representations to allow the simple formulas to capture their performance accurately. The wild impedances presented by the speaker and its passive crossover components are not an issue with the active crossover, as it is buffered from them by the power amplifier. And even within the active crossover, the varying impedances of its passive components are trapped in between active stages. All of which means that an active crossover can be made that easily fulfils its design expectation.

Design Goals
The active crossover must divide the full audio signal bandwidth presented at its input into sub-bandwidths for each of the four speaker drivers so that their summed output equals a flat frequency response.  It must also be able to shield its impedance sensitive components from the unspecified source impedance at its input. In addition, it must be able to drive the interconnect capacitance and power amplifier input impedance to full output. 

What Type of Crossover?
The theoretically pure crossover is the first order Butterworth. It alone is both minimum phase and  constant power. Unfortunately, few speakers are up to working with it. Because the crossover is of a single order, its attenuating slopes are only 6 dB per octave, which is just too gentle for many speakers.  But as this crossover feeds four drivers, no one driver need see too great a frequency band, which lessens the need for drivers with wide bandwidth and high power handling.

For less robust speakers, or narrower bandwidth drivers, the three order 18 dB per octave is a better choice. Its attenuating slopes are steep enough both to protect the high frequency drivers and to prohibit the low frequency drivers from trying to overextend themselves. 

Which circuit to use?
The active circuit used in an active crossover must have a high input impedance, unity gain, wide bandwidth, and a low output impedance. Sounds like the textbook description of the Cathode Follower. A White Cathode Follower would work just as well if not better, but would be more complex to design and build.

Cathode Followers sound bad, don't they?
"Not necessarily," is the short answer. Once again, a Cathode Follower need not sound bad; and, if properly designed, it can sound as good as any Grounded Cathode amplifier. A quick re-read of September 1998 provides more detailed information on the role of transconductance and current in determining the sound quality of a Cathode Follower.

6 dB Crossover Circuit description
In quality audio applications, less is more. Here the circuit has been pared down to the essentials only. Starting at the high frequency tweeter output, a single Cathode Follower is connected to the input and the coupling capacitor working into the resistor R in parallel with the power amplifier input impedance defines the crossover frequency. The math is simple: f = 159,155 / (R' * C) , where R' = total resistance to ground.

The upper midrange output is buffered at its input by a Cathode Follower to ensure that its low pass crossover frequency is not affected by the output impedance of the source feeding the crossover. The high pass frequency is set, as in the previous example, by its coupling capacitor working into the resistor R in parallel with the power amplifier input impedance. The lower frequency is set by juggling the input capacitor and resistor values:  f = 159,155 / (R * C).