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TiMBoZ 27th November 2012 06:59 AM

Alpair 12P Open Baffle/U-Frame with 8" sealed woofer. Passive Cross Over. Pictures
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Hi there! This is a quick Version 1 slap together prototype in 18mm MDF to test overall balance between drivers, the sealed woofer in my room, the acoustic roll-off of A12P in U-baffle etc. More details/comments to follow.

markaudio 27th November 2012 07:53 AM

Hi Timboz,
Looking good. By the way, I'm flying into Syd this coming Saturday morning.

TiMBoZ 27th November 2012 09:00 PM

I'd like to take the opportunity to express gratitude to Mark Audio, Troels Gravesen's great DIY site, MJK, Monte Verdi & the entire DIY internet audio community, both private & commercial - without whom the conception and execution of this project would not have been possible. Thank you.

So I've gone with the Alpair 12P because they're the biggest (Sd) of the MA range. To me, seemed the best on paper and in 'XLBaffle' simulation: seemed like the easiest, least critical XO to execute, with best possible chance of success ie: the right driver for the job.

The woofer was chosen because it's suitable for sealed box, has a lower nominal impedance than 12P, similar power handling (not too power hungry) a nice biggish 45.5mm voice coil, is available locally and isn't too pri$ey. Also the paper cone should integrate well with A12Paper cone. Boxes ticked.

At moment I just have a rough XO: with 6.9mH inductors on woofs and 100uf caps on 12Ps for running in purposes. The woofers are nominal 89db (approx 6.4 Ohms at XO) and the Alpair is 92db (approx 10 Ohms at XO). At XO approx 60% of amp power goes the woofer, 40% goes to the Alpair. The sensitivity balance between the woofer/FR drivers is fine TO MY EARS. In my room, there is ample bass. These speakers are sounding pretty BIG!

Personally I'm not concerned about the effect of dipole or bipole or getting too technical about what happens to the (sound of) rear wave when it's reflected back. I'm just exploring NO BOX FULL RANGE because I like the sound. Ideally, I'd have NO rear wall or rear reflected sound at all, if possible. I have absorbers on the wall behind the speakers, which are too close because of room limitations. Ideally I'd have a very large library room only half filled with books. All the random, half empty book shelves would be like diffusers. With a stereo and sofa smack bang in the middle. Wakey wakey McFly says Bif.

Lessons learned so far:
The driver choice/matching is two thumbs up. The speakers probably are a bit too big for the room/space they're in. I'll just ignore that for now.

The undamped 18mm MDF box/baffle is very resonant and live sounding. Not so much just in the U-frame, as you might expect - but everywhere. I haven't put much effort into damping/bracing at this stage. That will be in version 2, which will have a 32mm thick MDF baffle/sections proper damping etc.

Also the higher the XO point goes, the closer the two drivers need to be together on the baffle. I'll need to locate the woofer closer (higher up) to the A12P on the baffle, which won't look quite as cool. Next experiment will be with guitar and listening 'guestimating' the acoustic roll-off of A12P.

Now onto something more technical/theoretical: I read somewhere (on the MJK white paper/case study for OB U-frame, I think) there's a formula/theory that says that the wings of the U-frame should not be more than quarter the wavelength of XO frequency. For example: at 200Hz the wavelength is 1.7 metres x 0.25 (quarter of wavelength) = 42.5cm. So the wings of U-frame should be less than approx 42.5cm. Something like this? I could've got this be wrong. Understanding the some of the maths, theories, formulas etc. aren't a strength for me. Feel free to pipe in if you can articulate this basic aspect of OB theory better, more correctly.

Otherwise any concerns, questions, suggestions?

planet10 27th November 2012 10:50 PM


Originally Posted by TiMBoZ (
The woofer was chosen because it's suitable for sealed box, has a lower nominal impedance than 12P, similar power handling (not too power hungry) a nice biggish 45.5mm voice coil, is available locally and isn't too pri$ey. Also the paper cone should integrate well with A12Paper cone.

What woofer? I'm guessing it was mentioned in the other thread, but it wouldn't hurt to mention it here.


