Class-D amplifiers, which are used in devices such as smartphones and audio equipment, have the advantages of being compact and highly efficient, but crucially require countermeasures against noise generated by their high-speed switching. TDK has an extensive lineup of products which can provide effective noise countermeasures while ensuring high sound quality and without affecting signals. This article describes the applications and effectiveness of these products in speaker lines for various devices which use Class-D amplifiers.
Class-D amplifiers, whose features include compact size and high efficiency, are employed in audio amplifiers for speakers in devices which require extended periods of battery-driven operation as well as small sizes, such as smartphones and AI speakers.
A basic block diagram for an AI speaker is shown in Figure 1.
In audio devices which require high sound quality, the properties of noise suppression filters which are inserted in their speaker lines are extremely important. The speaker sound quality is also affected by the properties of inductors used in LPFs (low pass filters) at the output stages of Class-D amplifiers.
This article focuses on describing noise countermeasures in speaker lines for Class-D amplifiers which are used in devices such as smartphones, AI speakers, tablet PCs, and various types of audio equipment.
TDK offers a lineup of filters ideal for suppressing noise in speaker lines, which include the MAF series (multilayer type) and the VAF series (wire wound type) of noise suppression filters. As inductors for LPFs, the VLS-AF series, which uses high-performance ferrite materials, is recommended.
Chip varistors are often used as measures against ESD (electrostatic discharge) in speaker lines. Chip varistors have the characteristic of absorbing abnormal voltages such as ESD and surges, while also being effective at noise suppression since they normally function as capacitors. This provides the advantage of a single component which is able to function as a countermeasure against both ESD and noise.
Since these products are suitable for use in speaker lines for Class-D amplifiers, they can effectively ensure high sound quality, suppress radiation noise, and provide ESD countermeasures. Their basic application methods to achieve these functions are summarized below.
In speaker lines for smartphones and other devices whose speaker output is relatively small, Class-D amplifiers without LPFs are most commonly used, and insertion of MAF noise suppression filter can effectively suppress noise (Figure 2).
For devices with speaker output in the 2W to 20W class, such as AI speakers, tablet PCs, and audio equipment, it will be necessary to provide inductors for LPFs externally. The VLS-AF series of inductors for LPFs also has the ability to effectively suppress radiation noise (Figure 3).
The use of VAF noise suppression filters together with VLS-AF inductors for LPFs can enhance their effectiveness even further (Figure 4).
The effectiveness of these products in individual applications is specifically described below.
High-frequency noise caused by the switching of Class-D amplifiers becomes radiation noise which is emitted from speaker lines.
The simplest method of countering this is the insertion of chip beads into the speaker line. However, even though chip beads are effective at suppressing radiation noise, they also introduce the problem of distortion of the signal waveforms, which causes sound distortion. With chip beads, it is very difficult to both suppress noise and ensure high sound quality.
The MAF series and VAF series have been developed as noise suppression filter products for use in audio lines to address this issue by employing proprietary low-distortion ferrite.
Both of them can achieve low Rdc (DC resistance) and high rated current, making them ideal for speaker lines with large current requirements. Their insertion will not cause any effects such as distortion of sound, allowing them to demonstrate outstanding effectiveness at suppressing radiation noise.
The level of sound distortion occurring in audio lines is generally expressed in numerical form as THD+N (Total Harmonic Distortion + Noise), with lower values indicating better sound quality.
Figure 5 shows the THD+N characteristics versus output for chip beads and MAF noise suppression filters in speaker lines for Class-D amplifiers (LPF-less), and measurement examples of the frequency spectra of their output signals.
When chip beads are used, increasing the output causes the THD+N value to rise as well, but with the MAF the THD+N characteristics are equivalent to a case with no filter. This indicates that even if the output is increased, there will be no effects on the signals and no sound distortion will be generated. If this is checked using the frequency spectra for the output signal (1kHz), the level of harmonics is shown to be significantly high when chip beads are used, and this harmonic component is what is heard as distortion. In contrast, with the MAF the level of high frequencies is equivalent to the case with no filter, so only the true 1kHz signal can be heard.
Even though the insertion of harmonics is effective at suppressing radiation noise, it is also a cause of signal distortion leading to degradation of sound quality.
In an actual listening comparison test of high-quality sound sources, for cases using chip beads and cases using MAF series noise suppression filters, clear differences can be found in the sound image localization and sound field impression. Therefore, the application of the MAF series in speaker lines for smartphones and other devices is an advantageous solution to suppress noise while ensuring high sound quality.
