The cut-off frequency of the filter shown in the figure can be adjusted to values between approximately 239 Hz and 2.8 Hz using a double potentiometer. Care must be taken that when turning the potentiometer, the two sections have the resistance increased or decreased at the same time. The value of this component can also be modified, according to the desired range. The suggested operational amplifier is the 741 with a symmetrical power supply from 6 + 6 V to 12+ 12 V, but equivalent operational amplifiers can also be used. The frequency range can also be modified by changing capacitor values. Capacitors, however, must be the same. We observed that the maximum operating frequency of this circuit does not exceed a few hundred kilohertz. For higher frequencies, appropriate operational amplifiers should be used. It is also important to note that, when operating with audio signals, shielded cables should be used so that humming does not occur, as the circuit is of high input impedance.
