ABSTRACT
This chapter aims to give information on audio applications. Passive filters do not use active electronics, and this is a crucial advantage in some applications. They are not subject to slew rate limiting, semiconductor non-linearity, or errors due to falling open-loop gain, and this makes them the best technology for roofing filters. Preamplifiers for vinyl disk usage are commonly fitted with subsonic filtering, often below 10 Hz, to keep disturbances due to record warps and ripples from reaching the loudspeakers. Bandpass filters are principally used in mixing consoles and stand-alone equalisers. All-pass filters are so-called because they have a flat frequency response, and so pass all frequencies equally. Low-pass filters used to define the top limit of the audio bandwidth are typically second-order with roll-off rates of 12 dB/octave. At first an active filter that is also a differential amplifier, and thereby carries out an accurate subtraction, sounds like a very exotic creature.
