In this article a simple third order passive low pass filter project is shown. The filter is supposed to be placed before an audio amplifier driving a subwoofer speaker. The circuit, which works with line level audio signals, has a 200Hz cutoff frequency. Since no active components are present, it can be easily assembled, but the cut frequency depends on the source output impedance and on the amplifier input impedance. In this article typical values for these parameters will be assumed. In case you want a simpler version with an adjustable cutoff frequency you can check Subwoofer active low pass filter, while Subwoofer passive low pass filter explains a more complex, yet precise, solution.
The output impedance RS of modern amplifiers, such as mp3 readers or computers, is very small, about a few Ohms, and therefore in this project it has been neglected. It might be useful, during the assembling, to substitute the R1 resistor with a potentiometer of the same value: if the source impedance is high, the value of R1 can be lowered.
On the other side, the typical input impedance of an audio amplifier is quite high. A common value is for instance 20KΩ, which can be found in Fenice 20, a T class hi-fi amplifier with a 15W+15W output power, based on the Tripath TA2024C chip. This value was used in this project.