making an external subsonic filter

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=69627
Printed Date: May 06, 2025 at 12:41 PM

Topic: making an external subsonic filter

Posted By: black78ta
Subject: making an external subsonic filter
Date Posted: December 31, 2005 at 12:55 AM

Hey guys, I've got a question for you. I just bought my girlfriend a six channel amplifier, but the amplifier does not have a subsonic filter. I'm going to bridge the last two channels at four ohms to get about three hundred watts. How can I make a sort of external subsonic filter. I'm looking for something rather cheap, as I already spent enough with this amp and a sub for her christmas present. Thanks in advance guys!

Replies:

Posted By: DYohn
Date Posted: December 31, 2005 at 1:15 AM

You can purchase one far more cheaply than you can make one. A Harrison Labs HP-Sub crossover module is a good choice. Plus a passive subsonic filter would not only be expensive to build it'll eat up amplifier power.

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Posted By: black78ta
Date Posted: December 31, 2005 at 1:17 AM

Thanks, but I forgot to mention that I'm only going to be using 4-channel RCA cables for all four speakers and the amp is able to use the summed inputs to make channels 5 and 6 for the sub, so a pass-through type device wouldn't really work for my setup.

Posted By: menace2sobriety
Date Posted: December 31, 2005 at 6:17 AM

use a sealed sub box that way you dont need a subsonic filter.

Posted By: DYohn
Date Posted: December 31, 2005 at 12:06 PM

black78ta wrote:

Thanks, but I forgot to mention that I'm only going to be using 4-channel RCA cables for all four speakers and the amp is able to use the summed inputs to make channels 5 and 6 for the sub, so a pass-through type device wouldn't really work for my setup.

Sure it would. Just put it on channels 3 and 4. But as mentioned above, you don't need one unless your sub is ported.

If you really want to build a passive subsonic filter, a high-pass @ 12db/octave with knee frequency of 20Hz for a 4-ohm woofer would require about 1400 micro-farads of capacitance and 45 mili-henries of inductance. I'd expect you'll pay on the order of $250 or more to assemble those component values (and you'll have to build a capacitor array to get to 1400uf, and have a large iron-core inductor built to get 45mh.) Here's your circuit:

posted_image

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