How to filter DC noise

Written by alexander rudinski
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How to filter DC noise
DC noise is caused by poorly grounded circuits and can be attenuated with a high-pass filter. (electricity image by Mat Hayward from

You may have noticed it in your own car, or in someone else's, a whining noise coming out of the radio that seems to increase as the car accelerates. This is a classic example of DC noise, most likely resulting from a circuit that contains a ground loop, which occurs when an audio system is grounded in two different places. While fixing the ground loop would be the most direct method of fixing the problem, a high-pass (or low-cut) filter can also be introduced as a less invasive measure for eliminating the noise.

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  1. 1

    Make sure the circuit is sufficiently grounded. Grounding issues can cause DC noise, so you'll want to recheck your components to make sure everything is as it should be before proceeding.

  2. 2

    Separate the signal-carrying wires from the power-carrying wires. If these wires are too close together, it can cause interference which will result in the trademark whine of DC noise.

  3. 3

    Install a capacitor in the circuit. The capacitor must be installed in series with the other components and should be installed immediately after the power source. The capacitor must be rated correctly for your system and needs to block signal below 50Hz, which is called the "cut-off" frequency. To determine this, use the following formula: 2 x 3.14 x Resistance x Capacitance = 50Hz. Solve the equation for capacitance, which will likely be in the area of 100 to 500 microfarads.

  4. 4

    Alternatively, you can purchase a device called noise suppressor that is essentially a very large, very powerful capacitor used to suppress DC noise. These suppressors are available at most electronics stores and are installed in the circuit immediately following the battery. Make sure the model you purchase is rated to handle at least 30 amps.

Tips and warnings

  • If the filter does not work or is insufficient, reground the circuit so that all the components are connected to the same ground at the same location. This will eliminate the ground loop that is likely causing the DC noise.
  • Whenever working with electronics, make sure that the power source is disconnected from the circuit that you are working on.

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