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Digital amplifiers are increasingly popular for home audio applications. In many cases, power is supplied from an AC outlet for home audio, and the conductive noise in the power supply cable is regulated under EMC standards. Therefore, it is necessary to prevent noise from leaking into the power supply line during audio playback. Here is an overview of noise problems in home audio cables and their solutions.
The quality of the power supply is important to output superior audio from an audio device. Figure 1 (below) shows the THD + N results when another device is connected to the power supply outlet of a home audio device. While the THD + N deteriorates across the entire audible range when suppression is not implemented in the power supply line, applying suppression improves the THD + N result and the audio quality (Figure 2, below).
Murata’s PLT10HH Large Current Common Mode Choke Coils and BNX block-type EMIFIL® filters are effective for providing suppression from low to high frequencies in power supply lines. Both series are ideally suited for use in various sets to suppress power supply noise.
Example of noise interference from power supply circuits and countermeasures
Audio quality measurement results with power supply circuit suppression
An AC outlet typically provides power to home audio applications. The conductive noise in the power supply cable is regulated under EMC standards, making it necessary to prevent noise from leaking into the power supply line during audio playback.
Methods of cutting the noise are generally introduced into the AC adapter or the AC-DC power supply section. However, it is easier to avoid the noise issue by applying suppression in the power supply section of the digital amplifier that is the source of the noise (Figure 3, below).
Figure 4 (below) illustrates the results of measuring the noise leaking from the power supply line. It shows the higher harmonics of the digital amplifier switching frequencies are conducted as noise. Equipping Murata PLT10HH choke coils and BNX filters achieved an improvement of more than 20dB.
Using both series enables designers to suppress noise interference from the power supply circuit to the audio equipment as well as suppress noise leaking from the audio equipment to the power supply.
Example of countermeasures to deal with noise leakage from a Class D amplifier to the power supply circuit
Power supply noise measurement results before and after applying suppression products
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