ABSTRACT
This chapter looks at that people could select convenient resistor values for a 2nd-order Sallen & Key highpass filter by selecting the gain in the filter. A 4th-order highpass filter is usually built by cascading two 2nd-order highpass stages. A 5th-order highpass filter is usually built by cascading two 2nd-order highpass stages and one 1st-order highpass stage. A 6th-order highpass filter is usually built by cascading three 2nd-order highpass stages. The chapter focuses on the Butterworth filter characteristic. Highpass filters used for subsonic protection are usually 2nd-order or 3rd-order Butterworth types with roll-offs at 12 dB/octave and 18 dB/octave. In some cases it is possible to economically combine highpass and lowpass filters into one Sallen & Key stage using only one opamp. Active crossovers for sound-reinforcement applications commonly have variable crossover frequencies so they can be used with a wide range of loudspeaker systems.
