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PFM Special - a simpler and more affordable DIY loudspeaker design

So, everybody has no excuse not to build these speakers...
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I suppose the woodworking part seems daunting enough to put most potential builders off. It's a pity, because with just a low-end jigsaw and router (the quality of blades and bits used matter more than the tools), it's possible to turn out very precise work. The precision comes from making accurate measurements and finishing each edge with a router guided by a straight (or curved) edge. There is no need for a high 'skill' level - the secret (if there is one) is simply the need for enough practice runs to get the knack.

That said, if enough people show an interest, it might be feasible to get a quantity of basic 'flat pack' cabinets made up. The builder would still have to assemble and finish the cabinets, but without the need for anything fancier than an electric drill and sander. Just a thought.

Mr Tibbs
 
Mr Tibbs said:
There is no need for a high 'skill' level - the secret (if there is one) is simply the need for enough practice runs to get the knack ... That said, if enough people show an interest, it might be feasible to get a quantity of basic 'flat pack' cabinets made up.
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I'd echo Mr Tibbs' views. The PFM-Special was conceived to be easy to build and easy on the pocket. Compared to the E-IIIs and E-Vs for example, they are a doddle. The hardest part is routing the driver holes and rebates. You won't be able to do without a router. A batch of pre-manufactured MDF panels (including CNC'd holes) would certainly broaden the appeal of this design, but I reckon a huge part of the satisfaction is the work one puts into building it from scratch.

James
 
Would there be any of you guys with wood-working experience willing to briefly document stuff like "how to cut a perfectly round hole in a sheet of MDF and router the rebates", or "finishing edges the easy way". I reckon this would significantly lower the bar for budding builders to take a chance on making speakers, especially if the only power tools required are things most people already have (jigsaws) or at least those that are cheap enough to purchase (routers).

Carl
 
Routing a driver-mounting hole in MDF the easy way.

All that is required is a plunge router with adjustable depth-stops and a circle-cutting jig. The latter can be bought e.g. Jasper Circle Jigs or made up as I had done using the router’s trammel bars and a piece of shaped clear polycarbonate.

The first step is to mark the centre of the hole with precision. This is especially important when cutting close to the edge of the material as a 1mm deviation from the centre-line means it’ll be 2mm out on one side compared to the other. With the centre accurately marked, scribe or pencil with a compass the outer limit of the rebated hole. This should correspond to the size of the driver flange, plus maybe 0.5mm for a comfortable fit. The driver flange diameter is usually provided on spec sheets. Getting this right is important and the scribed/pencilled mark will let you know if you're wrongly set before you cut.

Drill a hole with an appropriately sized bit (I use 1/8”) to match the template holes for a snug pivoting point. The same drill bit can also be used as the pivot.

With the circle-cutting template or jig in place, and a 18mm or thereabouts straight cutter fitted, set the cutting radius and cutting depth (based on driver flange thickness). Plunge the cutter into the work and cut clockwise to the desired depth. Normally for a cut no deeper than 10mm, a single pass will suffice. Use dust extraction if you can. This is where a router design such as a Dewalt DW621 makes for a virtually dustless cut.

Once the rebate is cut, reset the cutting radius and depth to cut through the hole progressively. For a double thickness of MDF, you will need a longish bit and a router with sufficient plunge capacity. Failing that, a jigsaw can be used to cut the hole as a slightly ragged edge on the inside doesn’t matter. Just make sure the flange-mounting rebate is not damaged.

Always cut the flange first, before the hole. In using the router for the latter, make sure the final through cut is done carefully because the centre piece (of waste) to which the router is referenced, will be free to move around. I often cut to a depth just 1mm shy of breaking through, and simply knocking the centre-piece out from behind, and then using a round-over bit to clean up the opening.

Easy-peasy.
 
Wood working wise I learnt as I went. I did make a few mistakes, most you can't see, a couple are visible but you have to get up close.

I did spent many hours thinking about construction; working through the steps, checking the sequence of construction. And it does pay off. Figuring out how to transfer measurements from one piece of MDF to another, or marking a measurement once across many pieces was the biggest learning curve for me. Many times I had to stop and think through the process.

Cutting circles in the MDF and the Oak was the most 'exciting' part of the build. Biscuit joining bits of MDF is all well and good, but routing a rebate and a driver hole lets you start to imagine that this pile of MDF could really turn into a speaker.

And I still hate painting....

As for construction I know that my construction was slightly different from James's original, for example James's Specials had the internal baffle mounted between the internal sides/top/bottom, whilst mine had the internal baffle on top of the internal sides/top/bottom. The plans I drew up have different measurements. I'm happy to share the document I put together.

Mr Tibbs said:
I suppose the woodworking part seems daunting enough to put most potential builders off. It's a pity, because with just a low-end jigsaw and router (the quality of blades and bits used matter more than the tools), it's possible to turn out very precise work. The precision comes from making accurate measurements and finishing each edge with a router guided by a straight (or curved) edge. There is no need for a high 'skill' level - the secret (if there is one) is simply the need for enough practice runs to get the knack.

I totally agree. I found that good router bits were a must. I started to cut out the rebate in the Oak and realised that the bit that came with the router was pretty poor and blunt.

