DIY low pass

It's easy enough with a dual coil driver. Just treat each coil as a separate entity.
Done it once to fix a Tandy sub of that type. Used a big 15mH cored inductor on each woofer input, I had a couple of 9mH there and simply wound as much wire as I could onto each core using 2 lots of enameled wire and measured afterwards.
Wired as a .5 and a pass to the other speakers; so 2 in and 2 out but only one box, I could have done it as a second order but not cheaply.
But SLLXO I have no idea, Dave is the expert there

But it's so much cheaper and easier to do it in the digital domain
 
If there is a dedicated amplifier (or amplifier channel) available for the subwoofer, then a line-level filter makes more sense than a loudspeaker-level filter. If not, I doubt that an extra 1000 W amplifier is cheaper than a couple of big inductors and capacitors.
 
In less than a minute...it just does not allow me to post that fast.
Oh cool, I wasn’t sure if those were speaker or line level. Thanks!

I’ve decided on the Behringer FBQ1502HD, which has a sub XLR output with crossover, and 2 15 band EQ channels, so I can EQ my passive and active speakers seperately. For £90 it actually doesn’t seem too bad


Line level channel mixing + low pass filtering + single power amp + any available subwoofer, no need for dual coil or any special feature: 100 times easier cheaper faster than going the passive route.

Just saying.
I need to go for a dual voice coil sub so that I can change the impedance. In series 8ohm for home use and then wired in parallel for 2ohms for car amps
 
I need to go for a dual voice coil sub so that I can change the impedance. In series 8ohm for home use and then wired in parallel for 2ohms for car amps
Cool, but then you can not use dual coil to mix right and left channels as was the original purpose,

* If it's a single subwoofer for stereo equipment, there is also the issue of mixing stereo down to mono. You need a double-voice-coil driver or two drivers for that when you use passive filters after the amplifier.

* It's easy enough with a dual coil driver. Just treat each coil as a separate entity.
Done it once to fix a Tandy sub of that type. Used a big 15mH cored inductor on each woofer input


it ends up becoming a single speaker which can handle a single amplifier, only its single impedance becomes 2 or 8 ohms.

You can use them as "2 separate left+right woofers" OR a single dual impedance one, but not both wirings at the same time.


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Also, maybe for HT I'll need decent SPL, 105dB at a few metres
Ridiculous. The accepted exposure time for this level in any 7-day period is 8 minutes... Do you understand the damage you would be doing to your hearing?
If you haven't yet, have your hearing tested to get an idea of any damage already inflicted so you can take steps to protect yourself, as aging also takes its toll regardless of noise exposure.
 
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