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PFM Special - a simpler and more affordable DIY loudspeaker design

"The reason seems to be that it is difficult to get good subjective dynamics from this driver ... I would have expected to see a 3 or 4" cone in your design."

Markus, thanks for your observation. I'm not quite sure what other applications you have seen the D75 in, where dynamic performance is marginal. In my world of loudspeaker theory, a useful measure of dynamic behaviour is voltage sensitivity, which is directly proportionally related to motor strength and inversely proportional to moving mass. The D75 has an exceptionally light dome and consequentially has a sensitivity of 92dB/2.83V. Based on this, I expect it to be every bit as dynamic as any other midrange of choice. However, as you point out, what's on paper might not be borne out in practice. I am concerned with what is undoubtedly a very narrow bandwidth.

I have on hand another pair of drivers that can be used as midrange units. These are the Vifa M13SG, which offer a much wider bandwidth but lower sensitivity. Comparing them side by side with a low-level full bandwidth signal, it's immediately obvious that the D75 offers better clarity in the upper reaches.

"If it has a role at all, it's as a "filler driver" in a relatively narrow passband between the woofer and tweeter, but in that application it's more trouble than it's worth, IMHO."

I don't mind the challenge, and it's always worth a try before I even consider throwing the towel in.

"Remember that the SBL Xover has an impedence trap for the crossover resonance. If the resonance of the new tweeter is at all different (Q or frequency) then the resonance trap will be off and this is one area where component value being off will have major response issues."

Hi Richard, you raised an absolutely pertinent point. Which is yet another reason I don't generally recommend driver swaps without XO tweaks. Especially if the SBL tweet is a non-standard issue D2008. Otherwise, the on-paper specs between the standard D2008 and OW2 look remarkably similar.

James
 
Now that the E-Vs cabinets are almost finished, I really need to get moving with the PFM-Specials. I can confirm that my version of these budget loudspeakers will be clad with marine ply over an MDF carcass. Still need to decide if I should persist with the D75 dome mid or something else. I'm almost inclined to try the new Seas MCA15RCY

It sports the same chassis as the CA15RLY, which gives such a good account of itself on my E-IIIs and other designs such as Dennis Murphy's CAOW-1. But as a pure midrange design, it sports a slightly lighter paper cone and consequently a higher sensitivity (90dB/2.83V) that will better complement the Vifa woofer and tweeter. If this is chosen, I may have to consider a sloping baffle to better align acoustic centres.

Best I made some pricing enquiries ...

James
 
Not one to faff about, I've just ordered a pair of Seas MCA15RCY for the PFM-Special. They should be in my grubby mitts by the end of this week. This driver covers 100hz to 5kHz with ease, and so I should have much wider options to cross with low order filters. I fancy 2LR at 300Hz and 3kHz. Not a bad choice for under USD50 each plus shipping.

The bad news is Vifa has apparently discontinued the M26WR-09-08. Well, certainly Parts Express no longer stock them saying that they are a deleted line. If this is the case, it's a real shame as all this effort in developing a cheap but highly effective loudspeaker system for PFMers might not come off. Let's hope sufficient stocks remain at other distributors long enough for others to get started.

For those that are genuinely keen, I suggest you get to Madisound/Wilmslow/WES/Others fast. These 10-inch drivers retail for under USD100 each and represent a fantastic bargain. We can worry about making them sound good in time - mainly mine ;)

James
 
James,

Is the design still flexible (very flexible)?

What about an omni-directional, perhaps along the lines of the shahanian Arcs? Or could this be a new range.

Ashley
 
Ashley,

I've yet to make my first cut in MDF for the PFM-Special so I guess we still have time to revise the design to some extent. The only thing that I'm not inclined to change is the choice of drivers. I think an omni will require multiple tweets and mids, and I'm already kinda skint from all this power-tool and loudspeaker bits buying frenzy. Besides, the PFM-Special is supposed to be an easy to build design. This invariably means a regular rectangular box with parallel sides and flat faces.

But I'm open to ideas ... as long as they are simple in concept and cheap to execute.

James
 
My 2p - it would be a shame if the driver you want to use is a discontinued line. Even if there are stocks around, it means that the PFM Special will have a fairly short "shelf life", as those stocks will inevitably become depleted. I guess an alternative driver may become available that, with crossover work, would do as a replacement. Just seems a shame to start off with an already discontinued driver....
 
Well the Arc is a three way design using single drivers, but it's not quite a regular box shape, certainly not the sort of shape the DIY superstores could cut for you.

But I'm open to ideas ... as long as they are simple in concept and cheap to execute.
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How about mounting the woofer in its own rectangular/square enclosure and sitting on top of this at an angle say 45 degrees an open baffle configuration for the mid and tweeter?

