Schematic Diagram of Infrasonic and Ultrasonic Filter for Hi-Fi Audio


For high fidelity audio system, harmonic distortion level should be kept as low as possible. The response of many high fidelity audio systems is designed beyond the audio range, including the capability to handle infrasonic and ultrasonic signals. This is to make sure that the amplifier handle the signal perfectly. Even though this capability is useful in making sure the audio signal (20Hz-20kHz) signal is properly processed, sometimes the existence of unwanted high frequency ultrasonic and very low frequency infrasonic signal should be avoided.These unwanted signals consume the electric power without being noticed by the user. It could burn the tweeter of the speaker, or just distort the audible signal. Rejection filter circuit for ultrasonic and infrasonic signal are used to overcome this problem. Here is the circuit diagram :

Infrasonic and Ultrasonic Filter for High Fidelity (Hi Fi) Audio System

Infrasonic and Ultrasonic Filter for High Fidelity (Hi-Fi) Audio System

Infrasonic rejection filter shown in the first circuit. The filter circuit is a low-pass filter with third order Butterworth response. A high pass filter used as the ultrasonic rejection filter, and the response is designed to match the fourth order Bessel filter. The two filter circuits should be cascaded and connected in series to give the full frequency response, as shown in the last figure. This is to reject the unwanted signals. To attain the accurate response, 1% tolerance for all resistor is suggested. The full frequency response have to be a wide audio band pass response. [Source : National Semiconductor Application Notes]


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