Low-pass 8 ohm Subwoofer Filter

Thread Starter


Joined Feb 25, 2013

I've been reading through this thread: https://forum.allaboutcircuits.com/threads/subwoofer-passive-low-pass-filter.118184/ with interest.

I'm looking to do something very similar, using an 'off-the-shelf' filter from: http://www.avsl.com/product/900.584UK-subwoofer-filters

The filter has all the right parameters apart from it's designed for a 4 ohm speaker and I'd like to use it with an 8 ohm speaker. I've seen kits of eBay and Amazon etc. where the 400W version (http://www.avsl.com/product/900.582UK-subwoofer-filters) is sold with a 47uF 100V Bi-Polar Electrolytic capacitor, which when swapped for the 188uF (already solder onto the board) makes it suitable for use with an 8 ohm speaker, but alters the cut-off to approximately 300Hz, instead of 120Hz.

I'm not concerned about the cut-off, as the subwoofer is happy with any frequency below 1kHz. However, what I don't know is the value of the capacitor on the 600W version and whether or not swapping it for a 47uF 100V Bi-Polar Electrolytic capacitor will be suitable?

I did contact the supplier (AVSL), but they were unable to provide any technical information and just referred me to the re-seller.

I'm assuming that the capacitor in both filters is the same (although unmarked on the 600W version), with the only difference being the inductor using a heavier gauge of wire for the winding, as it's listed as being 60 grams heavier?

Any advice would be much appreciated. I look forward to hearing from you.

Kind Regards



Joined Oct 2, 2009
Here are some cross-over designs for 12dB/octave.

Reference: Building Hi-Fi Speaker Systems, M. D. Hull, Philips, 1969

Thread Starter


Joined Feb 25, 2013
@MrChips Thank you for your reply, sorry for the late response. I decided to change out the original capacitor (on the filters) for a 47μF Bipolar 100V electrolytic and it seems to work with no issues... I don't fully understand how or why, but never mind!

A functional test of the completed sub-woofers (a pair of Bazooka 10" 8Ω - EL1008s) was successful. They were designed to increase the bass response of a system, when used in parallel with existing full range 8Ω speakers and were tested with a 380W RMS stereo amp. The sub-woofers are rated at 200W (300W with liquid cooling), but obviously when connected in parallel with the full range speakers, they should handle less than half the rated output of the amp (overall impedance per channel approximately 4Ω).

Whereas previously, the bass was boosted on the mixer, to give an acceptable frequency response, this can now be achieved with the tone controls set flat :)

I've attached a selection of pictures, showing the construction of the sealed cabinets... Ported cabinets were considered, but required 4x the internal volume! As it was, we were able to make the concrete floor vibrate considerably, which makes you wonder why people install these speakers as part of a car audio system!