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Low-pass filter
AudioPhool's Circuits 101 on youtube explains that a low-pass filter makes use of the fact that the impedance of a capacitor varies with frequency. In fact, the 3dB cut-off frequency of a LPF is 1 / (2 * pi * R * C). So, with a variable resistor, you can make a variable low-pass filter. The wikipedia article is useful for more detail.
In the video Simple RC filter, AudioPhool demonstrates building a LPF with a .1uF capacitor ( 0.1 x 10^-6 F ), a 1.5k resistor (later a pot.), gives us 1000 Hz LPF . Talks briefly through output and input impedence, and shows the use of an op amp to buffer the filter circuit. He also mentions, briefly, that a high-pass filter can be built by reversing the two components.
