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Old 4th December 2012, 11:40 PM   #1
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Default Question about 3way passive crossover design

Hi,

I am currently desiging some crossovs for a 3 way set of speakers I am building. I need a question answered that I cant use my powers of google to find.

My Low sections has 2 x woofers in series both are 3.7ohm resistance for a total of 7.4ohms resistance.

My questions is when desiging the x-over for the low section do I use the total resistance for the speakers (i.e. 7.4) or the resistance of an individual speaker (3.7)


thanks>
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Old 5th December 2012, 12:03 AM   #2
micfre is offline micfre  Canada
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Definitely the sum of the resistance in series, 7.4 ohms.
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Old 5th December 2012, 12:11 AM   #3
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It is usually advised not to run woofers in series but IF you are the, total impedance at the desired XO frequency ( not the DCR)
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Old 5th December 2012, 12:20 AM   #4
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Originally Posted by Moondog55 View Post
It is usually advised not to run woofers in series but IF you are the, total impedance at the desired XO frequency ( not the DCR)

Thanks for the replies.

What is the reasoning behind not wanting to run the woofers in series?

I will need to run omething in series as 3 x componets in parellel (8,5,8 ohms) would give a load of only 2.2ohms which would blow my amp.
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Old 5th December 2012, 02:13 AM   #5
Jay is offline Jay  Indonesia
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You better do it in parallel if there is no issue with impedance. With parallel you get more efficiency and probably more.

The reason to put in series is to prevent the impedance of the speaker to go very low. Four ohm woofers are usually must go with series to give the "standard" 8 ohm woofer. If you put it in parallel it will be 2 ohm or so.

Many amplifiers cannot handle low speaker impedance. Or huge amount of current will be drawn from the amplifier, causing heat and transistor destruction.
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Old 5th December 2012, 09:07 PM   #6
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You better do it in parallel if there is no issue with impedance. With parallel you get more efficiency and probably more.

The reason to put in series is to prevent the impedance of the speaker to go very low. Four ohm woofers are usually must go with series to give the "standard" 8 ohm woofer. If you put it in parallel it will be 2 ohm or so.

Many amplifiers cannot handle low speaker impedance. Or huge amount of current will be drawn from the amplifier, causing heat and transistor destruction.
if MY calculations are correct using 3 x items in parallel that are 8,8 and 8 ohms wil present a 2.2 ohm load to my amp.

This is too low, or is there something that I am missing?
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Old 5th December 2012, 09:58 PM   #7
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You are missing how passive crossovers work. Here is impedance of a speaker with a 2-way crossover. The blue line is the woofer impedance with the crossover in-circuit, and the red is the same for the tweeter. The black line is the total impedance. You can end up with the drivers essentially in parallel over some ranges (and thus too low impedance) if the crossover is not designed well, but it's not how it's supposed to work.

Click the image to open in full size.

This is a totally separate issue from series/parallel when you are trying to use two drivers as one driver, i.e. play the same frequency range as each other. When you're doing that, you treat them as one driver, with combined impedance, and a crossover for that impedance.
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Old 5th December 2012, 11:21 PM   #8
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re:"What is the reasoning behind not wanting to run the woofers in series?"
- for guitar or PA systems, small variations in Re will mean uneven power handling, which may lead to catastrophic failure. For domestic systems, this shouldn't be a problem
... unless you like it really loud....

But Moondog's right, when calculating your low section use twice the impedance value of the speaker AT THE CROSSOVER FREQUENCY... you'll need an impedance graph of the driver to get this...
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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

Last edited by PeteMcK; 5th December 2012 at 11:24 PM.
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Old 6th December 2012, 06:02 AM   #9
mdocod is offline mdocod  United States
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Hi Jester,

You have DC resistance measurements. They will not be very useful for crossover design in and of themselves unless you have some plans to extrapolate from that data, combined with other data, some realistic impedance around the intended crossover.

I'd be willing to bet that the impedance you will want to use for component selection will actually be quit a bit higher than ~8ohm. Learn to use a crossover simulation program for best initial results.

Eric
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Old 6th December 2012, 06:16 AM   #10
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Sometimes tho, the DCR and the impedance at around 300hz are close .
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