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Mix and Match
Active filters can be cascaded to yield a degree of frequency tailoring that is all but impossible with passive filters. The passive filter works best with known, fixed input and terminating impedances, which a cascading passive filter cannot readily do.
For example, two active 3rd order filters cascaded equals a 6th order filter. An active highpass and a lowpass filter cascaded can yield bandpass filter; and these same filters fed into a mixer can yield a notch filter.
Conclusion
In this article we have looked into the why bother with active crossovers and filters? We have seen that active crossovers and active filters in general offer some real advantages over their passive brethren, as they are more accurate, flexible, and easy to implement. In the next article, we will look into designing tube crossovers.
//JRB
Suggested Readings
Active Filter Cookbook
By Don Lancaster, 1975, 95, 96
IC based active filters explained in detail. A classic that is readily available new and used.
Audio/Radio Handbook
By National Semiconductor, 1980
This book has a chapter whimsically named "Floobydust," which covers some interesting active crossovers, including asymmetrical filters.
Passive and Active Filters
By Wai-Kai Chen, 1986
The book to own, if you are serious about filters. A classic, but not for those fearful of math.
Understanding Electronic Filters
By Owen Bishop, 1996
This book is the best place for the novice to start. Should be read from start to finish.
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