How to Reduce FM Radio Interference From Class D Amplifier?




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If you’re a car driver and music enthusiast, then you already know the importance of having a good sound system.

But what do you do when your FM radio starts to interfere with your class D amplifier?

To fix the class D amp interference with the radio, try the following:

#1. Install the amp as far as possible from the stereo.

#2. Ensure correct ground

#3. Install ferrite clips.

#3. Shield an amp with copper foil.

#4. Replace the amp with AB Class one.

Tip: Many car audio amplifiers made in class D have an issue with radio interference. While there are several ways to reduce the effect, the easiest is to replace it.

Try one of the following D-class car audio amps, which a few years ago helped to solve a problem with the FM interference in my car (links to Amazon): Power Acoustik RZ4-2000D or Soundstream PN4.1000D.

In this article, I will detail this issue and will show you how to reduce FM radio interference from class-D amps.

Why D Class Amps Interfere With Radio Frequencies?

Class-D amplifiers work differently than class AB amps, constantly switching the output transistors on and off.

Although the switching reduces the heat generated by the transistors and improves efficiency, the switching frequency not only overlaps with the AM or FM frequencies but also creates a lot of electromagnetic interference (EMI).

The EMI produced by class D designs can interfere with nearby electronic devices, including radios.

Class AB amplifiers do not produce as much EMI because the output transistors are not switching, and the overall amplification logic is different.

Class AB amplifiers use a continuous flow of current. This design produces less EMI and is less likely to interfere with radios.

Do All D Class Amplifiers Have This Issue?

Not all class D amplifiers produce the same amount of EMI.

Some class D amplifiers are designed with special filters that help to reduce the amount of EMI produced, but this, unfortunately, depends on the processor type, so that some amps may cause more issues than others.

When the amplifier does not have an output filter, the speaker cables start acting like an antenna for the modulation frequency (300KHz-600KHz).

The amount of interference depends on all three elements, the amplifier, stereo, and antenna type.

It is essential to have a good quality shielded cable going from the stereo to the antenna and from the stereo to the amp.

If you have a standard copper wire antenna, try upgrading to a better quality one, like the Votex Antenna (link to Amazon), or any other car specific but with the full copper internal wiring or spring if it is a flexible antenna.

Sometimes people add antenna amplifiers or other signal boosters, but although this help with increasing an FM reception, it will not solve the interference issues caused by class D power amp.

How to Stop the Amp Interference With Radio Frequencies?

Although interference with the audio signal can be annoying, it is not the end of the world. The good news is you can do a few things to reduce the amount of interference your class D amplifier produces.

#1. Ensure Good Ground Connection for All Car Audio Elements.

Make sure that your class D amplifier is properly grounded. Good ground will help reduce the amount of interference your amplifier produces, which is related to more than just grounding the amplifier.

 All other elements like radio or sound processors need to have good ground as well.

The best way to achieve this is to connect all the elements to the car chassis. The car chassis provides a good ground connection and will help to reduce interference, but connect the amplifier to a different place than the radio.

If you cannot find a good grounding point within the car chassis, connect both positive and negative wires from the amplifier directly to the battery.

#2. Do Not use a Class D Amplifier Close to the Radio.

Try moving your class D amplifier as far as possible from the head unit and the antenna and away from other electronic devices in your car.

If there is room, try to rotate the amplifier or put it in a separate compartment or trunk because such separation will help reduce interference, especially with FM frequencies.

As you may know, the FM tuner creates signals with a length between 9 and 11 feet (2.78 and 3.41 meters), therefore class D amplifiers should be placed as far away from the radio antenna as possible, ideally in the trunk.

Most cases of substantial interference happen with the amps installed below the passenger seat, so if you are facing such an issue, move the amp to the back of the vehicle behind the rear seats.

If you have no choice but to keep the amplifier close to the car stereo, try to put it on the opposite side of the car.

Even slight movement may cause to strengthen the FM bands, and it should minimize interference, so it is worth trying different positions. However, there is no guarantee about removing the issue entirely.

