The disadvantage of Class D amplifiers is that they are noisy, so how to reduce them?

Due to its small size and high energy efficiency, Class D amplifiers have become fixed devices in smartphones and battery-driven artificial intelligence (AI) speakers, where space and power budgets are very strict. They have replaced Class A and Class AB amplifiers, both of which have excellent linearity, high gain and low signal distortion levels, but are very power-hungry.

Due to its small size and high energy efficiency, Class D amplifiers have become fixed devices in smartphones and battery-driven artificial intelligence (AI) speakers, where space and power budgets are very strict. They have replaced Class A and Class AB amplifiers, both of which have excellent linearity, high gain and low signal distortion levels, but are very power-hungry.

Class D amplifiers are one of the key technical elements that make consumer demand exponentially grow. These non-linear switching amplifiers can theoretically reach 100% efficiency. Unlike Class A and Class AB, current only flows through the conducting transistor. However, the significant advantage of Class D amplifiers also has a disadvantage, that is, their high-speed switching can generate potential noise (Figure 1).

The disadvantage of Class D amplifiers is that they are noisy, so how to reduce them?
Figure 1 High-speed switching can generate noise in audio equipment such as smart speakers. Source: TDK

Therefore, design engineers must take effective noise countermeasures. However, at the same time, these measures can neither increase the layout area, nor affect the signal and audio quality, which means that the performance of the noise suppression filter inserted into the speaker line is very important. The sound quality of the loudspeaker is also affected by the performance of the inductor used in the output stage low-pass filter (LPF) of the Class D amplifier.

In addition, usually anti-static measures (ESD) must be taken in the speaker circuit. Here, the ESD notch filter with multilayer varistor technology can protect both ESD-related transient overvoltages and wireless communication-related filter noise sources.

ESD notch filters also play a key role in suppressing radiated noise. Multilayer varistors have specific parasitic capacitances and are designed to work with filtered noise sources related to cellular, Bluetooth, or Wi-Fi band interference (Figure 2).

The disadvantage of Class D amplifiers is that they are noisy, so how to reduce them?
Figure 2 The cooperative work of MAF and AVRF series filters can reduce insertion loss. Source: TDK

The basic application methods to realize these functions are summarized below.

100 mW to 2 W class audio

In the speaker lines of smartphones and other devices with relatively small speaker outputs (100 mW to 2W), Class D amplifiers without LPF are usually used. The degree of sound distortion is usually expressed in digital form as total harmonic distortion plus noise (THD + N); the lower the value, the better the sound quality.

If a universal chip bead is used in the speaker circuit, the output will cause the THD + N value to increase, thereby reducing the sound quality. However, for a noise suppression filter, the THD + N characteristic is equivalent to the characteristic without the filter. Even if the output power is increased, it will not affect the signal and will not produce sound distortion. Using the frequency spectrum of the output signal (1 kHz), when using chip beads, the harmonic level is significantly higher; this harmonic component is called distortion. Conversely, when using a noise suppression filter, high frequencies are not affected, so only a clean 1 kHz signal can be heard.

Compared with not using the filter, the effect of the noise suppression filter in radiated noise suppression is equivalent to the effect when the class D amplifier is turned off. For example, TDK’s MAF series filters are multilayer chip components that use a new type of ferrite material to achieve low distortion while maintaining its noise cancellation characteristics.

2 W to 20 W class speaker wire

For devices with a speaker output of 2 W to 20 W, such as AI speakers, tablet computers and other audio devices, it is necessary to provide an external inductor for the LPF to accommodate large currents. Consider that these inductors have been inserted into the speaker lines, so they must not affect the signal in these lines.

Metal inductors are made of metallic magnetic materials. They can accommodate large currents; however, the THD + N value increases as the output power increases. Inductors with a wire-wound shielded magnetic structure using ferrite can provide many advantages, including low DC resistance, Rdc and the ability to adapt to large currents. Therefore, when the speakers are plugged in, the THD + N value changes only slightly.

As with smartphone speakers with lower output power, adding a noise suppression filter to the speaker circuit with a power of 2 W to 20 W will avoid any possible degradation of the audio output caused by radiated noise.

Similarly, the THD + N characteristics of the wire-wound noise suppression filter are equivalent to those without the filter, and the harmonic levels are almost the same. These results clearly show that replacing the magnetic bead speaker wire with a wire-wound noise suppression filter is very effective in reducing distortion and improving sound quality (Figure 3).

The disadvantage of Class D amplifiers is that they are noisy, so how to reduce them?
Figure 3 Noise suppression filter can effectively reduce distortion and improve the sound quality in smart speaker design. Source: TDK

The comparison of the noise intensity of the loudspeaker circuit with the frequency characteristics shows how to reduce the noise intensity in the 100 MHz to 400 MHz frequency band. It also shows that the high impedance of the wire-wound noise suppression filter in this frequency band makes it suitable as a noise countermeasure for class D amplifiers, can easily meet the CISPR Class B standard and have a certain headroom (red dotted line).

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