The line level audio signal enters the buffer where it is inverted out of phase. This is to compensate for the filters which will invert the phase again thus returning the signal to it's proper phase. The filters are more-or-less standard low and high pass filters respectively however, extra capacitors and resistors have been added in order to get the tolerance down to a minimum using only E12 components.
This circuit may not be of terribly much use to anyone as it was specifically designed to suit the applications. It is not variable or switchable for two reasons. Firstly, it didn't need to be since the properties of the speaker system were already known. And secondly, the extra complexity was put into the accuracy of the crossover rather than it's flexibility. The filters are matched in respect to each other as well as can be expected using only E12 components. If switching were to be employed to give a range of frequency options, a compromise would have to be made on filter accuracy.
The only other way to make a universal crossover design and maintain accuracy would probably be to use Switched capacitor filter building blocks. In theory, two 18or 24dB per octave filters could be constructed such that they tracked each other by being locked to the same clock. This would be a nice experiment but I haven't had the opportunity to prove this technology for this kind of application. My primary concern was the break through of switching noise using such a design.
Therefore this design represents the simplest approach I could take and still get the desired outcome.
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