ABSTRACT
The filler-driver crossover concept was introduced by Erik Baekgaard of Bang & Olufsen in 1977 [9]. It uses an extra drive unit in order to obtain a linear-phase response. This is one of those not-too-common situations where wading through the complex algebra that describes the crossover response is instructive. If you take the equation for the response of a second-order Butterworth crossover, and then compare it with the corresponding mathematical description of a linear-phase crossover, there is a missing term in the former. The idea is to add an extra drive unit, called a “filler-driver” that will supply that missing term, which is equivalent to a bandpass filter of low Q centred on the crossover frequency.
The second-order Butterworth alignment is normally used with one output inverted to get somewhere near a flat response (without frequency offset it gives a broad +3 dB hump at crossover). If the outputs are in-phase then there is a deep notch at the crossover frequency. This notch is filled in precisely by the filler drive unit, as shown in Figure 4.63, where the summed response is exactly flat, and additionally we get a flat phase response at 0° and a flat group delay response. If we assume the filler drive unit has a flat response then it must be fed via a bandpass filter that is −3 dB down an octave away from the centre frequency on each side; this corresponds to a Q of 0.6667. Ways of realising the required filter appear
