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ACTIVE FILTERS

        Active Filters originally found fame in professional sound systems i.e. cinema's and theatres where electronic filtering of sound sources at low signal level was developed to replace high power passive crossovers. The resulting signals were  individually amplified and fed to each driver.  This system whilst offering significant acoustic advantages was naturally expensive and did not find general acceptance in the domestic environment due principally to the location and cost of suitable amplifiers. With the advent of multi-channel audio systems now in abundance this problem has largely disappeared and QTA are pleased to offer a DIY evaluation kit, documentation, circuit layout and PCB.    

        Shown is a typical stand-alone two way active filter which is supported by an independent power supply (not shown). Two inputs from the signal source, left & right or 1 & 2, are processed at low signal level to provide suitable crossover characteristics for the loudspeaker. This is done with chips and transistors whereas the normal crossover board  contains large coils, caps and resistors operating at relatively high power.

        As the signal is split before amplification and routing to the speaker, it follows that each driver requires an amplifier. Hence for a two way system their are four channels, and for a three way their would be six.  Each filter is tailored or matched to the  mechanical / electrical properties of the loudspeaker model and like existing passive designs the filter cannot be substituted into other systems. Amplifiers of similar performance / characteristics should be employed for best results. Volume / gain controls are required either at source level or at the pre-amp / main amp interface.

        For the DIY enthusiast, a set of circuits, filter component tables and a PCB are available for experimentation. Power supply, components and hardware are not included although advice is available on request.

        In order to achieve workable designs, you should be familiar with frequency scaling of the filter block component values. This will enable you to modify the crossover frequencies. A good book on the subject is:- Active-Filter Cookbook by Don Lancaster, published by SAMS, ISBN 0-672-21168-8.

The key benefits are as follows :-

        (1) Faster circuitry, resulting in improved transient response.
        (2) Increased / improvement in dynamic range. 
        (3) Lower IM Distortion due to reduction in required cable bandwidth .
        (4) Improved crossover performance due to constant load.
        (5) Subjectively sounds better than high power crossovers.

All Prices include VAT @ 17.5%.   See carriage charges for further information.

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