The passive filters are easy to construct and require only simple components.
Step 1: Circuit construction
The highpass filter will attenuate (reduce) lower frequencies, the reduction getting bigger as the frequency falls. Higher frequencies will not be attenuated as much.
The low-pass filter will do the opposite and reduce higher frequencies while letting bass pass through unmolested.
Step 2: Calculating cut-off frequency
At the point where the voltage has been reduced -3db (aprox. 0.7V out when input is 1V) is called the cut-off frequency. It is calculated using the formula 1 / (2 * pi * R * C). In the example circuit the result is about 159 Hz:
1 / (2 * pi * 1000 Ohm * 0.000001 Farad) = 159.15 Hz
Step 3: DC offset and how to remove it
The audio signal you want does not have DC offset, meaning that it is centered around 0V, the peaks go equally into positive and negative voltages. If a DC offset is fed into an amplifier, it will be amplified and can hurt your speakers.
Feeding the signal through the HP filter will remove the DC offset, as shown in the graph. The green line is a signal with a 1V offset, the blue is the output with the offset removed.
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