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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    15 Discussions

    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?

    1 reply

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

    K so what i have is 2 nice laptop speakers powered by an amp from powered desktop speakers in a box that runs off batteries. I'd like to have the low pass filter after the amp. I was going to tap into the left and right channels to power the larger speaker but only want the bass sounds to come through. I also have another question. To reduce the distortion in the smaller speakers should I put high pass filters on them?

    1 reply

    If you tap both speakers, then left and right are connected! :O
    All these filters are better suited for putting before an amp, not in the output.
    But if you just connect the sub in series with one of the speakers, then put a capacitor in parallel with the sub, it will mean that as frequency rises, less power is used on the sub and more on the other speaker.

    ok so ive made a simple 2 speaker "boombox" using harman/kardon laptop speakers from a toshiba satellite p300, i want to add another 3" speaker to it but what i want that 3rd speaker to do is act as a subwoofer. and from what ive read online 60 hz is about optimal for bass sounds, ive looked all over and dont really understand the calculations, or rather, dont want to cuz its late. what would the resistor and capacitor values be for making a low pass rc filter of 60 hz?

    1 reply

    I need more information. :)
    Are you going to have a separate amp for the sub, or are you going to connect the sub in series with the other speakers?