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PFM Special - a simpler and more affordable DIY loudspeaker design

Richard,

"Have you looked at the mids with a built in enclosure? Peerless M 122 for example? US$36.85 from Madisound."

It's not so much the added complexity of building a separate chamber for the mid that's the issue, but rather what I can do a little different this time around. At first, it was going to be the use of a limited band-width dome mid, but I've thought better of it. And rather than design and build yet another closed-box 3-way system, I thought a partial dipole could be fun.

That way, I can more easily change the shape and size of the mid/tweet baffle and not have to rebuild a new box for every iteration of the PFM-Special.

James

PSSST: One of the reasons for going dipole is an opportunity for me to use the primo Seas Excel M15CH001 driver that I had hoped, but failed to, retrofit onto my E-IIIs. This shares the same chassis as the MCA15RCY and should be a direct swap pending minor filter changes. The beauty of the M15 is the tiny motor system and open back design, which means it should excel in a dipole application. However, it is not cheap by any stretch of the imagination and time will tell if it is worth the extra money for PFMers wanting a little bit more performance. Heck, I might even swap in a Scan-speak tweeter or two that I have in my drawer of spares.
 
Here is what I had in mind. A 40-ish L sealed box measuring (external) 550mm tall x 400mm deep x 330mm wide loading the Vifa M26WR-09-08 woofer in acoustic suspension. This box is topped by a sloping open-backed baffle supporting the Seas MCA15 mid and Vifa tweeter. The exact shape of the upper baffle will depend on how the midrange driver response is affected, but I'm expecting that 45-degree 'wings' on the edges will help yield an even response in the 300 - 3kHz range. Time will tell.

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I'm still not sure where the XO will be placed. The worst place is within the woofer enclosure due to microphony, so I'm thinking I might put the XO 'box' in the space behind the open baffle, or separately behind the loudspeaker cabinet altogether ala E-II, E-III and E-Vs.

I shall now go get a fresh supply of 18mm MDF and see what I can knock together over the weekend ...

James
 
We have the basic ingredients for the PFM-Special Woofer Cabinet. A series of 18mm MDF panels cut to size, hole-sawed and biscuit slotted.

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A closer view of the biscuit slots. I estimate this approach will save no less than three or four hours of effort compared to router-cut trenches.

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Liberal application of good woodworking glue, careful assembly and plenty of clamps look something like this ...

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After 30 minutes of clamping, we have the basic carcass looking a bit like this from the front ...

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... and from the back ...

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The rear 'shelf' will eventually be a separate chamber in which the XO will be placed. This location gives good proximity to all drivers and enables me to swap parts in and out during the voicing stage without needing to turn the loudspeaker upside down (ala E-IV) or taken apart and compromising the air-tight seal.

Next stage will involve shaping the marine ply cladding, which I will leave for another day or evening.

James
 
Since the Seas MCA15RCY midrange driver to be used in this design goes up to 5kHz, I don't have a strong case to use a one-inch dome tweeter that has more limited dispersion characteristics than, say, a good 3/4-inch dome. I'm thinking that perhaps a Scan-speak D2010 might be a good alternative as it (or a variant thereof) seems to be the tweet of choice for the likes of the NBL, SL2 and DBL.

Thoughts?

James
 
I haven't been able to post for a while due to various reasons but I would like to add my encouragement (not that it's needed) to the open baffle speaker. You move fast James:eek:

You might yet get me making speakers other that for guitar and bass, I can't wait to hear Ashley D's.

Cheers,
 
James,

if I was the designer, I'd stick to 25 mm, again for reasons of dynamics. Have you considered this Peerless tweeter? Cheap as chips, very low distortion and has been sounding good in a number of designs I've heard. It would free up a little money for the midrange driver for some people, I'd imagine.
 