#3. Install EMI Filters.

Consider using an EMI filter to reduce the amount of interference that your class D amplifier produces.

There are several different filters available, and ideally, if the amplifier you want to buy has one already installed.

Car Power Filter EMI Noise Ground Loop Isolator Suppressor for Auto Stereo Audio Radio Stereos Amplifier Amp Speaker Equalizer Subwoofer 12V

If not, you can have one installed by a professional, or if you are handy with a soldering iron, you can install one yourself.

The most common type of filter is the low-pass filter, which helps to remove high-frequency interference, and this is the one you will need to use.

The heart of a Class-D amplifier filter is an L-C low-pass filter. The corner frequency of the filter is chosen so that the filter will have minimal effect on the desired output frequency range while attenuating the switching noise as much as possible.

You can find low-pass filters online or at your local electronics store. Be sure to get one rated for class D amplifiers, as not all low-pass filters are created equal, and ask an electronic specialist for support with the installation.

However, before deciding on adding a separate filter, consider replacing the amplifier with a higher model initially equipped with all the needed filters.

#4. Cover Power and Signal Wires With Ferrite Clamps.

If filters installed in your amp do not stop the signal interference, add ferrite clamps to your class D amplifier’s power and ground wires.

Ferrite beads are often used on USB cables, so you may already know how they look.

Ferrite Ring Core Black RFI EMI Noise Suppressor Cable Clip-20Pcs

Ferrite clamps help to reduce the amount of electromagnetic interference your amplifier produces by absorbing the interference and dissipating it as heat.

They are available at most electronics stores or online and are relatively easy to install. Slide the ferrite clamp over the power or ground wire and close it until you hear the “click” that indicates it is locked in place.

#5. Build a Faraday Cage Around the Amplifier With a Copper Tape

Building a faraday cage around the amplifier is not difficult. The only thing you have to do is to cover the amp with copper tape.

This will create a Faraday cage and, in effect, block all outcoming frequencies from reaching the antenna. Easy right? Yes and no.

Although wrapping the amplifier may bring the effect because all created waves will stay within the amplifier, the visual effect is not what I would be looking for.

It will work like a charm if you are willing to experiment, so if the visual look of the amplifier is not important to you, this method may be the easiest way to reduce interference.

#6. Try Different Class D Amplifier.

Try to use a different D class amp. Some models are better than others in terms of producing less interference because of the design and materials used.

Finding the amplifier fitting in your system may require some research. Still, if you are facing a radio signal loss or making a new system from scratch, I recommend trying one of the following D-class amps (links to Amazon):

Power Acoustik RZ1-2300D 2300W Class D Monoblock Amplifier,Black

Soundstream PN4.1000D 1000W 4-Channel Picasso Nano Series Class D Amplifier, Gray

Both amps are powered by MOSFET with Pulse Width Modulation (PWM). 

Pulse Width Modulation is generally used to help regulate the voltage in a switching power supply.

This is necessary when the current demand on the power supply or the charging system’s supply voltage varies, which is the case in most class D amps.

In short, PWM may help to reduce FM interference with the antenna, 

#7. Replace an Amplifier With AB Class.

If, after all trials, you are still looking for a suitable amplifier for your system, the best option may be to use a class AB amplifier.

Class AB amplifiers are not as efficient as class D amplifiers, but they do not produce as much electromagnetic interference, making them a perfect choice.

In addition, AB amplifiers are better for sound quality, which may be critical in multi-channel amplifiers.


Class D amplifiers have high efficiency and produce less heat, making them a popular choice for car audio systems. However, they can also create a large amount of electromagnetic interference, which can interfere with your FM radio reception.

Several ways to reduce the amount of interference produced by a class D amplifier include installing output filters, ferrite clamps, or a Faraday cage. 

You can also try using a different class D or if nothing else helps, swap it with a new amp made in AB class.

By following these steps, you can significantly reduce or even eliminate the amount of FM radio interference your class D amplifier produces.