Passive High- and Low-pass Filters





Introduction: Passive High- and Low-pass Filters

This is a short guide to passive high- and low-pass filters, explaining what they do and what to use them for.
The passive filters are easy to construct and require only simple components.

Step 1: Circuit Construction

Both filters consist of a capacitor and a resistor - the difference is the placement of the parts. The 1K resistor and 1µF capacitor are filter components, the 100K resistor represents a device receiving the output of the filter.

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

The curves show the voltage of a signal coming out of the filters, being 1V on the input of the filters. The blue line is a high-pass and the red is low-pass.
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 high-pass filter is often used in amplifiers even though you don't want to reduce the bass. The reason for this is to remove a DC offset in the input signal.
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.

If this was useful to you, please leave a comment! :)



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    Thanks! Fantastic :) We used these in signal condition but I never really understood how they worked (only in principle).

    so do i have to have i high pass filter b4 the pam8403?


    Very good tutorial !

    I want to use your design on a project i'm working on but need to confirm some info, looking at your sketch above you have a high pass and low pass. if I select the correct high pass and low pass components to provide me with a signal from the input from 20Hz to 20Khz. Can I combine the signals after filtering into one signal again and send it into a single amplifier and then on to speakers ? will the high pass filter already fix the voltage offset or is an addition offset required ?


    I have a question im making a shed stereo and have a car headunit a component speaker and a sub.. i was wanting to separate the low frequencies to only go to the sub?

    I have question.I made an ir receiver & I want to make a filter for it.but my filter doesnt work!!!!!!

    do I need anything like peak detector??

    I made both of them but it didn't work. Can I get some supports?

    What do you want to know?

    came here for something else, but really enjoyed your post. Thanks! =]