The effectiveness of radiation noise suppression using an MAF is shown in Figure 6. The noise intensity for the case with no filter is suppressed by use of the MAF to a level equivalent to that occurring when the Class-D amplifier is off.
Since inductors used in LPFs are inserted into speaker lines, it is essential that they do not affect the signals in those lines. VLS6045AF products are inductors for LPFs with a wire wound magnetic shielded structure using ferrite, which provides benefits including low Rdc (DC resistance) and the ability to accommodate large currents.
Figure 7 shows the THD+N characteristics versus output for the VLS6045AF and metal inductors, and measurement examples of the frequency spectra of their output signals.
Metal inductors have cores made of metallic magnetic materials. Their features include the ability to accommodate large currents, and they are preferred for use as power inductors for power supply circuits. However, while the THD+N value of a metal inductor increases as the output becomes larger, this value changes only slightly with VLS6045AF, so there are almost no effects associated with their insertion.
Also, the frequency spectra show that with metal inductors the level of harmonics in their output signals (1kHz) is significantly high, but with the VLS6045AF the harmonic level is low, close to that of the case with no filter.
The effectiveness of the VLS6045AF at suppressing radiation noise is shown in Figure 8. Radiation noise is greatly suppressed over a wide frequency range, meeting the limit values specified by the CISPR Class B standard.
The VAF series consists of noise suppression filter products developed for use in audio lines with 2W to 20W output.
Figure 9 shows the THD+N characteristics versus output of chip beads and the VAF201610FA, and measurement examples of their frequency spectra.
When chip beads are used, increasing the output causes the THD+N value to rise as well, but with the VAF the characteristics are nearly equivalent to those of the case with no filter, indicating that no distortion will be generated by their insertion.
If the frequency spectra of the output signal (1kHz) are examined, with chip beads the level of harmonics is extremely high, but with the VAF it is nearly the same level as the case with no filter. These results clearly show that replacement of chip beads with a VAF to serve as a filter in a speaker line is exceptionally effective at reducing distortion and improving sound quality.
Figure 10 shows the impedance-frequency characteristics of the VAF201610FA series, and the effectiveness of noise suppression with the combined use of VLS-AF and VAF.
The graph of noise intensity vs. frequency characteristics shows examples of the measurement of radiation noise intensity in speaker lines for Class-D amplifiers, for cases using only a VLS-AF, and a VLS-AF and VAF combined. With the VLS-AF alone, the results are slightly insufficient to meet the CISPR Class B standard (red broken line), but it can be clearly seen that by combining the VLS-AF and VAF, the noise intensity in the 100MHz to 400MHz band is significantly reduced.
This indicates that the high impedance of the VAF in the 100MHz to 400MHz band makes it ideal as a countermeasure against noise for Class-D amplifiers.
Although it has been shown that combinations of a VLS-AF and VAF in speaker lines with 2W to 20W class output are effective at suppressing radiation noise, this effectiveness can be enhanced even further by also using chip varistors.
Chip varistors are frequently used in electronic devices as elements to protect circuits from phenomena such as ESD (electrostatic discharge) generated by human bodies. They are represented using equivalent circuits where bidirectional TVS diodes (Zener diodes) and capacitors are connected in parallel. Because they function as capacitors in cases other than when invading ESD, etc. is bypassed to ground, using them in combination with VLS-AF and VAF products in speaker lines can improve the effectiveness of noise suppression even further.
Figure 11 shows the THD+N characteristics versus output of a circuit with a VLS-AF (for LPFs) combined with a VAF and AVR chip varistors; and a comparison of the effectiveness of radiation noise suppression for the cases of no filter, and various combinations consisting of VLS-AF, VLS-AF+AVR, and VLS-AF+VAF+AVR.
It can be seen from the comparison graph of noise suppression effectiveness in Figure 11 that when either a VAF or AVR chip varistors alone are used together with a VLS-AF, there are almost no differences between them in the resulting THD+N characteristics, with no negative impacts on sound quality. However, if AVR chip varistors are used in combination with the VLS-AF and VAR together, the radiation noise is greatly reduced.
Since AVR chip varistors can improve the effectiveness of radiation noise suppression in this way while also functioning as an ESD countermeasure, the combined use of VLS-AF+VAF+AVR is an optimal solution which can offer various advantages.
A guide to the application of the VLS-AF series of inductors for LPFs, and the MAF and VAF series of noise suppression filters, regarding their use in speaker lines for Class-D amplifiers, is shown in Figure 12. These products should be selected and used according to speaker output and application.
A guide to the application of the AVR series of chip varistors, which are effective as countermeasures against ESD and noise in microphones, headphones, and speakers, is shown in Figure 13.