ABSTRACT
This chapter shows the classical method of making a 3rd-order lowpass filter by cascading a 2nd-order stage and a 1st-order stage. It discusses a single-stage 3rd-order Butterworth lowpass Sallen & Key filter with unity gain. In lowpass Sallen & Key filters the resistors are almost always made equal. The chapter looks at that there was little to be gained by using non-equal resistor values in 2nd-order Sallen & Key stage. A very desirable feature of active crossovers for sound reinforcement is bandwidth definition, to keep subsonic and ultrasonic signals out of the amplifier/speaker system. Subsonic signals of sufficient level will cause mechanical and possibly thermal damage to lowpass filter drivers, but even smaller amplitudes will erode precious headroom. The design of lowpass filters to stop subsonic signals and highpass filters to stop ultrasonic signals is always something of a compromise, because people must steer a course between most effective filtering and intrusion on audio bandwidth that people want to keep.
