The Crossovers

As mentioned at the start, the BBC design used 3rd order Butterworth crossovers.  These were also fitted with extra adjustments for various 'characteristics' of the drivers BBC used, an anti ringing circuit and an adjustment to match the bass & treble driver sensitivities.

In my design I figured I would not have the means to measure driver response well enough to match them, and I did not have any specific driver 'characteristics' to compensate for, so these extra features were not needed.  The basic 3rd order Butterworth set for 3kHz cross over is as follows:

The capacitors used were 150v polyester except for the 15uF & 4.7uF which were bipolar electrolytics.

the inductors were 'roll your own' wond with 24g enamel wire on cotton reels.  The number of turns was calculated from online resources and checked with a multimeter with inductance range.  The capacitors, as can be seen beside the circuit, were constructed from 3 values in parallel, and checked with a capacitance meter.  Yes some adjustments were necessary so final combinations were a little different to the theortical values shown above.

A quck check with an audio oscillator and oscilloscope showed all was well, and ready to assemble in the enclosures!

The crossover built on vero board.