building a bandpass passive crossover for midbass speakers

[jht3]

i have a pair of speakers i'm thinking about installing in the rear to add some bass fill. it's not gonna add much, but it should help a bit. at least better than not having them powered. i don't want to add the weight of a 2nd amp, 10" sub, and box.

2 of the 4 channels on my amp are unused so my idea is to bridge those channels and connect my 2 speakers in series. they are 4-ohm speakers so that will make the total impedence 8-ohms.

my amp doesn't have any crossovers so i'll need a bandpass filter, 100-500hz . it doesn't look all that complicated to build a passive bandpass crossover (and maybe a little fun too), but i'm a little confused. i'm assuming i should buy capacitors and coils meant for 8-ohms. so by my calculations for a 12db bandpass crossover i'll need 18.01mH and 140.63pF for the highpass and 3.6mH and 28.13pF for the lowpass

am i on the right track?

[KenC]

What is a 'bandpass' crossover? I've built my own passive crossovers before, it's pretty easy.

I would just make a low-pass one at 500Hz, don't worry about making a high-pass one at 100 since the frequency response that low will be pretty minimal.

Here are the capacitor and coil values for a Linkwitz-Riley crossover with a 12db/oct slope:

Capacitor: 1.99E-05 farads

Inductor: 0.0050928 henries

parallel the cap with the speakers, and run the inductor in series with the positive lead between the amp and the capacitor.

[jht3]

i only want to limit what freqs my speakers will see. hence the band pass filter to only allow 100-500hz to the speakers. these aren't large speakers. i'd go w/ a 8" or 10" sub if i wanted to get some real thump.

i did some more research, and it looks like i got my calcuations wrong since the freq range is so close. but the same idea applies. they should actually be 14.4 millihenries and 35.2 microfarads, and 4.5 millihenries and 112.5 microfarads.

a lowpass would be MUCH simpler. and i can't say i'd be able to notice a difference w/ the extra freq cutoff. i also might find my speakers aren't up the the task. but 1/2 the fun is finding out

[Mystikal]

I agree that high-passing them at 100hz seems a little overkill, but you can't knock fun. ;)

[chuck90m3]

x over values

if you are looking for the formulas and values for passive xovers go to www.selectproducts.com find the location on the left side of the page where you can choose the application guide. download the crossover charts, get the values for the high pass from where you want your speaker to start playing from. then get the low pass value from where you want the speaker to stop playing to. wire them both in parralle and you have a bandpass xover for the speaker. if you want to look at the layout of the xovers on the select products page, go to the menu and pick select products, crossover calculator.