Feedback type tone control circuit

In the figure, for medium and low audio signals, C3 can be regarded as an open circuit due to its small capacity. The resistance value
is also large and can also be regarded as an open circuit, so the figure can be simplified as Figure 1-74a. When the sliding arm of the potentiometer RPz moves to point A,
cl is short-circuited , thus forming the bass boost circuit shown in Figure b.