Markus,

I've just priced the Scan-speak D2010, and I feel it falls outside the budget criterium for this design. So instead of collecting yet more drivers in my drawer, I think I'll use the Vifa D27 tweeter I bought. It's cheap, effective and doesn't sound half bad. Also, the extra crossover latitude it brings will be welcome. I might even try 1st order acoustic cross between the mid and tweet.

James
 
Nice work James,

You are so quick, right tools for the job and all that. It's worth pointing out that fellow PFMers will not have to use the biscuit jointing technique, a simple glued and screwed joint using the appropiate glue will work just fine.

I think you are right to go with the cost effective drivers, prove the principle first ;) .

Ashley

(measured twice and cut once, but measured from the wrong end, another upper baffle blank to make. I can laugh about it now :p .
 
Ashley,

"You are so quick, right tools for the job and all that. It's worth pointing out that fellow PFMers will not have to use the biscuit jointing technique, a simple glued and screwed joint using the appropiate glue will work just fine."

You raise a very valid point. A biscuit jointer is not essential. I have not taken into account that I've built several pairs of loudspeakers and some of the techniques I use whilst second nature to me will seem foreign to others. Thanks for bringing me back down to earth.

The problem with plain-butted joints using wood glue is the tendency to slide out of alignment, especially when clamping pressure is applied. Predrilling with screw pilot holes (essential with the end-grain) will help immensely, as will routed trenches. I don't advise glue and clamps without the aid of keyed locating mechanism.

"I think you are right to go with the cost effective drivers, prove the principle first."

I'm also keen to prove that good sonic results can be obtained from cost-effective drivers carefully implemented with a thoroughly designed crossover network.

" ... measured twice and cut once, but measured from the wrong end, another upper baffle blank to make. I can laugh about it now ..."

I bet you didn't at the time ;)

James
 
I'm also keen to prove that good sonic results can be obtained from cost-effective drivers carefully implemented with a thoroughly designed crossover network.
Click to expand...

It will be very interesting to read how it sounds. This is the principle used by most commercial manufacturers, understandably so as they have to sell in competition to make a profit, and is a major driver in DIY.

Would you like to estimate a commercial price, as if this were to be sold by a major manufacturer?

cheers,
 
"Would you like to estimate a commercial price, as if this were to be sold by a major manufacturer?"

Hmmmm .. tough question. It's really hard to estimate as different manufacturers have different methods of pricing their models. Some price their by performance. Others by what they think the market will bear. Others yet are motivated by volume sales and therefore allow a smaller profit margin.

Also, there is very little in the market that is remotely similar to the PFM-Special in concept or execution - so your guess would be as good as mine. But just to illustrate the range possible, I understand Linn Komri uses the same woofer and we know what the Linn sells for. At the same time, the Vifa tweeter is a fairly common item amongst budget PMC loudspeakers, but I'm not aware of any commercial designs using the MCA15RCY. I guess it's too new.

If I were to sell this commercially and I could manufacture cheaply (two very important caveats), I'd pitch it at around GBP1K - 1.5K.

James
 
If there is enough interest, I can investigate how much it costs to get all the panels cut on CNC router. I know just the place..

This would bring the project within the capabilities of the average DIYer...
 
Hi James,

Super work on the carcasses, and very quick if you don't mind me saying so.

I wanted to ask about the biscuit joints actually (for another project I am about to embark on). What power tool are you using as I understand that you can buy a dedicated biscuit cutter machine (bit like an angle grinder if I understand it), or just the tool itself to fit into a standard power tool/drill. I also believe that you can do this with a router (or am I off the mark here).

I'd also appreciate your advice on buying a router, I can get Makita tools very cheap from Thailand (about 25% of the UK cost generally) so I fancy that route, the Makita Router is about 2.25 HP, 11 Amps, and has both 0.25 and 0.5 collet capability I think - what do you think ??

Good luck with the PFM special James, I have no doubt in my mind that I'm going to build one someday when I have more time.

