subwoofer bass extension active filter?

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A sealed box is probably the best way to achieve good transient behavior. Unfortunately, especially with low Q designs, the corner frequency is pretty high.

A special passive or preferably active filter could compensate this and extend the corner frequency considerably by driving the coil harder at lower frequency.

There are various hints about this forum or in articles on subwoofers, but I haven't really seen a circuit nor a desciption on how to design these bass EQ circuits.

I could imagine using another low pass filter with high Q would do the trick because of the hump it develops. But I doubt this is optimal from the impulse response point of view.

Anybody have a link to a good primer on the subject?


Hi Eric

I f you have a low QTC (around 0.5 - 0.6) and want to keep this and you need only a lower cutoff frequency then use a shelving filter consisting of two series connected active lag filters (can even be done with one op-amp only).

Another possible solution would be the ELF circuit which is also an LF EQ circuit. It is basically two active "integrators" (PT1 to be correct) in series. The combined transfer function of the ELF circuit and the driver is a bandpass whose upper cutoff frequency is determined by the lower cutoff frequency of the original speaker/box combination. But maybe that doesn't offer you enough flexibility.

A third solution would be the use of an active highpass subsonic filter with a hump, but this would definitely raise the system order.


make life simple. see if you can lay your hands on a Audio Control C101 or simialr EQ. it has a XO and bass EQ built in and it is flexible.
Make life CHEAPER and better and DIY(and likely it`s simpler too). Build the simple Linkwitz circuit or the EAS according the links provided above. They can be made flexible too. Just change a couple of caps and resistors (could be done by small DIP switches also) and You have another transfer function.
No need to shoot on sparrows with cannons.;)
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