The circuit of a noise and rumble filter is shown in Fig. 18.9. Bass and treble cut are produced by an RC network connected between two emitter-followers, and a feedback loop from the output to the input through a second RC network. In this way, a high slope of about 13 dB per octave is achieved.
The frequency limit of the rumble filter is fixed at 45 Hz, and the noise filter can be switched to limits of 16, 12 and 7 kHz. The voltage gain is 0.95, and the total distortion at 1 kHz and an output voltage of 2 V is 0.35%, falling to less than 0.1% at 1 V. The input and output impedances are 1.7 MΩ and 450 Ω respectively.
Fig. 18.8 Frequency characteristics of low-pass/high-pass filter
Fig. 18.9 Noise and ...