P.S., I have been looking at wood from America recently, a bit expensive on postage but the selection is awesome, have a look at this beauty , not that you could use it on a speaker, but it is gorgeous don't you think ??.

Try the US Ebay site and search for wood of your choice under the Woodworking, Lumber section - topnotch stuff.

Keep up the good work James

Mike
 
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Mike,

Most of my power tools are Makita and for pro site use they are unbeatable. Since DeWalt was taken over by B&D quality has gone down from the Elu days, but to be quite honest they are still superb for more intricate work. I have a Makita that you mention and it is a brute, just what is needed for forming kitchen worktops day in, day out. It does not have a 1/4" collet but a reducing bush, not a very good arangement. I would quite often like something a little more manageable for finer work.

You can biscuit join with a router, its just slower and more hassle than with a dedicated B/J. You will need to use a 4mm cutter and a staight edge to put slots in the middle of panels of course.

If I had, say, £250 to spend on power tools I would spend the lot on a good router rather than compromise on 2 lesser quality machines.

IMHO of course!


Paul.
 
Hi Paul,

Which model do you have, only one I have seen from Makita is the RD1101 which I rather fancied for myself, other options in Thailand are Bosch (not nearly so bargain-priced) and B&D believe it or not.

Can you give me any leads on the dedicated B/J you mention, I have searched the net to no avail on this.

Do I take it that you don't recommend the Makita router then, or only not for B/Jointing ??

Mike
 
Hi,

I have the 3612c router its a beast, but I would like the RP1110C for smaller jobs, so it depends on what you want to do the RP1110C is 1100W so it should most jobs apart from some large panel razing bits you shouldn’t have any problems. I would go for a biscuit joiner as its so much easer to use in the middle of panel which you will be doing on a speaker cab.


pete
 
"If there is enough interest, I can investigate how much it costs to get all the panels cut on CNC router. I know just the place.. "

That's a fantastic idea, Richard. Once I have determined all the dimensions (now awaiting trials with the upper baffle), I will post an updated plan with all the numbers.

"I wanted to ask about the biscuit joints actually (for another project I am about to embark on). What power tool are you using as I understand that you can buy a dedicated biscuit cutter machine (bit like an angle grinder if I understand it), or just the tool itself to fit into a standard power tool/drill. I also believe that you can do this with a router (or am I off the mark here)."

Mike, I use a DeWalt Biscuit Jointer. Not overly expensive, but not exactly cheap either. If I had money burning a hole in my pocket, I'd get a Swiss-made Lamello - but I know enough not to compromise of quality tools. The main advantage of a proper biscuit jointer is the alignment fence that they have, which makes it amazingly easy to make accurate edge and surface slots, not to mention mitred (45-degree) ones as well. Well worth it if you are planning to make a few pieces of 'furniture'. A router can be used to make biscuit slots too, and in fact that is what I have been using for my solid-wood panel joints until recently. However, a router will be fiddly to use when lining up edge-to-face joints.

DW682K.jpg


"I'd also appreciate your advice on buying a router, I can get Makita tools very cheap from Thailand (about 25% of the UK cost generally) so I fancy that route, the Makita Router is about 2.25 HP, 11 Amps, and has both 0.25 and 0.5 collet capability I think - what do you think ??"

I started with a baby Makita plunge router with 1/4-inch collet and later got a DeWalt DW621, which in my neck of the woods comes with both 1/4-inch and 1/2-inch collets. The DeWalt is a fantastic router for my purpose because it has through-column dust extraction along with all the other bits like variable speed, soft-start, etc. I wanted a Makita 3612C, but found it a bit big and unwieldy, as Pete does.

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James
 
Mike,

I have the 3612c that Pete and James mention, it is siimply to big and unweildy for intricate work. It would be fine in a router table however.
James' De Walt or the RP1110c would serve you well.
Ive never owned a biscuit joiner, but have hired a Makita a few times (cant remember the model) and it worked very well.

Paul.
 
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