Crossover Types  69

disadvantages. The6 dB/octave slopes are too gentle for use with normal drive units, and time alignment is frighteningly critical. Despite these disadvantages the positive features of a 1st-order crossover are so powerful that there does seem to be a distinct trend towards investing in driver technology in order to create units that are suitable for 1st-order operation. A recent example is the

KEF Q-series of loudspeakers. [3]

First-Order Solen Split Crossover

The so-called Solen split 1st-order crossover is a variation on the standard 1st-order crossover (Solen is a company that supplies passive crossover parts). This scheme attempts to improve the poor slow crossover from one drive unit to the other by putting the crossover point for each filter at −6 dB rather than the usual −3 dB, as in Figure 4.2. This applies a frequency offset to the cutoff (−3 dB) frequencies of each filter, so that the lowpass filter now has a cutoff of 579 Hz and the highpass filter now has a cutoff of 1.726 kHz, pulling apart them apart by a factor of 2.98 times or 1.68 octaves; the pullingapart process is presumably where the term “split” comes from. It may somewhat ease the demands on the drive units, but what of the amplitude response?

Figure 4.8 shows that, as we might expect, pulling apart the two cutoff frequencies has caused the summed response to sag in the middle, by 6 dB in fact. This is obviously going to sound like rubbish, but we get a better result if we reverse the phase to one of the drive units—a common manoeuvre in crossover design.

Figure 4.8: Frequency response of Solen split 1st-order 1 kHz crossover; both filter outputs plus their in-phase sum.