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Old 4th January 2003, 11:14 PM   #1
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Default xover with woofers in parallel + tweeter

OK, the system is going like this:

two Eminence ALPHA-8A 8" woofers (290-404)
and one titanium compression horn tweeter (260-080) in a 12"x4.5" horn (270-096).

The internal dimensions are 34Hx14Wx12D giving a volume of 3.31 cu.ft. tuned to 55 Hz.

The two woofers will be in parallel for a 4-ohm load and the tweeter is 8-ohm. This will be run with a tube amp, or VERY high current vinatge SS amp- whichever I find at the most affordable price later on. For now I guess they would be used on the B outputs of my 65WPC Onkyo receiver.

Question, though: What does the xover schematic look like using two woofers in parallel? I am having some difficulty imagining it. I can guess what it looks like, but would really like to check to be sure. Also, is there any way to work this so I get 8-ohms total? What impedence do I get like this? Thanks
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Old 5th January 2003, 12:27 AM   #2
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Correction: Those were Eminence BETA-8A drivers.
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Old 5th January 2003, 12:45 AM   #3
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It looks just like a crossover with one woofer, but the
values are adjusted not only for the load being halved,
but also the different acoustic output of two drivers.
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Old 5th January 2003, 01:08 AM   #4
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So, could this be what it looks like? (file is attached) And it would give a 4-ohm load from F3-2000Hz and an 8-ohm load from 2000-20000Hz? Correct? BTW, it isn't marked, but the top wire is + and the bottom -. I read somewhere you had to reverse the polarity on a tweeter, so that is why it is like that.
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File Type: jpg xover.jpg (22.1 KB, 138 views)
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Old 5th January 2003, 04:07 AM   #5
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This is a second order linkwitz-riley network so that is true, you have to reverse the polarity of the tweeter.

Your nominal impedence will be around 3 ohms. Keep in mind that the impedence depends on the frequency and on your crossover network. To get the real value you need to run simulation.

I will be the bad guy by saying that you should add an impedance compensation network to your woofer to be sure that your network behavior correspond to the designed values.
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Old 5th January 2003, 07:01 AM   #6
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I was afraid of that... the only real problem I have with adding the network is figuring the values since the woofers are in parallel. Also, when I figure xover values, I enter the woofer impedence as 4-ohm and the tweeter impedence as 8, right? So I get:
C1- 4.98
C2- 9.95
L1- 1.27
L2- 0.64

Is this correct? How would I figure the values for the impedence correction network with the woofers in parallel? I am basically clueless here about xovers. Any help is much appreciated.
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Old 5th January 2003, 03:12 PM   #7
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The impedence used to compute the values is the impedence seen at the crossover point, this is really different than the nominal impedance. I usually measure this with speakerworkshop. If you use the nominal impedance, you will think that your crossover cut at let say 3Khz but in fact, your crossover point has shifted to a different value.

This is the same thing for the impedence compensation network. This network will ensure that you have an almost linear impedance curve.

It doesn't matter if your woofers are in serie or parallel, you have to see them as one woofer and do all your design/measurement with the final configuration.

Please take a few minutes to read this: http://sound.westhost.com/lr-passive.htm

And consider buying a good book like the one of Vance Dickason (Loudspeaker design cookbook).

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