716 Chapter 24
24.1.4.7 Active Crossovers
The active crossover (fi rst suggested by Norman Crowhurst in the 1950s) takes the
preceding concept a stage further. Frequency division is accomplished actively. This means
using active devices—and a DC power source—to provide fi ltering in a highly predictable
manner. For example, the fi lters are able to work in an ideal environment, having well-
defi ned and resistive loading. This and the fi lter function are defi ned potentially very
precisely by active electronics, usually employing high NFB ( Figure 24.6 ).
The main disadvantage of active crossover systems is cost, not just of the active crossover,
but of the added amplifi cation and cabling. In DIY domestic setups there is also the bulk
of equipment (if using, say, three stereo amplifi ers, placed centrally, or six mono block
amps and two mono crossovers, half to be placed by each speaker) and their cabling. Such
inconvenience is irrelevant in concert sound systems, and even in recording studios. It is
High pass Low pass Line driverHF
power amplifierHF driverSet HF
level4 kHz 150 kHzHigh pass Low pass Line driver
Midrange
power amplifierMid driverSet MF
level250 Hz 4 kHzHigh passHigh pass
bypassLow pass Line driverBass
power amplifierBass driverSet LF
level5 Hz 250 HzBalanced
to unbalInputFigure 24.6 : Classic three-way active crossover.