Alpair 12P Open Baffle/U-Frame with 8" sealed woofer. Passive Cross Over. Pictures
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.
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.
Attachments
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?
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?
What woofer? I'm guessing it was mentioned in the other thread, but it wouldn't hurt to mention it here.
dave
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.
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.
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.
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.
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.
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.
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.
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.
OK.
So with a inline cap to the ribbon (ie 1st order XO) and the A12P driven full range (ie no XO) then the ribbon is 90-deg out of phase with the A12P. You still need to swap the +/- terminals on the ribbon to get both drivers back in phase You'll also need a L-Pad across the ribbon to bring this inline with the dB of the A12P. Perhaps the reason you are hearing too much HF?
At 0.82uF the ribbon is XO at around 23kHz.
So with a inline cap to the ribbon (ie 1st order XO) and the A12P driven full range (ie no XO) then the ribbon is 90-deg out of phase with the A12P. You still need to swap the +/- terminals on the ribbon to get both drivers back in phase You'll also need a L-Pad across the ribbon to bring this inline with the dB of the A12P. Perhaps the reason you are hearing too much HF?
At 0.82uF the ribbon is XO at around 23kHz.
What if the ribbon fires upwards? Do I still need to swap the +/- ? I will try swapping them anyway. I have a 5.6 Ohm resistor on input of tweeter, ahead of the capacitor. So it goes 5.6 Ohm resistor, then 0.82uf cap, then tweeter is 10 Ohms @ 20Khz where it's about 92db. At 20Khz the A12 is 89db. So 10 Ohms tweeter @ 20Khz cap value 1.0 or 0.82 or a 0.68uf should be about right. Perhaps an L-pad of a 2-3 db? Also my room is fairly dead. Lots of soft materials around. So the resistor value will just be to 'taste' by ear. Sound ok?
The polarity flip is just a text book recommendation, and to be even valid assumes the natural inductance of the woofer is equivalent to a 1st order filter which it is likely not for this driver. Many factors contribute to whether or not it's needed, including:
- Acoustic center;
- Phase of the individual drivers at the cross over frequency;
The acoustic center of the ribbon is set back because it's up firing. And probably fairly in line with the woofer. So the text book example may be somewhat reliable in this case.
The phase of the drivers is a total crap shoot. The alp12 is in breakup way up there and I doubt it would have linear / minimum phase relative to its frequency response. So the drivers may be in phase, or out of phase. You can not know without measurements. I have examples of this for more typical 2 ways I can show. I've never done one this high, but the application is the same.
Now, the good part is you're crossing so high I doubt you could tell. The reason being is:
- It's really hard to hear up there;
- Vertical lobes are very tight.
Because all your hearing is "air" I doubt improper phase matching would ruin the sound of this speaker.
The vertical lobes will be very very tight because of the wavelengths and separation of the drivers. All you have to do is slouch and you'll be in a null, or maybe a peak. I can show you some simulations if you'd like. Centimeters will make a difference. Which is why I don't bother with high cross overs. And why I don't like dual Full Range designs.
So in summary, I don't believe the polarity matters. Or the L-pad for the same reasons.
- Acoustic center;
- Phase of the individual drivers at the cross over frequency;
The acoustic center of the ribbon is set back because it's up firing. And probably fairly in line with the woofer. So the text book example may be somewhat reliable in this case.
The phase of the drivers is a total crap shoot. The alp12 is in breakup way up there and I doubt it would have linear / minimum phase relative to its frequency response. So the drivers may be in phase, or out of phase. You can not know without measurements. I have examples of this for more typical 2 ways I can show. I've never done one this high, but the application is the same.
Now, the good part is you're crossing so high I doubt you could tell. The reason being is:
- It's really hard to hear up there;
- Vertical lobes are very tight.
Because all your hearing is "air" I doubt improper phase matching would ruin the sound of this speaker.
The vertical lobes will be very very tight because of the wavelengths and separation of the drivers. All you have to do is slouch and you'll be in a null, or maybe a peak. I can show you some simulations if you'd like. Centimeters will make a difference. Which is why I don't bother with high cross overs. And why I don't like dual Full Range designs.
So in summary, I don't believe the polarity matters. Or the L-pad for the same reasons.
