Schematic Diagram

The switchable rumble filter circuit shown provides a cut-off at 25, 40, or 80Hz. C1 and C2 in con- junction with R3 - 9, form second order Butterworth filters with l2db/ octave roll—off below the turnover frequency.
Unlike most designs, the feedback is taken from the inverting input. ln practice this works well once the signal at this point follows exactly that at the non-inverting input. _ A useful feature is the deep bass boost provided by the feedback loop proper. S2 in position 3 gives a +3db point at l50Hz whilst position 2 provides a +3db point at 150Hz. A supply 6·35V DC at 10mA is required.