The trimming bit has been my saviour, especially as I discovered that a few of the MDF pieces were cut 2mm too big all round.

And I have to do a music report:

Bass thump has gone, mid has settled. Oh and the highs are just super.

Timing is just spot on, really impressive. I just can't describe the real rhythm these speakers have.

Tonight's music menu was:

David Bowie - Ziggy Stardust - Green RCA (reprint) - Soul Love is such a great song)
Gary Numan - Living Ornaments '79-'80 - 'Cos I'm sad.
The Mighty Blue Kings - The Mighty Blue Kings - Even my wife danced around the room.
Spirit - Adventures of Kaptain Kopter & Commander Cassidy in Potato Land - To be heard to be believed. Ya gotta love LSD!
GroundHogs - Split
Boxer - Absolutely

I've not even started working through my Japan albums.

Looks like the grass is going to remain uncut for another weekend, ho hum!
 
chiily said:
Bass thump has gone, mid has settled. Oh and the highs are just super.

Timing is just spot on, really impressive. I just can't describe the real rhythm these speakers have.

Looks like the grass is going to remain uncut for another weekend, ho hum!
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I'm glad to hear that, Garf. The PFM-Specials prove you can have pitch, rhythm and timing accuracy in addition to spatial resolution. Middle Earth is where you can have it all. Someone should buy up that lot of Vifa M26WRs soon before they disappear for good.

James
 
Two things I'd add to James' post above:

1) Drill the pin hole for the jig with a drill press, to ensure that it is perfectly perpendicular to the workpiece. If it's not, the driver will not sit square with the baffle.

2) Attach the workpiece to a piece of scrap with double-sided carpet tape, so that the pin is held in place even after you've cut through the baffle, thus avoiding that annoying glitch at the end of the cut.
 
RustyB said:
Drill the pin hole for the jig with a drill press, to ensure that it is perfectly perpendicular to the workpiece. If it's not, the driver will not sit square with the baffle.
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Buy a trammel bar with a pin in it as they are so cheap it's not worth bothering making anything.

RustyB said:
Attach the workpiece to a piece of scrap with double-sided carpet tape, so that the pin is held in place even after you've cut through the baffle, thus avoiding that annoying glitch at the end of the cut.
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If you screw the central disk to the surface you are routing on, via the two visible countersunk screw holes shown in the photo, and clamp the baffle board(not shown) to the sacrificial routing backing board, you will have a safe and stable method of routing your speaker holes.
 
The trammel bar idea that Mark put forward is a good one, but is ultimately limited to routing bigger holes. For tweeter and mids, I've found that you need to pivot within the confines of the router footprint, which requires the use of a jig.

My jig uses the trammel bars for adjustment and so is an adaptation of that idea. I'll try put up an FAQ with pictures this weekend.

James
 
James said:
For tweeter and mids, I've found that you need to pivot within the confines of the router footprint, which requires the use of a jig.
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Agreed, I machine up jigs for smaller circular holes using a rotary table on a milling machine as this gives fixed repeatability...especially useful if you cock up somewhere else and have to redo the panel. :D



eg. to give a rebated seat for a tweeter sitting on a 3mm neoprene gasket.



looking like this when finished(without gasket).

 
chiily said:
I should really pull my finger out and sort out a pair of modified McBs... You did sent me the masks yonks ago. Did you settle on how to incorporate that little daughter board, or did it remain an attachment? Any pictures?
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Sorry for the late reply. I think I did complete the artwork incorporating the daughter board but this would be around the time of "The Big Unbacked Up Hard Drive Crash", I'm afraid so whatever I had was lost. I am continually reminded of the event.:(
 
Well done Garf, excellent job, you'll enjoy them for years. My speaker search finished when I got my Ergo's up and running, it will only start again if I get a much bigger room for the music system, Jame's would then be my first port of call.
Enjoy them,
Ashley
 
AshleyD said:
It will only start again if I get a much bigger room for the music system...
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Ashley,

The more I listen, the more similarities I find between the E-IVs and E-VIIIs. They are both boogie-meisters with startling clarity, but the latter obviously has considerably more scale and gravitas. I think I've got your next Ergo when you're ready. ;)

James
 
James, you are certainly waving temptation at me! I lived without bass for many years when I had the SBLs, the E-IVs have given me an appreciation for a fuller frequency experience, may be the E-VIIIs could take me to the next level? What sort of breathing space do they need?

Garf is quite, he must be enjoying the music.

Ashley
 
Ashley,

The E-VIIIs are huge, and they'd certainly dwarf the E-IVs. They do like being out and away from the walls, but that could just be my room. I'd suggest you'd need a room no smaller than 6m x 5m to take advantage of their ability to move air with absolute authority. In my estimation, the E-VIIIs offer an extra half-octave of extension over the E-IVs, and an additional 2-3dB of voltage sensitivity. I'm seriously considering making them (rather than the E-IIIs) my reference design until the next one comes along. I'll see if I can take a picture of an E-IV next to an E-VIII for your reference soon.

If Garf is distracted by the music emanating from the PFM-Specials, I'm only too pleased for him.

James
 
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