Biquad-audio-filter

  
This universal filter offers low-pass, high-pass, bandpass, band elimination, and all-pass functions. The Biquad consists of two successive integration stages followed by an inverting stage. The entire group has a feedback loop from the front to the back consisting of Rl which is chiefly responsible for controlling the center frequency, W 0. The first stage of integration is a poor integrator because R2 limits the range of integration. R2 and C form the time constant of the first stage integrator with R3 influencing gain H almost directly.
Biquad-audio-filter - schematic

The band-pass function is taken after the first stage with the lowpass function taken after the third stage. The remaining filter operations are generated by various combinations of three stages. The Biquad is orthogonally tuned, meaning that W 0, Q, and gain H can all be independently adjusted. The component values known will allow W 0 to range from 40 Hz to 20 kHz. The other component values give an adequate range of operation to allow for virtually universal filtering in the audio region. W 0, Q, and gain H can all be independently adjusted by tuning Rl through R3 in succession.




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