Audio Line Solution Guide Using Noise Filters

Solution Guide

Summary of the audio line solution guide using noise filters

If no countermeasures are taken, electromagnetic noise will be radiated from the lines such as the speakers and earphones of smartphones. This noise interferes with the built-in antenna and reduces the reception sensitivity, so chip beads are generally inserted to suppress the noise. However, although chip beads can effectively suppress noise, they may cause problems such as sound distortion in audio lines. Therefore, TDK developed the MAF series of audio line noise filters as a solution through a new product concept. Due to the excellent noise attenuation effect in the cellular frequency band, it can greatly improve the reception sensitivity and solve the problem of sound quality degradation caused by the previous insertion of suppression components. In addition, it is also very effective for countermeasures against harmonics of Class D speakers used in smartphones, etc.

Background of the development of the MAF series of audio line noise filters

It is difficult to suppress noise while keeping sound distortion to a low level using chip beads made of conventional ferrite materials. A solution is required to solve both the problem of deterioration of reception sensitivity and the problem of sound distortion in audio lines.

Noise filter commercialized using unique low-distortion ferrite material

Newly developed ferrite material that achieves low distortion while maintaining noise-cancelling properties.

1. Background of the development of the MAF series of audio line noise filters

Why is noise suppression in audio cables important?

Figure 1 shows a block diagram of the audio line of a smartphone. In smartphones, digital amplifiers such as class D amplifiers are used in the power amplifiers for the speakers. Class D amplifiers are also called switching amplifiers. They use PWM (pulse width modulation) technology using switching devices (MOSFETs, etc.) to convert audio input signals into pulse signals, restore them to analog signals, and then output them to the speakers. However, since pulse signals have many harmonic components, if no countermeasures are taken, the wiring connecting the class D amplifier and the speaker will become an antenna and radiate electromagnetic noise, which will interfere with the built-in antenna and degrade the reception sensitivity. This is the so-called "self-poisoning" problem. Since class D amplifiers are small in size and have excellent power efficiency, they are used in devices such as smartphones that are becoming more multifunctional and consume more power to extend battery life. The noise generated by the headphone cable can also cause the deterioration of the reception sensitivity.
In addition, the noise countermeasures used in audio lines in the past distort the waveform of the audio signal, causing problems such as sound distortion. As high-resolution smartphones and headphones that support Hi-Fi audio are gaining attention, there is a demand for solutions that can simultaneously solve the problems of deterioration of reception sensitivity and sound distortion in audio cables.

Figure 1: Block diagram of a smartphone audio cable and the "self-mediation" and sound distortion issues

Problems with noise suppression using chip beads

In order to suppress the noise radiated from the audio line, chip beads are generally inserted into the output stage of the class D amplifier. This is a chip component that forms a coil inside a ferrite component by means of a multilayer process. The impedance of a chip bead is expressed as the reactance component and the AC resistance component of the coil. In the low frequency range, the reactance component mainly works to emit noise, while in the high frequency range, the AC resistance component mainly works to absorb noise and convert it into heat energy. The characteristics of chip beads are closely related to ferrite. Chip beads with a large AC resistance component are used in power supply systems where large currents flow. Large currents also flow through the speaker lines of smartphones. However, chip beads with a large AC resistance component cause sound distortion to increase, and chip beads using conventional ferrite materials have difficulty in suppressing sound distortion while removing noise.

Noise filter commercialized using unique low-distortion ferrite material

Figure 2: Comparison of characteristics between the MAF series noise filter and chip beads

In order to solve this difficult problem in chip beads, TDK has developed a new ferrite material that can achieve low distortion while maintaining noise removal characteristics by using technologies such as material design that have been accumulated over a long period of time. In addition, based on a new product concept, a multilayer chip component that suppresses noise in audio lines such as smartphones has also been developed. This is the MAF series of audio line noise filters.
Figure 2 shows a comparison of the characteristics of the MAF series of noise filters and chip beads. The MAF series of noise filters is a product with a unique positioning that can achieve low distortion characteristics while maintaining excellent noise removal effects.

