Building a small speaker for my Alpair 12p's

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Hello all, I've been a member here for a while but this is my first post.

This is a project I have been working on recently- a pair of 20 litre speakers for the Alpair 12p drivers from Mark Audio. I bought the drivers directly from Madisound 18 months ago when I was over in the States and I originally planned to build the M12-A12 floor standing speaker from Brines Audio. In fact, I bought the plan from Bob Brines but lack of time meant that things got delayed and I decided to build these cabinets as a quick project to get the drivers run in.

The design is by Scott Lindgren and and I kept things simple by using 18mm MDF left over from a home diy project. All cutting was done at home using basic diy tools + table saw, router etc. There are no fancy joints, just edges butted up and glued together. It's a small box so it comes out pretty rigid. The whole lot is lined with left over carpet felt and I bought two ports and a pair of speaker terminals from eBay for a tenner.

I initially ran the drivers in for 100 hours on low volume acoustic music just in the cardboard boxes they came in, then for another 100 hours or so in the bare MDF cabinets before I added a secondary baffle and a coat of off-white paint sprayed on. The secondary baffle is just a 6mm piece of mdf glued on to pretty things up a bit. The paint was left over from a built in wardrobe project. I admit there's a bit of a keep-the-cost-down theme going on here.

Total running time is now is about 250 hours and I've begun to introduce some blues + jazz and a little bit more volume. (Elland Fitzgerald, Average White Band, Robert Cray etc)

So, how do they sound so far? They have bags of low level detail, a wide, deep soundstage and a crisp, musical and tuneful presentation. Bass isn't room shaking but it is tuneful, rythmic and not boomy. I've been listening to mainly to vinyl- records that I haven't played for ages and I'm happily letting it run through both sides just enjoying the music.

At the moment I'm running them from a diy 'Rocky' integrated amplifier rated at 4 watts/channel and I also have Quad 57's, LS3/5a's and Tannoy DC6's that I listen to in rotation, albeit powered by different amplifiers.

They fit right in and I'm well pleased!



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Nice work indeed; glad you're enjoying them. That's the presentation targeted; lightly damped alignment, they don't do sub-bass but they're practical & suit near-wall placement. About 15.7 litres volume, once driver & duct are accounted for. A simple box, nothing fancy really, but I've always rather liked them. The combination of that box & driver gells rather nicely; it takes off like a whippet.
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Thankyou both for your comments and also to Scott for putting this design out for free download. Feel free to use the images- I'm flattered that they are good enough.

The speakers keep getting better as the drivers run in. We had a garden party yesterday and half the people were camped in the lounge going through my record collection.
I lost count of the times folk commented on how 'real' the music sounded.

The Mark Audio driver in this cabinet really does sound superb. I think it's an excellent way for a beginner to try out a diy speaker.

Thanks again.
These look really nice, how did you finish the paint job on them Dave? I'm currently on with some small sealed boxes for my Pluvias, but prepping the MDF to be completely seam free is proving to be a bit of a time sapper. I was hoping to put a piano gloss finish on them, but I'm not sure I have either the skills or the patience to pull it off!!
I've had a bit of practice painting MDF in diy projects and my technique is always to seal the edges, apply water based filler, sand and then apply a priming coat followed by a couple of top coats. All sealers, paints and fillers are water based.
To seal the edges of MDF I use a mortar additive called SBR. (Silicone Butyl Rubber). SBR looks and behaves a bit like PVA but the significant difference is that it dries waterproof. That means that once the edges are sealed, subsequent water based paints don't raise the surface of the MDF.

In this instance , after I'd glued the boxes together, I ran a flush trim router bit round all the edges to get it everything evened up, then I sanded it all over with 400 then 600 grit wet or dry. I then sealed the cut ends of the mdf with a 50/50 thinned solution of water/SBR.

All gaps were filled and the radius on the front baffles were made using Everbuild universal water based wood filler, then the whole lot was sanded smooth with 240/400/600 wet or dry.

At every stage, sanding is done dry

Two coats of white primer and two coats of Dulux water based eggshell were applied using a HVLP spray gun. (High Volume Low Pressure). The paint was thinned 20% with water and sanded between coats with 600 grit. I leave 12 hours minimum drying time between coats.

It sounds long winded but once you get in the groove it isn't too much trouble.

I'm very pleased with the results but eggshell is a lot more forgiving of surface imperfections and blemishes than a high gloss piano finish.
The biggest difference for me is in the scale of presentation. I have owned this pair of ls3/5a's for 14 years and for much of that time they were my only speakers. I listened in a smaller room, drove them with a solid state amplifier and the speakers were pulled right out into the room- about 4 feet away from where I was sitting.
With female vocal music, acoustic blues etc they are spellbinding. For listening to plays etc broadcast on R4 they are superb. With larger scale music played louder such as 70's rock, RnB etc they sound more ordinary, less special. They sound compressed and the lack of low bass was obvious.
That said, they are still a fine speaker and I have no intention of selling them!

These Alpair 12p's are in a totally different set up, a much bigger room, a flea-powered SE valve amplifier, different cables etc, so that must be taken into consideration when making comparisons. Right now I'm very happy with the sound from them.:)
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