Alpair 12P MLTLs - Part 3

Day 3 - 4 hours

Today was all about learning how to use the router guide and some large circle cutters. It was the first use for the router too, a £12 special from either Aldi or Lidl, I don't remember which. It's not worth thinking about how they managed to manufacture and ship it half way across the world for 12 quid, even when reduced to half price.

So, nearly 8 degrees C today.

First job was to finish off the remaining three braces including a light sand. I might just rout an arris around the internal edges later.

Rather than bodge some rather rough holes for the various bits and pieces that are to be mounted in the cabinets I wanted to make a proper job so used the template guide that came with the router. This of course means I need some hole templates, so I bought an adjustable hole cutter from Toolstation for 8 quid.

Decent ones look to be four times the price so I was a little wary about its quality. As I haven't used any others I can't compare, but it's a bit fiddly to set up (so might others) and though it reckons to cut up to a depth of 30mm I struggled to do much more than 3mm in mdf. The cutting tips are pointed, and once most of the point has entered the mdf the mdf started to burn.

Maybe more coarse materials would cut better but I simply flipped the mdf over and cut from the other side. The battery drill struggled a little to drive the cutters but it's a lot easier to hold. When I cut the larger holes for the 12Ps I think I'll probably need a mains drill.

Now to check to see if the template is the right size. I couldn't find much scrap wood, apart from a rather narrow piece, but it worked just fine. The template guide is shown on the underside of the router below.

The 6mm router cutter...

First pass of the router, about 6mm deep

After a couple of passes, 12mm ish

Three passes and done. There's a slight lip at the bottom around about a third of the circumference. I tried cleaning it up but the router didn't touch it so I must have managed to get a little bit of twist in the router. But it's not an issue because it will be covered. If I was going to use it anyway.

Pretty much perfect size, about 1mm larger than the speaker binding tray.

I cut two more templates, one for the ports and one for the "supertweeter" L pad attenuators. The port template produces a snug fit, maybe just too snug, another half millimeter clearance would be better I think. On the other hand the L pad attenuator was completely wrong and I had to cut another. Just shows how easy it is to get it wrong, and how important it is to check before assuming it's correct and ruining a precious piece of birch ply!

Alpair 12P MLTLs - Day 2

Cabinet build - Part 2 (2 hours)

Just a short day today, only a couple of hours spare in the morning to play.

The next job is to cut the internal braces. The braces add a little stiffening, but perhaps more importantly they should help assembly by keeping the sides square and of equal width. One per cabinet would probably be enough but I had plenty of spare mdf so cut two per cabinet.

Then a quick trial fit with the front and back and a single side. The braces were a pretty good fit, only one needing just a little easing.

Meanwhile on temperature watch we have a heady 7 degrees C today :-).

Once cut, the braces need the middles removing. Four holes cut with a 19mm wood drill bit in the corners first, and then the rest removed with a jigsaw.

And that's all there was time for.

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!