Hi-Fi Loudspeaker Projects

I have in my garage a pair of speakers that I really love. They are large, loud and quite pretty in an old style sort of way. They are also totally impractical in a modern house. They were built with the idea that, eventually I would earn enough money and have a big enough house to have a dedicated room just for them. Well, its not likely to happen but I do have the advantage of a good functional although small workshop in my garage which is ideal to use these monsters.

I started off with a pair of Goodmans Audiom 12 inch 15 ohm drivers with Axent 100 15 ohm horns. They were supplied as junk from a friend of a friend along with a BTH valve pre and power amp combo which is to be one of those refurbishment projects later in the year (or decade more like!). Cost was £15 in around 1973-4. The cabinets were pretty disgusting being 38 inches tall, 24 inches wide and 16inches deep. They were ported with a hole in the baffle some 8 inches square. Finished in a dark oak veneer with a recessed front panel. They sounded horrible! Slow, i.e. over damped, no stereo image to speak of due to the front panel recess (over 2 inches at the top because of a sloping baffle), nasty treble rather like those early metal dome tweeters we had at the start of the digital age, poor bass extension, recessed midrange with some peculiar lumps and bumps in the frequency response. I used them as they were with minor xover tweaks  for about a year then the bug over took me and I had to do something. A visit to Henry's in Edgeware Road got me two 12 inch Dalesford bass drivers rated at 200 watts. These were the for-runners of the later spec Cobex cone version that appeared in the Dalesford speakers of the early 80's I believe. A pair of 5 inch paper cone mid range and Seas H107 ferro fluid tweeters and I was off into another world. The new mids and treble units were placed in a pod on top of the main cabinets so they were rather like an even bigger Kef 105! All told the sound became more "Hi-Fi". Things improved so much that a radical re engineering task began. First job was to rotate the cabinets and install the drivers in the sides which of course now became the fronts if you see what I mean. The top pods were removed and are now part of another set of speakers. Now we had the narrow face to the front and a much better looking cabinet albeit very large. Internally, with the front and rear old baffles removed, the cabinet was split into a bass enclosure and a midrange enclosure by adding a braced 1 inch chipboard divider from the rear top edge to approximately 18 inches down the front baffle. A batten supports the edges on all sides of this panel with a stiffener bar running from the front to the back of the enclosure at an angle to offset any resonances in the wood.
Directly below this was the port. This took some designing. It needed to be about 3 inches deep to start with and various changes were made until a suitable cut-out and depth was arrived at. A 4x2 inch cross member was added about mid panel below and behind the port to give some additional structural strength and to dampen down any panel resonances caused in the bass cavity. This was glued in and screwed to the cabinet sides with 4,  6 inch N0:10 wood screws.  
This is the basic layout for the speaker. Due to their age there have been some changes to the drivers recently. The bass drivers eventually went the way of all foam edged drivers after 15 years or so. With just the paper of the cones left I decided that it was time to replace them. I now use a much more advanced poly cone 200watt driver. I have installed it in the box to get it run in before I make any drastic changes to the box. I intend to fit new suspension to these old drivers from probably one of the suppliers from EBay                                              
Midrange has also changed due to the same problem, i.e. the failure of the foam suspension. So the new drivers are larger, 6.5 inch units with bright glass fibre yellow cones and rubber surrounds as seen in B&W speakers of the recent past. This of course meant that new top baffles were required. The baffles are in effect double thickness three quarter chipboard. The baffles are made to fit the drivers such that they are all front mounted whilst the rear baffle is a cut out in the slab of the enclosure that just allows the basket of the drivers with a little clearance to poke through. This means that there is ample rigidity at the all important driver interface.
The tweeter has always been consistent. It is very smooth and predictable and has none of the odd response spikes that seem to be part of the design when looking at metal domes or other less refined designs. I would say that, if anything it could do with having rather more output as it is hard pushed to produce levels that match easily with the other drivers. This however isn't a problem if you are sitting at the correct level to speakers. The dome is of treated man made fibre, probably some sort of silk equivalent and the coil sits in a ferro fluid gap. I think these tweeters will be staying in these speakers for the next few years!
With the baffles both standing proud of the shell of the enclosure I have made an edge of quarter round hard wood leaving a gap of some 12mm between it and the baffles. This has given me the ideal means of securing the front covers whilst minimising the dispersion effects of parts of the enclosure standing proud of the tweeter dome. A 12 mm wide frame was built such that the grill cloth could be squeezed between the parts leaving a finished 4 X 20mm outer frame on a trimmed 8 X 18mm former. The 8 X 18mm shape was made first, the cloth being stretched over it then the outer wood glued and screwed through the frame so that the good wood was on show. In effect the drivers front edges sit only 1mm behind the grill cloth and the front of the speaker is thus flush with the exception of 2mm of protruding edge timber. No clips or clamps are needed to secure the grill as it is a smooth but quite tight fit in the recess in the front of the speaker.
Then once dry, square and working, use 1/8 or 1/4 ply with a good grain structure to provide the covering. Corners can be easily constructed by sanding rather than trying to cut it with a saw so you need only some simple experiments to see how well you can do it. Glue the panels down and use fine panel pins to secure. Edge can be fitted using either off the shelf hardwood moulding or can be cut from stock timber and planed / sanded into shape. Ensure the edges are in real hardwood as they get the most knocks. Once this is all done and you have no resonances due to inadequate glue etc you can rub down, fill, rub down again, stain, rub down again, re-stain again and then varnish. Its a long process but it is worth it. Especially on a big unit like this!


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