TiMBoZ 28th November 2012 02:29 AM

Woofer is SB Acoustics SB23NRXS45-8, 8" Woofer

TiMBoZ 5th December 2012 01:32 AM

Update: experimenting with an upwards firing 'omni' tweeter mounted to top corner of prototype U-frame. Small cap values passing thru v/high frequencies >5K. Initial impressions are favourable. Up close you can hear the 20cm space between the 2 drivers. But from the listening position, it doesn't seem to interfere/compete too much with the primary sound coming off the A12. It just compliments it in a discreet, pleasing way - adding 'air' and 'spatial presence' when seated in the listening position. Will have to live with it for a while to make sure it's not just a novelty/commit to it permanently.

TiMBoZ 18th December 2012 12:52 AM

Update: currently I have no caps/filtering on A12P. Just wired directly to amp. I have a ribbon tweeter (preferred over a soft dome I also tried) mounted on top of baffle firing upwards with the aperture of ribbon parallel to the baffle. So it would be firing upwards roughly in a V shape, front and back with the nulls to left/right of each speaker in line with the sides of the U-frame. I'm experimenting with 2-3uf cap values and resistors around 3 - 6 Ohms to get the balance with the A12 just right. Nice n discreet.

The idea of trying to measure the roll-off A12 in baffle with a guitar...didn't work. B-( Open G-string sounds exactly the same volume as open-E.

The good news is that although the A12 doesn't seem to have a lot of excursion, it generates enough bass body/fullness on it's own that I can listen quite happily late into the evening with the woofer disengaged. Woofer is filtered with 9mH coil when it's engaged. This is really handy, because I live in an apartment complex. The sound of A12 is a lot fuller, more useful in OB than A10 from my experience so far. I'm GUESSING A12 starts rolling off around 175 - 150Hz or so on the baffle I have it on. You can hear upper bass quite well and you can still feel the 'pulse' of the music enough to be satisfied. Not thin sounding at all. I'm really happy with aspect of it. It's quite versatile. I'm going to have two inputs. One with the woofer bi-passed.

There is some fairly noticeable 'MDF/boxy flavoured' resonances colouring the sound at moment. I'm currently working on the woodwork for the proper cabinet which is quite involved and has been a bit of a learning curve for me in terms of woodwork. But once done, I'm confident the cabinet will be cured of audible 'MDF flavoured' resonances I hear at present.

TiMBoZ 19th December 2012 07:57 PM

Update: the jury is still out on the ribbon tweeter....

On the surface the tweeter is good, it ads a degree of air, and sparkle, not present in the A12P. But on the other hand, it's more fatiguing: your ear/brain has to sum together the 2 drivers. And it obscures/confuses the 'recording' a bit, not the 'sound' so much, but the 'recording'. It takes away from the depth at which the speakers communicate; the ambience/the mood/the artistic intent of the recording is better served without the tweeter. Just the single driver. But at the same time it doesn't 'sound' as shiny/as alive as it does with it..perhaps the tweeter filter is too indiscreet?

At moment I've got a 2.2uf with a 4.7. I'm going to back down to a 0.82uf for a while.

Toppsy 20th December 2012 06:46 PM

Not sure what XO you are using 1st order or 2nd?

Most ribbon tweeters that I know require at least 12dB slope filter ie a 2nd order XO.
So if you have a 1st order on the A12P and a 2nd order on the ribbon you need to swap the +/- terminals on your ribbon otherwise it'll be 90-deg out of phase.

TiMBoZ 21st December 2012 01:02 AM

The ribbon is basically a super-tweeter. There is no XO. The capacitor just passes very high frequencies the to the ribbon with a 6db roll in. The A12 plays full range. With a 2.2uf capacitor there was too much overlap/interference with sound from ribbon competing against sound from A12. I was going for too much sound from the ribbon. I can't really say exact values, I'm just guestimating and going by ear. But it's working much better with cap value at 0.82uf, so you only hear the ribbon very discreetly at the very top end of the treble, the 'air'. The trick is to not spoil the good stuff about 'full range' but just to add back in the 'air/sparkle' to the sound that only light/small tweeters can provide.

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