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21st December 2012, 07:49 AM  #11 
diyAudio Member
Join Date: Apr 2009
Location: Lancashire, England

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 90deg 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 LPad 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. 
21st December 2012, 10:04 AM  #12 
diyAudio Member

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 Lpad of a 23 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?

21st December 2012, 06:19 PM  #13 
diyAudio Member
Join Date: Jan 2011
Location: Ladysmith, BC

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 Lpad for the same reasons. 
21st December 2012, 11:25 PM  #14 
diyAudio Member

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?

21st December 2012, 11:37 PM  #15 
diyAudio Member

Sorry: correction at 20Khz the tweeter is 10 Ohms from spec sheet, So I need a resistor value that increases the impedance of tweeter to roughly 15 Ohms to align with A12 and @ 20Khz then also reduces the output of tweeter down a few db give or take.

22nd December 2012, 03:54 AM  #16 
diyAudio Member
Join Date: Jan 2011
Location: Ladysmith, BC

It doesn't work like that, although the electrical side of things, isn't my thing. Afaik, the lower impedance side will draw more current, but the amp will just offer more current. Neither driver is "robbed" of current.

22nd December 2012, 06:24 AM  #17 
diyAudio Member

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 23 db higher at 20Khz.

22nd December 2012, 02:56 PM  #18 
diyAudio Member
Join Date: Jan 2011
Location: Ladysmith, BC

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. 
22nd December 2012, 10:14 PM  #19 
diyAudio Member

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. 
23rd December 2012, 12:13 AM  #20 
diyAudio Member
Join Date: Jan 2011
Location: Ladysmith, BC

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. 
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