This circuit shows both sections of an LM833 that is connected as a filter to reject undesired ultrasonic signals. The filter characteristic is 4th-order Bessel with the -3-dB frequency at about 40 kHz. Resistors and capacitors should be 1%-tolerance components. This circuit can be cascaded with that of Fig. 7-43 (with the low-pass preceding the high-pass) t..