The circuit for Scratch Noise and Rumble Noise filter has been described in the attached link. This filter is used in the series of any audio pre-amplifier stage to filter the scratch noise as well as rumble noise. By using this circuit Bass and treble cut are produced by a 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 12 dB per octave is achieved.
The frequency limit of the Rumble filter is fixed at 40 Hz and the Noise filter can be switched to limits of 16, 12 and 8 KHz. The resulting frequency characteristics are shown in the below figure -1. The approximate voltage gain shall be about 0.95 and the total distortion at 1KHz and the output voltage of 2Volts is 0.3% that also falling to less than 0.1% at 1Volt. The approximate Input and Output impedance are 1.7 M ohms and 460 Ohms respectively.
Frequency Characteristics response curve of the Scratch and Rumble Filter. The measurement starts at 18Hz for four curves. The curve-2 shall be 7.9-KHz, curve 3 shall be 8.2 KHz and curve-4 shall be 20 KHz.