With modern Operationsverstaerker Active filters can be realized with high accuracy and goodness. Even with high-order filter can be represented.

Here you will find Schematic Circuit Diagram for DIY and further information on Active filters
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A very good introduction to passive filters, active filters, Switched Capacitor Filters	www.national.com/an/AN/AN-779.pdf
Software for the calculation of active filter, supports many Operationsverstaerker	www.aktivfilter.de