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The TDA8571J has reached the end of its life and its manufacturing is discontinued by NXP Semi. The two largest electronic parts distributors in North America have none.
Its datasheet says its typical input resistance was 30k ohms (the resistor was shown) so the 470nF input capacitor was calculated to produce -3dB (half power) at 11.4Hz.
Don't you know the important simple calculation for a single pole RC filter?
f= 1 divided by (2 x pi x R x C). f is the -3dB frequency. R is in ohms and C is in Farads.
1 divided by (2 x pi) is almost 0.16 times so I use the simpler formula 0.16 divided by (R x C). Then 0.16/(30k x 470nF)= 11.35Hz. If the capacitor is 220nF then the frequency is 24.24Hz.
An online calculator can also be used but some of them might be wrong.
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