Would someone check my math please?

John Larkin

Sublime Master of Turbology
May 25, 2001
I am trying to do homemade inline crossovers and have used the proper formulas, I think. The parameters are as follows: Tweeter: 4ohm, 4000 - 20000Hz, Woofer: 4ohm, 200 - 4000Hz, Sub: 4ohm (??, I haven't bought this yet so don't know the ohm level or if it's available), 20 - 200Hz.

Here are the components I plan to use: A highpass filter on the tweeters using a 10uF non-polarized capacitor, a bandpass filter on the mids using 150uH inductor and 220uF non-polarized capacitor in series, and a lowpass filter on the sub using a 330uH inductor. I had to convert the values to microHenris since Radio Shack didn't list milliHenris. All the charts use milliHenris.

Does this sound right? I checked a few of the crossover websites and their charts seem to support the numbers. I am still a little shaky on the bandpass configuration but the schematic the website shows is the way I've listed the components. Best part is despite the pleas of several members to buy ready made, all this stuff will run about $25.00 including tax. The goal is to lessen the work seen by each speaker so each can work more efficiently within their intended range. I pretty much arbitrarily picked the crossover points. ;)

Also, can I verify my speaker's ohm rating by testing them with a multimeter across the terminals?
Home-made passive x-overs are the way to go. They are cheap and fairly easy to build. Active x-overs are still the best, but sometimes passive are more practical

I SERIOUSLY recommend you using a 12db/octave x-over though.....6 just doesn't give you any real benefit for your work.

okay, do you know your spkrs Fs? Lowest Hz before its response starts to drop off? You need to know this to actually get it completly correct, but if you are close then it will be fine.

Before we get into this, for your own benefit (if you like this type of stuff and would do this more than once) then get Advanced Speaker Designs by Ray Alden. Its put out by Prompt Publications and can be bought @ www.partsexpress.com (which is where you can also get all your caps and inductors for a good price, along with anything else electrical u can thnk of, and better selection than Radio Shack)

Your HP is correct and the sub's low pass comes out to 318uH. The diff is prob just rounding error (from pi) and doesn't really matter. Go to Parts express and look for one closest to that if Radio shack doesn't sell it. The bandpass came out to 200uF and the inductor came out to 159uH. Still that little diff won't make that much of a diff, but try to get as close as possible

Now like I said, I say go with 2nd order. Its MUCH better. First order WILL NOT properly protect your tweeters, 2nd order barely will. here are the values (and formula's) that I get for it.

Cap: 5uF Eqn: C = [1/(4*pi*Fc*Rt)]*10^6 Fc=x-over freq ; Rt=impedance
Coil: .318mH or 318uH L = [Rt/(pi*Fc)]*10^3

This book doesn't have the 2nd order bandpass, I am not totally sure of its wiring method so I won't say which way to do it. I don't have the book that does have that info with me and dont remember the title to give to you. The internet should have that info though.

Cap: 99uF same eqn as with tweeter just substitute the woofers imped. with the tweets
Coil: 6.37mH

I must ask, what is this to be used for? Car or Home? If its car then that sub freq is too high. It needs to be under 120 at the absolute most. Thats even true for Home, but it doesn't matter as much in that case (i think u will find the sound to be unfavorable there too).

Good Luck!! and have fun with it!

Oh, I almost forgot, if you go the 12db route then remember to SWITCH THE TWEETER POLARITIES. The nature of passive x-overs (2nd order) cause the tweeter to be 180 degrees out of phase, to correct this just switch the leads at the tweeter itself.
Thanks for the detailed reply! well, it is my first attempt and maybe my last but how really knows. Anyhow, the speakers are factory stock in my wife's car. The rears are a 4X6 coupled with a separate 1" tweeter. The fronts appear to be 3.5" round coupled with a 1" tweeter. So I plan to split the two speakers in each corner of the car and then ultimately install a sub in the trunk to round out the bass. I have looked in the trunk at the setup and there is already a factory crossover of some sort in there. I have no idea what it does, nor do I really care. The sound is way too treble in the car and distorts rather easily. This is with a head unit that puts out 25X4. My initial plan was to make the 6db first order type system. I won't be adding an amp to these speakers. Matter of fact, I only plan to run a little Rockford Fosgate 30W bridgable amp to the sub. It's a small car and competition is the farthest thing from my mind. I just want to overcome road noise and get a nice full sound out of the system. The 12db second order stuff might be overkill for me. I had the head unit in her other car and with 4 dual cone speakers, a pair of 4X6's and a pair of 5" plus a 10" sub, it sounded darn good. So I know the head unit is up to my requirements. I do like your idea of running the tweeters out of phase. I read this and seems to be an installers trick to bring down the harshness of the tweeter. These probably need this since they will be bouncing sound off the windshield. Outside of that, I guess my final question is, should I recalculate and bring the mids down to the 100 -125Hz crossover you recommend for the sub's upper cutoff, or just lowpass the sub at 100 - 125 and leave the mids at a reasonable 200Hz bottom figure? I don't want to leave a hole in the sound spectrum. Thanks again for all the info. Sorry for the length. :)
I would still say go with the 12db/octave. Its not overkill, its protection, esp for the tweets (and give you a better sound). Plus, its not really any more difficult (or expensive) to go the 12db route (win win situation ;) ). Now for that mid, go for the 6db. Mids can handle a 100 or little less and not be screwed, but a tweet needs to always stay above its lower limit as much as possible, they are a little more delicate.

