BBC LS3/5A is a pair of legendary speakers. Since 1970s more than 60,000 pairs were being built and sold (most of the speakers were sold in Asia !). You still can buy a new pair of LS3/5A today (as of 2016) from Falcon Acoustic (Falcon Acoustic has a long history with LS3/5A and once they provided crossover to Goodmans.). Founded by Malcolm Jones, the father of T27 and B110 driver units, FA was sold to Jerry when Malcolm was fully retired in 2009. Recently, FA started to reissue the T27 and B110 with original recipes and specification. The brass grille for the treble unit is from the original brass sheets which were found in a warehouse of a Denmark factory.
After I moved to Canada, I started to learn woodworking, building all kinds of furniture and renovating the house so I have all kinds of tools. Instead of buying a new LS3/5A, why not to build one. So I got all driver units and crossovers from FA. Thanks Jerry! The only thing I need to do is to build the speaker cabinets.
First of all, I need a plan. No problem, here is the LS3/5A cabinet drawings (click here http://www.g4dcv.co.uk/ls35a/lsdrgs.html). Second, to find a right materials. No problem, 12mm Baltic birch plywood* can be found in selected Homedepot (but it is a BB/CP grade plywood with many voids. only size 2x4 feet is in-stock. I will get a better plywood from the local woodmill for my production run). I got a beech wood from a chop board and used it to make battens. If you cannot find the beech wood. Try use red oak since it has the similar density and stiffness/strength (see below table). The original LS3/5A uses bitumen pad as damping material. You can get a better performance damping material by using 3M Sound Reduction Mat which is used for car audio application.
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| Angle-joint with batten support, the most simple form of box making. |
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| Adding the damping pad (3M Sound Reduction Mat) |
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Since I treated it as a prototype so no exotic wood veneer was used. The birch wood itself is beautiful enough so I just use Tung Oil and wax to protect the box.
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| Apply Tung oil after sanding with 140/400 sand paper. Wax and polish with Finishing wax |
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Each cabinet will has its own resonance frequency and it will "Colour" the sound of the speaker. So, it is very important to match the cabinets even you have a matched pair of driver units. The resonance frequency is a combined effects with case volume, material density, weight, shape and the variants of workmanship such as the tidiness of the joints and the amount of wood glue used. It is better to make several and match afterwards. But in reality, home brew project cannot afford to do so. Therefore, a carefully selecting the panels and batten (weight the panels one by one and match) for each cabinet is very important. The weight different between these two prototypes is 3g (out of 2250g). The most easiest way to check if they are closely match is to knock on their back panels and listen if there are any different. To have a better measurement, it needs to measure the working speaker.
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| It is the table from BBC research paper "Factors in the design of loudspeaker cabinets" showing oak can be used instead of beech |
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