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I'm wondering how an Impedance Equalizer (a capacitor & resistor connected across the speaker terminals) should be intergrated into a low pass circuit where two 8Ω woofers are connected in parallel resulting in 4Ω?
Should I use an Impedance Equalizer for each woofer (basing the values of the equalizer on 8Ω) or just one Impedance Equalizer prior to the branching off of the wiring to each woofer, where the impedance is 4Ω & obtain the values for the resistor & capacitor based on 4Ω & ½ the voicecoil inductance of a single woofer?
Also, correct me if I'm wrong, but as the parallel woofer connection will result in the Low Pass filter reading 4Ω, I am basing the values of the inductor & capacitor (2nd order Linkwitz-Riley) on that specification.
Thanks for any help.
Steve:
Follow Ups:
You should base your lowpass filter calculations on the actual equalized impedance of the driver(s), not the nominal impedance.
You need to know the Re and Le of your driver. Most manufacturers publish those specs. If not, you'll need an LCR meter to determine them.
If the drivers are identical, and you're hooking them up in parallel, cut both values in half. Then enter them into THIS calculator:
http://www.mh-audio.nl/spk_calc.asp#impeqalize
The calculated value of R-zobel will essentially be the real impedance of the equalized drivers. Calculate your lowpass filter into that.
(Don't want to confuse you at this stage, but there's a whole LOT more to crossover design than just plugging numbers into textbook formulae or calculators. Those give you textbook electrical transfer functions into a presumed flat impedance, but don't account for the acoustic rolloffs of the drivers, which combine with the electrical transfer functions to produce the actual crossover slopes. There is also the issue of baffle diffraction step, which textbook formulae don't account for. Read Ray Alden's "Loudspeakers 201" to get a better handle on the basic theory, and then look into downloading some good free designware. Jeff Bagby's Passive Crossover Designer goes way beyond textbook calculators, but does require accurate impedance and response-in-baffle data to start with. It interfaces with other FRD consortium freeware such as the baffle diffraction calculator and SPL trace, which allows you to use published measurements if that's all you have to work with. Don't be intimidated by all this. There's a learning curve, for sure, but if you're brave enough to consider building your own speakers, you're smart enough to learn it).
One Zobel network before branching off to the woofers seems like the sensible option. Although I might be tempted to use one per woofer to avoid the thinking required to determine the values for a "composite" driver. Or I'd hook up the IMP/MLS and use trial and error.
Two 8 ohm woofers in parallel gives you 4 ohms, so you should design the low pass filter for 4 ohms.
It's impressive how well even a rule-of-thumb Zobel improves the electrical response of a passive crossover.
Thanks for your thoughts bassbinotoko.
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