newbie, help with low-pass filter

[micfly]

I just want to ad a simple filter to a pair of small mid-bass speakers. These are in the rear doors of my truck. The rear seats stay folded down basically covering the speakers so all I want back there is bass. The speakers are 4 ohm 6.5" drivers rated at 50Hz-5kHz. These are drivers only with no built-in tweeters.

I would like to just cut out the highs to these speakers anywhere from 1-5kHz. I can only find select components at my local electronics store so not sure which way to go. What I bought was a choke rated at .68mH. This could be used for a simple first order filter by what I can figure should cut out about 1kHz and up.

Or, I can use a 10uF capacitor with a .33mH choke for a simple second order filter. What I can figure is I needed a 9.95uF cap with a .636mH choke. Will the 10uF/.33mH be close enough? If I use this should I swap the +/- wires for correct polarization?

Also, the capacitors are non-polarized and the chokes are RF Filters. Are these the correct parts?

Which way should I go, the first order or second order filter? Thanks for all help!

[Cal Weldon]

You can start with just the chokes you bought and if that doesn't do it then design it for a 12dB at say 1000hz. Remember your drivers are 4 ohms so:

20 uFd caps
1.3 mH chokes

If you use a 10 and a .6 your cut off will be 2kHz.

Are you using one of these?

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

Cal

[micfly]

Heh, thanks for the help. Yea, actually I was using this one: http://www.apicsllc.com/apics/Misc/filter2.html

2K is probably too high, eh? I was kinda thinking that. If I can find the right combo of caps/chokes what should I really shoot for? 1k or lower? Like I say, I just want bass back there. Thanks again for the help!

[Cal Weldon]

If bass is your desire then 500 with a 12 dB might be a good target. At the same time keep in mind, the lower you go the bigger and more expensive the components are. Is this your permanent set up in the truck?

[micfly]

Yes it's permanent. I replaced the front two speakers with a good set of co-axils that sound fine. It has a good sounding set of tweeters up front, too. And the factory 6-disk changer is also fine with me. I just thought in the back, with the speakers covered, just do the bass deal. When I ordered the drivers for the back I didn't realize they would not come filtered...duh :-( When I installed them they sounded terrible (distortion). So, on when my jorney for filters. I'll try the parts store again for the 500 range components. Thanks again for the help!

[Cal Weldon]

Originally posted by micfly with the speakers covered,

Now there's an idea. Why don't you stuff a blanket or pillow up against them and see what you think of the sound. Just don't impede the cone travel. The thicker the blanket, the lower the frequency.

When I installed them they sounded terrible (distortion)

Distortion?
What kind of distortion were you hearing?

[micfly]

I'm sure the distortion is from all the highs these speakers can't reproduce. It's just when I open the doors and stick my ear up near them, otherwise up front most people wouldn't notice.

BTW, still wondering, If I go with the 2nd order should I swap the +/- wires? Also, if I get close on the part specs is that ok?

[richie00boy]

If you're using RF chokes they will probaly saturate as soon as any real power goes though them and the result will sound bad and possibly be a fire hazard.

The distortion you can hear is because your amp/headunit is clipping. Fitting a low-pass filter will help this to some extent by rounding off the waveform, but it will not do sod all for the fundamental problem that you are overdriving your amp/headunit.