an effective scratch and rumble filter

an effective scratch and rumble filter Many home-constructed audio amplifiers incorpor- ate scratch and rumble filters, but quite often the roll-off of these filters is so weak their effect is negligible or they affect too large a range of the audio spectrum. The TDA 1028 is eminently suitable for the con- struction of a stereo scratch and rumble filter with a roll-off of as much as 18 dB per octave. The IC contains four electronic change-over switches and the wiper of each switch is connected to the input of a non-inverting unity gain opamp. Each elec- tronic switch therefore selects one of two inputs to the (vamp and, as they are controlled in pairs, only one external switch is required to insert a filter into each stereo channel. The first section of the circuit diagram shows the rumble filter. When switch 51 is in position b, both channel inputs are connected, via capacitors C2 and C6, to the inputs of the first two opamps. With switch 51 in this position the circuit will oper...