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Old 9th November 2003, 03:37 AM   #1
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Default Determining crossover values

I have a pair of Advent Legacy III speakers. I am sort of happy with them, but am investigation some tweeks to make them better. I disasembled one speaker and took a look at the crossover. It appears to me to be a third order crossover on the tweeter, with no low pass filter on the woofer. Using the diagram for a 3rd order crossover on the xover calculator at http://ccs.exl.info/calc_cr.html#third, C1 has a value of 5 uF, C2 a value of 32 uF, and the inductor, L1, has an unknown value, but is a small cylinder about 30 mm long with a diameter of 10 mm. All negative leads out to the woofers and tweeters are joined together, so I don't think the circuit has any affect on the woofer. This speaker has a dual voice coil on the woofer with a switch to select 6 ohms or 8 ohms. The 6 ohm circuit has a resistor in it, but I have always used the 8 ohm setting, so I think this circuit is irrelevant for my purposes.

Is there any way to determine the crossover frequency from the above information? Is there any way to determine the value of the inductor? I am thinking of replacing the components (bipolar caps) with metallized poly caps, and the inductor with an air coil. I am also considering replacing the tweeter with a Morel MDT 20 and crossing over at a slightly lower frequency.

Any help would be appreciated.
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Old 11th November 2003, 03:14 PM   #2
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I found out the published crossover is 2,500 Hz. Does anyone know how to determine the value of the inductor?

C1 has a value of 5 uF, C2 a value of 32 uF, and the inductor, L1, has an unknown value.
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Old 12th November 2003, 12:26 PM   #3
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You need either

LCR meter (just the L bit, actually)
signal generator and scope or decent multimeter

to measure out the inductor value.

Replacing with an air-cored version will (unless a monster sized one is used) raise the DC resistance of this component and may affect the crossover frequency and response. I personally prefer to use over-sized cored inductors as they have lower DCR and are not so wildy affected by atmospheric conditions.

If you replace the tweeter with another (different) unit, the crossover will be invalid anyway, as it relies on the characteristic impedance and response of the drive unit.

You can re-design the crossover network, but you need to kit yourself out with a signal gen and scope/meter at a minimum, to characterise the drive unit impedance curve. You can try doing it by ear but you will waste a lot of time and will probably end up with some weird response. You may like it, but it prolly won't be right
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