You can change the amount of attenuation by adjusting the resistor values, and you can set Q by varying the ratio of C 1 to C 2 . You must simply keep the product of the capacitor values constant to avoid changing the center frequency. Figure 3 shows the response of the circuit for two distributions of potentiometer values. Despite the considerable differences in response, the performance may be acceptable in some applications. This type of filter works well in audio applications for notching unwanted frequencies, thus fighting acoustic feedback.
Combo amplifiers combine the amplifier and loudspeaker in one enclosure.
These amplifiers are virtually ubiquitous, but the one shown specifically targets acoustic-guitar amplification. An electronic crossover is much cheaper and easier to design than a conventional passive crossover. The third stage is a 2.4-kHz lowpass filter, and the fourth stage is the power amplifier itself. Only a small difference exists between the frequency responses of circuits using separate and combined filter stages. With available filter-design software, you can easily figure out and minimize this difference if necessary.