i want to make a low pass crossover for a sub at about 75-100hz. I did some reading on them and from what i read; all i have to do is add the the proper size milihenries coil inline on the positive wire? is this correct? What octave slope should i be using 6db 12db 18db im thinkin 12db? the sub is 4ohm and will only have low wattage. using the chart on this site to crossover a 4ohm speaker at 100hz takes a 9mH coils with a 12db slope? any help appriated(sp?).
A single coil would be 6dB per octave, but at 100Hz, I would NEVER use a passive crossover. There will be so much wire, your DCR will be so high, you'll lose much of your power in the coil. It CAN be done, but the expense of a large wire-gauge coil will outweigh the disadvantages of buying a decent quality active crossover.
Why are you using a passive solution?
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
so what should i do im looking for the cheapest way to crossover a sub? the frequency isnt to important as long as its below 100hz but above 70hz. this isn't for my car. its for this ghetto system im rigging up in my garage with a bunch of old stuff i dont care about. my sub is currently runnig full range and sounds like total crap plus im sure its not good for it to be playing those high frequencies.
For a cheap and dirty 6 db/octave subwoofer passive crossover, use one of THESE in series with each sub's positive VC terminal. At 4ohms, this will low-pass at about 90Hz. I would never recommend an iron-core inductor for a high-end stereo system, and haemphyst is 100% correct that for a subwoofer low-pass active is a far better solution, but if your amp doesn't have a low pass built in and you want cheap, this is about the cheapest way to go.
For a 12 db/octave crossover by the way, you must use both a coil and a capacitor. Check the links to the left.
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Support the12volt.comk thanks guys this is exactly what i needed thanks for the help.