Sunday, 1 February 2015

Alpair 12P MLTLs Part 1

I've had a pair of Alpair 12Ps for well over a year now which have been "running in" in the Quasars. But the intention has always been to build a pair of MLTLs in an attempt at domestic compromise. It's a design by Scott Lindgren which he very kindly allowed me to use. Thanks also to Colin Topps for his invaluable advice, particularly on routing techniques.

The 12Ps are a nice driver with a decent if not stellar 92dB sensitivity and a well balanced sound. For a wide band driver they have a well judged frequency compromise and reports of this MLTL cabinet suggest there is good bass. But the 12Ps do roll off about 12k Hz which I find audible, missing that little bit of sparkle, so I'm going to use a little bit of support above this frequency.

Cabinet build - Day 1 (4 hours)

Starting the build on the last day in January is perhaps not the most sensible time as it was a little chilly, a heady 3.5 degrees C. Fortunately I was out of the wind at least. (The thermometer reads 5 degrees C too high. My better half never really liked it, which is why it's relegated to the garage, but whenever I look at it I think of beautiful Seville, from whence it came.)

So here's the starting pile of wood, a single sheet of 2440mm x 1220mm x 18mm solid birch ply.

I took the easy road and got the timber yard to convert the sheet for me to my cutting plan. The upside is it saved me a whole lot of effort for a tenner, the downside is I didn't get to control the cutting. 4 no. fronts and backs at the rear, 4 no. sides in front, 4 no. tops and bottoms to the left, and some bits left over to the right which I might use to thicken the base up.

Examining the fronts and backs I wasn't particularly happy as there were the odd marks on the faces, a couple of well filled but unsightly knots, and quite a lot of the grain had been ripped out by the saw cutting across the grain of the exterior plies. Still, no going back now, just pick the best compromise and get on with it!

The front baffles need thickening up with a second piece - this improves the rigidity of the baffle but is also required because the depth of the rebate to suit the 12Ps would leave precious little of the thickness left to attach the driver to. In an ideal world I would probably have used more birch ply but I had some 18mm thick mdf offcuts and wasn't about to spend another £55 on a second sheet of birch ply. It probably makes little difference to the sound anyway.

I drilled and countersunk the secondary baffle then brushed on some plenty of pva, perhaps too much...

And then clamped the first edge together and fixed some screws to hold the two pieces together, then turned them around to clamp and screw the opposite side. Perhaps just a little too much pva. But better than not enough I guess. I took quite a bit of care to ensure the screws didn't damage the fronts, but even so I was a little nervous until I was able to examine them later to put my mind at rest.

Both baffles completed and left to dry after quite a bit of repeated mopping up of squeezed out glue. Really the glue needs to dry overnight, and preferably 24 hours. And be a bit warmer.

It was about 6pm when I finished by which time it had reached a balmy 5 degrees C. Hopefully it'll be a bit warmer the next time!

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