3rd Order Filters
    Two ways of making a 3rd order filter exists. The first is to have a 2nd order filter cascade with 1st order filter. The Q of the 2nd order filter  must be made to compensate for the un-adjustable 1st order filter. The second approach is to place all the RC sections in series.

   What advantage does one hold over the other? For the cathode follower based filters, little advantage exist other than the two section method having one less active device in the signal path. But if the buffers were replaced with amplifiers, then the two section method allows for all the resistors to equal each other. How is that an advantage? If all the resistors equal, then an adjustable crossover point can easily be had by using a three deck potentiometer to set the crossover frequency.

4th Order Filters
     While 4th order filters can be made from a single section, it is much easier to cascade two 2nd order filters. Remember the Qs multiply against each other. Thus two Butterworth 2nd order filter with a Q of .707 will when cascaded yield a Q of .5, which is, by the way, the Q of the Linkwitz-Riley filter. So in order to make a 4th order Butterworth filter, one section's Q might be .84 and the second section would have to be also .84 to have the product equal .707.   

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