Passive Low Pass Filter for Audio Circuits (Free-Form RC Filter)




Introduction: Passive Low Pass Filter for Audio Circuits (Free-Form RC Filter)

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One thing that has always given me trouble when making custom electronic instruments is persistent noise interference on my audio signals. I have tried shielding and different tricks for wiring signals but the simplest solution post-build seems to be a passive device for filtering out the unwanted high frequencies.

This instructable will be a quick crash course for building one of these filters with a few components and a soldering iron.


-1 resistor (I am using a 1k but you can use whichever works for you. see next step for more information)

-1 capacitor (I am using a 1uf but again this can vary depending on how much filtering you are looking for)

-2 audio jacks (can be whatever kind you have, I am using 3.5mm jacks)

Step 1: Understanding the Circuit and Calculating Your Own

An RC filter is just a filter made out of a resistor (R) and a capacitor (C). It doesn't require power making it a passive component. The filter works by using the resistor to slow the charging of the capacitor. The output signal is not able to keep up with the sudden changes being made by the input signal, resulting in higher frequencies not passing through.

In order to calculate the frequency that is filtered out, we can use the following equation:

F=1/ 2π*R*C

where F is the cutoff frequency, R is the resistance value in ohms and C is the capacitance of the capacitor in farads.

So when I am using a 1uf capacitor and a 1k resistor, my formula plugs into:

1/ 2π * 1,000 * 0.000001 = 1/ 0.00628 = 159.236 ~ 160Hz

Meaning this combination filters out at around 160Hz.

For an even more deep dive into RC filters, I highly recommend this video by Afrotechmods

Step 2: Attach Resistor Across Signal Pins of Jacks

Solder the legs of the resistors across the signal pin (or pins) of the 2 audio jacks. clip off excess wire.

Step 3: Connect Capacitor to One of the Jack's Signal Pin

Connect the positive side of the capacitor to one of the jacks signal pin.

NOTE: The jack you are connecting the positive side of the capacitor to will be the output.

Step 4: Connect Negative Side of Capacitor Across Ground Pins of Both Audio Jacks

Using the negative leg of thee capacitor, bridge the 2 reaming 2 legs of the audio jack.

Trim excess wire.

Step 5: Clean Up and Mark Input and Output

Clean up any extra wires poking out and mark which jack is your input and which is your output.

NOTE: if you're not sure which is input or output, remember that the jack with the positive side of the capacitor connected to its signal pins is the output.

Step 6: Test and Adjust

Plug the completed filter in line with your audio device.

audio device --> audio cable --> RC Filter --> audio cable --> speakers or recording device

Turn on your speaker and see how the filter works! If you are still getting noise, you can play around with different values on your components. If your audio device is sounding muddy, try putting lower values for your components to get back some of the higher frequencies being filtered out.

The files attached here are an example of one of my own successful noise filters. The file "no filter.wav" is before I added the RC filter. Notice the high frequency blips and squeaks. The file "with filter.wav" is a recording made in the same environment by the same device but with the filter in line with the audio signal.

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    1 year ago

    Finally!!! Someone that knows how to make a simple design work!
    Thank you for you project.
    P.S to the other comment. Inductor is not needed. Yes you can add one if you like.


    2 years ago

    where are the inductors?