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7.2  Lowpass Filters

Lowpass filters have obvious uses in crossover design, keeping the MID and HF material out of the LF path, and keeping HF material out of the MID path. They are also used to explicitly define the upper limit of the audio bandwidth in a system, say at 50 kHz. This is much better than letting it happen by the casual accumulation of a lot of first-order roll-offs in succeeding stages. This bandwidth definition is not a duplication of input RF filtering which, as described in Chapter 16, must be passive and positioned before the incoming signals encounter any electronics at all, as active circuitry can demodulate RF. Lowpass filters are used in sound reinforcement systems to protect power amplifiers and loudspeakers against ultrasonic oscillation from outside sources or in the system itself. Lowpass filters are defined by their order, Q, cutoff frequency, and characteristic, e.g., Butterworth.


  

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