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Old 16th November 2009, 01:47 PM   #1
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Default crossover point - effects of impedance?

hi guys, a few quick questions.

I am putting together some speakers, out of some spare bits and pieces kicking about - nothing special, just be useful having some junk speakers kicking about.

I have 4 ohm woofers

i have crossovers that are designed for 4 ohm woofers and 4 ohm tweeters

but my tweeters are 8 ohms.

so my questions are:

1) what is the effect on the tweeter crossover point by using the wrong impedance tweeter? would the crossover frequency go higher? or lower?

2) is it just the crossover point of the tweeter that would be effected? or would the woofer crossover frequency be affected too? would it depend on the design of the crossover?

3) are there any other effects of doing this?

Thanks in advance

Andy
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Old 16th November 2009, 03:14 PM   #2
Pano is offline Pano  United States
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quick answers:

1) Lower - by an octave.
2) Just the tweeter unless it is some odd design. Probably isn't
3) Not really. Except that a crossover should really be designed with the actual driver impedance taken into account, as well as a bunch of other factors.

But for quick and fun, no worries. You could put ~8 ohms in parallel with the tweeter. Will get you closer to the right spot. But will drop the level by 6dB.
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Old 16th November 2009, 03:16 PM   #3
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You can use an 8 ohm tweeter with a 4 ohm woofer. I have done it on an AR3a using the original 4ohm woofer with a new Seas mid and tweeter.

Welcome to Carl's Custom Loudspeakers
Super-Mod development .pdf

Use one of the WWW free crossover pages to input your tweeter's Re and the xover point you want and the slope you want. Those are your choices. If it will model a woofer and tweeter, all the better. Then input the woofer's Re as well and it's xover point and slope.
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Old 16th November 2009, 03:58 PM   #4
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cheers guys.

so i guess reducing the crossover frequency of the tweeter will reduce its power handling?

not really a huge problem for me i dont suppose.
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Old 16th November 2009, 04:23 PM   #5
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If the high-pass part of the filter feeding the tweeter is second order or higher, the effect of having an 8R load instead of a 4R is likely to change the Q of the filter, probably resulting in a "peaked" response around the turnover frequency. Do you have the schematic of the crossover? I'm sure that I or someone else here could suggest some component value changes that would help.
Of course, if your tweeter is of a high enough efficiency, you could just put an 8.2R resistor across the tweeter to make the crossover see the expected load. You'll lose approximately 3dB of tweeter efficiency though.
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Old 16th November 2009, 09:45 PM   #6
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re: ' I have 4 ohm woofers... but my tweeters are 8 ohms' - no you don't, these are only nominal values, for an accurate crossover you need to use ACTUAL impedance values at the crossover frequency.
A good way to see the effect of impedance changes is to play with an online xover calculator: Crossover Design Calculators
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 17th November 2009, 08:41 AM   #7
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Thanks for the responses guys.

Yes i realise that 4 ohm and 8 ohm are nominal values, but its all i have to work with. As i said above, i'm just throwing them together with bits i already have. Even the wood for the boxes. The bass drivers are 'Average' at best, and the tweeters are nothing special either. the project isnt really worth any more effort that this hehe,

thanks again guys, i'll see how i get on.
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