This circuit passes frequencies in the 300Hz - 3.1kHz range, as present
in human speech. The circuit consists of cascaded high-pass and low-pass
filters, which together form a complete band-pass filter. One half of a
TL072 dual op amp (IC1a) together with two capacitors and two resistors
make up a second-order Sallen-Key high-pass filter. With the values
shown, the cut-off frequency (3dB point) is around 300Hz. As the op amp
is powered from a single supply rail, two 10kO resistors and a 10µF
decoupling capacitor are used to bias the input (pin 5) to one-half
supply rail voltage.
The output of IC1a is fed into the second half of the op amp (IC1b),
also configured as a Sallen-Key filter. However, this time a low-pass
function is performed, with a cut-off frequency of about 3.1kHz. The
filter component values were chosen for Butterworth response
characteristics, providing maximum pass-band flatness. Overall voltage
gain in the pass-band is unity (0dB), with maximum input signal level
before clipping being approximately 3.5V RMS. The 560O resistor at
IC1b's output provides short-circuit protection.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgmg6RnfYMow-bzx4dNHJYcucdEwf1bWvk2iS-1JgP6BCJ4s26dgsEfRinw6rHyybX3YzoFotV_1TwYePHoKKyDlRF8JYLpn5MV02Bd4qHH8XsRrqSW1GUolHyUc_MIh_w5RGcEUkQrrFM/s400/http---signatures.mylivesignature.com-54493-234-C69AC511AFC3EFFA644CF2190333122B.png)
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