Alpair 12P MLTLs - Day 8

Day 8 - 3 hours

Time to cut out the absorbent material and glue it in both cabinets. It's starting to feel like I'm getting somewhere now.

First, of course, temperature watch. Even I'm getting bored of this now. Anyway 9 degrees today.

Here's both cabs together after the cramps were stripped from the second.

The absorbent is old fashioned carpet underlay, roughly 10mm thick. It's not so easy to get these days, fortunately Dave the Bass had some spare he let me have. Thanks Dave :-). The top, back and one side is lined; here it's cut to size and trial fit.

I used some spray adhesive to mount the material. I was a little unsure what to use, some of the cans sounded like the bond might be a bit temporary or light, and I don't want the linings to come loose in the future. So I used a contact type adhesive, sprayed on to the cabinet and a couple of coats on the material. It comes out as a web and the fibrous nature of the lining meant it webbed up almost immediately without really making contact with the body of the material. On the other hand I found it difficult to get an even coverage on the plywood. I think it might have made quite a mess if I was trying to glue laminate, but it's good enough for this job.

Once the coats had dried so they were no longer tacky (the can says 2 to 5 minutes but it took more like 10 due to the low temperature) the pieces were placed carefully. As it's a contact adhesive it tends to grab as soon as it touches each other so steady and careful positioning was required. Then pressure was applied to ensure the two substrates made good contact. And that's it. Hopefully.

All done.

And here are the two cabinets together.

One last job for today, glue on the side of one of the cabinets. A little more time today to get a photo as there's a lot less glueing for just a side.

And here it is cramped up.

That's it for today, no more cramps.

Alpair 12P MLTLs - Day 7

Day 7 - 1 hour

Dad was round again yesterday so as well as clearing leaves from the gutters it was important I got the other cabinet glued up - it's always easier glueing with two!

First temperature watch, a little cool for glueing really, barely 5 degrees

Here's the first cab we glued last week now stripped of the cramps. Not bad, but there's a very slight bow in the front baffle. Either there's a slight bow in the bench or the composite of the ply and mdf has expanded or contracted at slightly different rates in the year it's been sat in the garage on the bench subject to the natural moisture in the air. This wouldn't have been a problem if I'd realised when glueing up, but I missed it and the side is a little "short" in centre of the baffle. It shouldn't be a problem as I think I'll have to make a cover for the baffle as the greeny grey look of the 12Ps hasn't gone down too well...

And here's the second cabinet glued up. Note the damp wood where the glue that's been squeezed out has been wiped off. Best not to get the wood unduly wet as it lifts the grain. Light sanding should fettle this.

And that's it for another day.

Alpair 12P MLTLs - Day 6

Day 6 - 1 hour

It's nearly a year since I last made any progress with these speakers. It's been a very busy time in the garden this year making trellis fences, gates, arches and a shed from scratch. But now it's winter it's audio time again!

After a long overdue tidy up in the garage I could see the bench again so I could finally start to glue up the first speaker. But first, temperature watch. It was a particularly mild and pleasant morning - 14 degrees in the still of the low slanting sun.

Not too much to talk about but here's a photo of the first gluing up. You can never have too many cramps. One side is left off as three of the internal faces are to be lined with absorbency material. Still a fair bit to do.

Alpair 12P MLTLs - Part 5

Day 5 - 5 hours

As is customary let's start with temperature-watch - much cooler today, 6 degrees.

The first job was to finish off the holes for the pseudo tweeter and port for the second baffle. And then rout the holes for the Lpad and speaker terminal in the rear panels. Here's the front baffles with the finished holes.

With all the holes complete the next job was to rout 45 degrees chamfers around the holes for the 12Ps on the rear of the front baffles. This allows the driver cones to "breathe" as the front baffles are quite thick.

Then it was time for a trial fit to make sure the 12Ps and Monacor RBT-95SQ pseudo ribbons fit okay. First a sand with 70 grit to clean things up a touch before mounting the drivers. Only one of the pseudo ribbons was tried as they needed unsoldering from the Quasars as well as unscrewing.

And then it was back to sanding. All surfaces are now sanded to 120 grit both inside and out, I'll finish with 240 grit when assembled. I did test a patch that will eventually be inside with the finer grit and finish was lovely and smooth.

It's taken a while but the speakers are starting to come together now.

Alpair 12P MLTLs - Part 4

Day 4 - 4 hours

Goodness, it's over 7 months since my last blog, and over 10 months since I last had chance to play with the speakers. It's fair to say audio has been on a back burner for much of this year, but today I made a little progress with the MLTL speakers :-). But first let's have a look at temperature-watch.

Parts of the country are experiencing terrible flooding, no doubt made worse by the unseasonably mild weather which is probably retaining a lot of moisture. 12 degrees C, mild for the end of December.

So first job was to have a quick tidy, then remember where I'd got to. I'd made a start on routing the first hole for the 12P at the end of the last session, but I was rushing and hadn't noticed that the template had slipped slightly and was no longer centred. Fortunately I noticed in time before serious damage was done. So I marked out the circles with a compass to be sure, then carefully reset the template and clamped well.

More haste less speed. Lesson learnt - measure twice cut once.

The first hole needed three phases. The first was to rout the inner hole for the driver to pass through, 170mm diameter. The plunge cutter only reaches part of the way through the baffle though, so needs cutting from the other side too - the third phase.

The second phase is to rebate the driver, 203mm diameter and 11mm deep. This was a bit of a laborious process as there was a lot of wood to cut away. Here's the finished hole. Note the misalignment from cutting from the front and then the back. This was largely due to the initial cut that had slipped. It shouldn't be a problem though, as long as the driver passes through and the driver makes an air tight seal.

Next was the hole for the Monacor pseudo ribbon I'm going to use to fill the treble out over 12k Hz. I started to cut away the all the wood so the unit sits within a cavity...

...but then decided that I was probably going over the top so then cut full depth before turning over and finishing the cut. Ideally I guess I would have preferred to isolate the driver from the main chamber and the air pressures within, but hopefully it shouldn't make too much difference. I might even add a plate to the back just to seal it.

And here's the two holes viewed from the rear.

Next the 3" port, which requires a 79mm diameter hole. This is perhaps just a little slack, but probably better than being too tight and struggling to insert the port. I can always use a bit of glue or silicon to secure them.

From the rear

And here's the first baffle with the initial holes made. It needs a good clean up and a rear 45 degrees chamfer cutting on the rear of the hole for the 12P. And a check to make sure everything fits of course.

For the second baffle I fully marked out all three holes both front and back first, now I had a little more confidence with what I was doing.

And started with the largest hole for the 12P. Here's the first 170mm diameter hole cut and the start of the wider 203mm rebate.

The finished hole from the rear.

And from the front.

And that was the end of the today's audio fun.

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!