Should be simple enough, no idea what it would sound like though.

Ashley
 
"I guess an alternative driver may become available that, with crossover work, would do as a replacement. Just seems a shame to start off with an already discontinued driver"

Richard, you cannot imagine how disappointed I was when I got the news. Clearly, this was after I had bought the drive units. I have it on fairly good authority that Vifa has not ditched the M26WR but they are not offering it as a standard line item. I suspect it is too cheap, because they have a Premium Line equivalent that is almost identical except for gold plated terminals and slightly different specs. Vifa will supply the M26WR in minimum order quantities of 108 pieces at a time. I'll see what others might work, but Ashley has given me good food for thought.

"How about mounting the woofer in its own rectangular/square enclosure and sitting on top of this at an angle say 45 degrees an open baffle configuration for the mid and tweeter?" - AshleyD

Hmmmm ... open baffle is do-able, but I'm not sure about the 45-degree incline. The rear wave might do strange things and make the system tricky in a different room. But I like the idea of an open baffle. I'll think along those lines for a bit and get back to the crew here.

James
 
Hmmmm ... open baffle is do-able, but I'm not sure about the 45-degree incline. The rear wave might do strange things and make the system tricky in a different room. But I like the idea of an open baffle. I'll think along those lines for a bit and get back to the crew here.
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The 45 degree idea was to mimic the Arcs and get a sort of omni, but you're the guru.

...something along the lines of this [a Basszilla thread on DIYAudio]?
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I should hope not :eek: them is one ugly looking speaker.

Ashley
 
The current and available-for-the-foreseeable-future Vifa PL26WR-09-08 is physically identical to the deleted M26WR-09-08, but has slightly different T/S parameters. The PL is less sensitive but goes lower. However, Hoffman's Iron Law allows me to equalise what are essentially the same drivers in different guises. Let me explain.

The PL version gives more extension by sacrificing around 1.5dB of sensitivity and requiring more enclosed volume to deliver the same system Q as the M version. However, by adding 1-ohm of series resistance to the M26, its T/S parameters are modified to more closely approximate the PL26. Preliminary modelling indicates that the M26 (with series resistance) requires 47L to achieve an F3 of 50Hz, which is close to the 48L required by the PL to achieve 49Hz. Heavy stuffing should reduce the volumetric requirement down to around 37L.

Both drivers have almost identical acoustic responses and impedence character so an XO designed for one should work for the other. In other words, it's not an entirely lost cause. I will proceed as planned, and design the PFM-Special with this woofer interchangeability in mind.

Do we have many takers for the open-baffle mid configuration, but one done more aesthetically pleasing than the Basszilla?

James
 
James,
Thinking about the 45 degree open baffle a little more, this may alleviate some of the placement problems with open baffles, which generally require a lot of space behind them. The 45 degree baffle would cause the back wave to bounce off the top of the woofer enclosure, then off the back wall, it would then run in parallel to the forward wave, thus we have an increased rear wave path which means the baffle can be placed closer to the wall.

I guess an open mid baffle would tolerate less distance to the rear wall than an open bass baffle would due to the shorter wavelength.

Ashley
 
Ashley,

You are not a fan of the Bose Direct/Reflected sound paradigm from their suspect X01 days, are you?

James ;) (just teasin')
 
Alrighty then, I guess we should stick to the design brief - a simple, cost-effective, easy-to-build 3-way system that do the groove thang with conviction in a wide range of rooms. Deal?

James

EDIT: I'm almost convinced that an open baffle is the way to go. At least it will be easy to play around with different baffle slopes if I take a prototypical approach. Note that the 'bass bin' will measure no less than 300mm wide x 500mm tall and 400mm deep to get the requisite internal volume. At least I won't have to worry about a separate air-tight chamber for the mid.
 
Of course it means that you'll be able to provide some of the resistance with air core inductors using thinner (cheaper) wire.

This is a big win for a low cost speaker. Also if the mid/top needs padding and people want to hear how low Q sounds, they can refigure the crossover to choice.
 
"Cool - nice and simple box with a bit on top. Almost sounds achievable!"

Stuart, the box might be simple, but I don't know if you can say the same for the bit on top. Might turn out to be a complicated affair with wings, complex curves and other acoustic treatments. ;)

"Of course it means that you'll be able to provide some of the resistance with air core inductors using thinner (cheaper) wire ... This is a big win for a low cost speaker. Also if the mid/top needs padding and people want to hear how low Q sounds, they can refigure the crossover to choice."

Absolutely, Richard. Do you have any thoughts on a dipole mid? I felt I should do something a little different this time.

James
 
Never been there. I like the "air spring" linearity gain from sealed boxes.

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