ejektor; active filters for the filler-driver principle

A loudspeaker `crossover` filter invariably consists of a pair of comp- lementary Butterworth-characteristic circuits, with stop-band slope of 12 or 18 dB per octave. The phase-character- istic of the complete system can be improved by adding a `filler driver` channel. The filler driver principle introduced by the Danish company of `B & O` adds an extra loudspeaker for each crossover in the system. The extra unit is only driven in a narrow range of frequencies, by means of an extra filter section. The active selective filters given here will enable this extension to be applied to the `active crossover filter` described· elsewhere in this issue.