Now if you go to the first order route for the tweets (which I would urge you not to) then the polarity doesn't need to be switched.

Experiance with 3.5" (in my Monte) shows that they could DEFINATLY benefit from a 2nd order, they will crackle and distort (even a very good quality like the Kickers I had) very easily also, its such a small spkr. But they could still live with the 1st order, just make sure its good and high. They definatly do not need to be x-over below 200Hz, the 4x6 can.

Don't think that if you x-over the mid @ 200 and the sub @ 100 that there will be a hole. 1 octave down from 200 (100) will only be 6 db less (or 12, in which case they do need to be the same point) at 100Hz. Then your sub @ 100Hz will be 6db (or 12db) less @ 200. So your talking about, if the theory and graphs are still correct in my head, between 3-6db dip between 100-200Hz. Now since your using stock spkrs, and in stock locations, I guarentee you that you have dips and rises of that magnitude elsewhere also (windshield reflection and etc). So you won't be able to notice a "hole" in your sound. X-overs don't "cut" out freq. they cause them to "roll-off" at a greater rate than normal.

Oh, and if you are planning on amping a sub then stick with a built in active x-over in the sub amp. Passive filters rob power and since you will be going with a low power setup on your sub then you won't want to burn any unnecessary power inside a passive filter.
You da man mcss383!!!!!

I was going to bust out my RTI training manual but I think you have it covered mcss383, good job on reply :cool: .
He he, last night you were looking for simple 6db bass blockers. Now you are building second order filters LOL.

Hey make sure you check out the Parts Express link cause when you use inductors the type and quality matter greatly. Air core vs. Iron core and wire gauge and type, such as ribbon vs. std. copper wire.

Quality and voltage rating of capacitors is impt. too.

At that site they have premade crossovers that will work both for home and the car with both 4 ohm and 8 ohm versions with quality components at a reasonable price.

Much more variety than Radio Shack.

Good luck 1/2pi*... man!!!!! :)
I got your 1/2pi.. right here... ;)

I learn fast! Ok, second order on the tweeters, put 'em out of phase, first order on the mids, keep in phase. Got it. I can do that, just got to alter my parts a little. I did find good parts at Radio Shack. I had to go to the catalogue, but the guy there was bored and seemed interested so we took a little time and found the right stuff. I went out after the last post and looked at that factory crossover. It has a 10uF cap and some air inductor ring about 1" across, but couldn't see a spec on it. I got tired of trying to figure out the little circuit board it was on. Maybe I'll get daring and hook the thing up A & B and compare my components to the factory before hard wiring it in. Lotta wires but at least I could see the differences if any on the spot. Then again, maybe I just have crappy speakers and this whole project is a waste! LOL! Oh well, the components won't be a waste anyhow; they will still serve a purpose.
what kind of car is this? I'm really suprised that there would be x-overs in a factory car. Of course the only experiance I've really had has been with my 86 Monte, my bro's 69 Chevelle, and a 94 S10 and our '92 Suburban. Not exactly luxury cars with premium systems (although the heads in the '96 newer Chevy trucks are GREAT).
It's a 1996 Saab 9000. It just had a standard cassette factory headunit. I am wondering if it has an auxiliary amp. The original radio died and the security code would not longer work. We took it to the dealer for warranty and they tested the head unit. They said it was fried and a replacement unit was the only solution. Since they wanted boo-coo bucks to change it out (~$600, ouch!), we decided to go aftermarket. I removed the socket from the back of the factory stereo and made my own adapter so I could just plug into the factory wires without hacking things up. But during this, I found that I had to run switched power to a power wire that was on the speaker plug. It's possible that this means the car has an auxiliary amplifier somewhere in the system. I can't find a shop manual for the car anywhere, so I am flying by the seat of my pants here. There is a thump when I turn the radio off and the speakers will not work without this power wire hooked up. Could this be causing the distortion? Am I feeding this amp an amplified signal and effectively overdriving it? I don't know much more than this at this point. The little crossover is kinda neat but it may or may not be of the standards that I am trying to achieve. Thanks for wading through this.
that could be the prob....i say just run a wire (outside the vehicle) from the head to the spkrs and then just listen to them. If when u do that there is no more distortion just run brand new wires from your current head to the spkrs eliminating anything factory.