Thank you tuxedocivic for sharing your expertise. Regarding the value/qty. of padding that 'works' I believe this is also related to the impedance of the A12 @ 20Khz. Roughly 15 Ohms from spec sheet. Because the current from the amp will go wherever the impedance is lowest to some extent depending on the output impedance of the amp. So without padding the 8 Ohm tweeter will 'hog' the current from the 15 Ohm A12. So I need a resistor value that increases the impedance of tweeter to roughly 15 Ohms to align with A12 and then also reduces the output of tweeter down a few db. A decent sized value of resistor seems plausible. Does this seem correct?
Ok, what happens if you wire a 92db 4 Ohm driver and a 92db 8 Ohm driver in parallel to an amp? The amp sees a 2.6 Ohm load. Are drivers the same volume or is the 4 Ohm driver louder? I'm not 100% categorically certain of this either, but this how I understand it. If we assume a 10 watts output from amp with the A12 and the tweeter wired parallel to the amp. At 20Khz the amp sees a 6 Ohm load. 4 watts is drawn/used by the A12. 6 watts is drawn to/used by the tweeter. The amp can't compensate/correct for the impedance mismatch between the drivers. The amp just sees a 6 Ohm load and supplies 10 watts. In this circumstance, power goes where impedance is lowest. Generally speaking. In the case of a valve amp, it's 10 watts of nice liquid 'low impedance' power. In the case of a solid state amp it's 10 watts of well controlled, well damped/rigid 'high impedance' power. If I add a 5 Ohm resistor onto the input of the tweeter the amp now sees 2 x 15 Ohm loads for total load of 7.5 Ohms. A12 uses 5 watts and tweeter draws 5 watts. The tweeter will still be 2 -3 db louder than the A12 because it's efficiency is 2-3 db higher at 20Khz.
You have to think in terms of voltage.
In your first example, if they are both 92db/2.83V/m, then they will be the same volume even though one is 4ohms and will "see" more wattage.
If it worked like you're suggesting, no crossover would work right. Because impedance is always changing across frequency. Don't worry about the impedance, only the frequency response.
As for tubes, I'm totally lost, but I still think it works as I say. Maybe a tube expert could be more insightful.
In your first example, if they are both 92db/2.83V/m, then they will be the same volume even though one is 4ohms and will "see" more wattage.
If it worked like you're suggesting, no crossover would work right. Because impedance is always changing across frequency. Don't worry about the impedance, only the frequency response.
As for tubes, I'm totally lost, but I still think it works as I say. Maybe a tube expert could be more insightful.
I can see what you mean, but remember this isn't frequency and impedance within one driver. It's impedance across 2 drivers simultaneously overalpping. Not an XO. Where you transition from one driver to another. I would just say; How do you make a speaker louder? you turn up the volume/voltage. Isn't it voltage quantity = loudness? Also in guitar quad box type speakers you don't mix drivers with different impedances because you get imbalances. With passive multiway speakers (quad boxes included) you can't just go throw any old combination of drivers together. The impedance relationships between drivers have to be considered. The imedance curve has to be manageble for the amp. Resistors get used to correct this. Impedance/resistance is used to attenuate multiple drivers when used in combination.
Anyways, I guess in context of this forum, it's probably not worth labouring over the theory of it over too much. Maybe an electrical engineer will pipe in and enlighten me/us.
Anyways, I guess in context of this forum, it's probably not worth labouring over the theory of it over too much. Maybe an electrical engineer will pipe in and enlighten me/us.
Sorry but that's wrong. This is a cross over.
As for the rest, the voltage increases so the current also increases. Sure, guitar cabs don't mix impedances because they want each driver dives equally hard. All things equal, the lower impedance driver receives more wattage/current but they're equally sensitive per watt. It doesnt really apply. But if they did mix impedances, both drivers would still play at their respective db/2.83V/m sensitivity.
I'm not sure why you need an engineer to tell you this. I've taken a few under graduate level EE courses if that helps. And I've tinkered with my share of cross overs and this has always been the case, unless the amp can't handle the low impedance.
As for the rest, the voltage increases so the current also increases. Sure, guitar cabs don't mix impedances because they want each driver dives equally hard. All things equal, the lower impedance driver receives more wattage/current but they're equally sensitive per watt. It doesnt really apply. But if they did mix impedances, both drivers would still play at their respective db/2.83V/m sensitivity.
I'm not sure why you need an engineer to tell you this. I've taken a few under graduate level EE courses if that helps. And I've tinkered with my share of cross overs and this has always been the case, unless the amp can't handle the low impedance.