pressures. To achieve this you need to use battens on the inside cabinet walls with cross bracing wall to wall. The idea is to have no more than 4" (100mm) of unsupported walls for 19mm material this puts wall resonance up as high as 2kHz or higher.

Whichever way you go keep cabinet dimensions close to the original but compensate a little where needed for wall thickness but within reason, it's not rocket science, just a speaker cabinet.

You can also make the cabinet deeper to improve bass response keeping frontal dimensions to specification.

25mm Foam lining can be used for standing wave control or I prefer long haired sheep's wool. The baffle must be around 9mm thick 19mm is NOT acceptable, the B110 MUST be rear mounted and seating on a gasket bolted to the rear of the baffle air sealing is important care is needed here, rear mounting absorbs a B110 chassis resonance. The T27 must be flush and 10mm square strips of felt arranged in a rectangle to control edge diffraction. 120 X 110 mm overall dimensions. The baffle should bed on foam strips with ample retaining screws. Polyurethane varnish several coats and rubbed to a smooth finish with 600grit Wet Dry carborundum paper buffed and finished with a teak oil looks classy and is my choice of finish.

Additionally pages relating to the KEF Constructor CS1A were also supplied by John Smith as part of this article on the LS3/5a. These included: Specification of the CS1A; Cabinet details; Crossover details.