active equalization help

[downward_dog]

I am new to the world of active electronics (I come from loudspeaker design and car audio), and I am interested in designing a simple active equalizer, similar to what Linkwitz uses for his Orion and Phoenix designs (see http://www.linkwitzlab.com/filters.htm).

I would like to start simple, with just some subwoofer equalization. Where could I find a calculator or tutorial that explains how to design active equalization?

Thanks for the help!