With modern operational Amplifiers you can build active Filters with high Quality and accuracy, even filters of very high order are possible.

Here you will find Schematic Circuit Diagram for DIY and further information on Actice Filter
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Very good introduction to Passive Filter, Aktive filter, Switched Capacitor Filter	www.national.com/an/AN/AN-779.pdf
Software for Calculation of Active Filters. Supports many operational Amplifiers	http://www.aktivfilter.de