This occurs because any Class D (digital) amplifier has a power supply with high-frequency switching, which can interfere with long-range FM signals. To avoid interference, prioritize local radio stations because their signals are stronger.
Another alternative is to mount the digital amplifier independently from the initial vehicle wiring, with the powering cables (positive and negative) taken straight from the battery and a well-shielded RCA (as short as possible) to prevent both resulting in a kind of antenna.
Noise that interferes with the FM signal is common on all class D amplifiers, regardless of manufacturer. Depending on the model, high-frequency switching from the amplifier's power supply leads to the development of more reliable amplifiers with lower consumption and higher power.
Here are a few tips to help you reduce this kind of interruption even further:
– Remove all contact between the amplifier and the car chassis;
– Lock the power cables and pull them straight from the battery to the low power amplifier (this benefits the FM signal);
– Determine the best location to attach the amp inside the car, and if it's RCA, use the shortest cable possible with high-quality shielding.
– Make good use of the high-performance CD player and antennas. This removes the possibility of FM signal interruption.
– Braiding the output wires to the speakers with the aim of canceling out high frequencies will also help.