Filter Equations.

[tade]

I would like to see a general discussion of the math behind crossovers.
I have scoured the internet and cannot find anything on them! I need a low pass filter with 12db roll off at 40hz. 12db because i heard that that puts the phase 180 out, which is easy to correct by swapping wires. what sort of components will it need?
I cannot hink of anything else to ask, so i will check back often!
thanks guys.

 

[bigmike216]

http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

 

[jcx]

you could go to "the source" - http://www.linkwitzlab.com
and see what the designer of the most popular crossover response has to say

 

[Cal Weldon]

Are you looking for a XO calculator?

http://www.mhsoft.nl/spk_calc.asp

Cal

 

[AndrewT]

Hi,
two pole filter is 180 degree phase shift only at the -3db point.
It will reduce gradually as you move in towards the pass band and at about F/10 phase shift is almost 0degrees.
Look up National Semiconductors application notes AN779, oa21, oa26, oa27, oa28, oa29, Burr Brown ab034b, ab017c,
Plenty reading & maths in that lot.

 

[theChris]

yes, how general are you looking to make the math.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1795
is about the best math-based site i can find.

 

[tade]

I see lots of good stuff on RC low pass filters, but i think you can build on using only an inductor, am i right? Aren't RC circuits **** por too because they load resistore which dont have anything to di with producing sound?
what value of inductor should i wire in series to get a crossover of 30hz?

 

[AndrewT]

Hi Tade,
do you want a passive low pass filter? or active?
Passive @ 40Hz into low impedance will require a large inductor!

 

[tade]

good point. so an active filter which would go before the amp will be cheaper perhaps? but require an extra amp.
(i have the extra amp but am not nuts about taking it back to school. the nine cubic foot sub box is bad enough!)
In my room it sits behind and to the left. I have it crossed over at about 200 and it is clearly directional, plus boomy because of the poor quality materials and general construction. Maybe i do not need 30hz?
PE sells an active crossover simulator which i believe you wire in between your source and the amp. I could buy a pair of those as a low pass. A solution?
thanks

 

[Bill Fitzpatrick]

May I ask why you have a 9 cuft sub enclosure?