We provide G-type and F-type products optimized according to the application

TDK's audio line noise filter MAF series offers G-type and F-type products in 1608 size (L1.6×W0.8mm) and G-type products in 1005 size (L1.0×W0.5mm) (August 2016). G-type products are for major cellular frequency bands such as LTE (700MHz~2GHz), have high attenuation characteristics, and can significantly improve reception sensitivity by inserting them into speaker lines or headphone lines. F-type products are used in speaker lines and have excellent effects on removing harmonic noise by inserting them into the output stage of a Class D amplifier.

2. Guidelines for use with audio cables

1 Solution ①
Improve receiving sensitivity and THD+N characteristics

The following is a detailed description of the excellent characteristics of the MAF series of audio line noise filters and application examples. Figure 3 is a block diagram when the MAF1608F and MAF1608G are used in speaker lines and the MAF1608G is used in receiver (headphone) lines.

Figure 3: Example of using MAF1608G/1608F in a smartphone audio line

First, Figure 4 shows the effect of using the MAF1608G, which has high attenuation characteristics in the cellular band. ◆ indicates the case without a filter, and ● indicates the receiving sensitivity-frequency characteristics when the MAF1608G is inserted. This is a measurement example in the 900MHz band, and it can be found that it is improved by about 8dB compared to the case without a filter. From the insertion loss-frequency characteristics in Figure 5, it can be seen that this is because it is optimized to increase the insertion loss in this band (=higher impedance).

Figure 4: Effect of inserting MAF1608G into speaker line (improving receiving sensitivity) ①
Receiving sensitivity-frequency characteristics

Figure 5: Effect of inserting MAF1608G into speaker line (improving receiving sensitivity)②
Insertion loss-frequency characteristics

It can be seen that using MAF1608G in audio cables is an extremely effective solution to improve the problem of deterioration of reception sensitivity of smartphones, etc. Then, will there be sound distortion after insertion?
The degree of sound distortion in audio cables is generally expressed in the value of THD+N (Total Harmonic Distortion + Noise). This indicates the proportion of distortion components caused by harmonics and other noise components (total harmonic distortion + noise) in the original signal components (in [%]), and the smaller the value, the higher the sound quality.

Figure 6: THD+N characteristics of MAF1608G

Figure 6 shows a comparison chart of the output THD+N characteristics of chip beads (TDK MPZ1608D) and MAF1068G (measured at a frequency of 1kHz and a load of RL=8Ω+33μF). For chip beads, the THD+N value increases significantly from an output of about 200mW. On the other hand, the MAF1608G is almost the same as the case without a filter at an output of 1000mW. This means that even if it is inserted into a speaker line, the sound distortion that occurs with chip beads will not occur. At the same time, the rated current of MAF1608G is 1.6A, which is a relatively large value, making it very suitable for speaker lines that require large currents.

DC resistance (RDC) is also an important characteristic. This is because the higher the DC resistance, the more power is consumed, which causes the signal level to drop. The MAF1608G achieves a low resistance of 0.06Ω (typical value). Therefore, the volume drop is minimal when plugged in, and it helps extend the battery life.

2 Solution ②:
Use MAF1608F in speaker lines

The THD+N characteristics of MAF1608F are more outstanding. Figure 7 shows a comparison chart of the output THD+N characteristics of chip beads (TDK MPZ1608S and MPZ1608D) and MAF1608F (measured at a frequency of 1kHz and a load of RL=8Ω+33μF).
In the chip bead MPZ1608S, the THD+N value is almost stable at the level of 1[%], and MAF1608F maintains excellent characteristics until it approaches 1000mW.
The S in MPZ1608S represents the type of ferrite material used, and the S material is a standard type with similar frequency-impedance characteristics to ordinary ferrite.
As can be seen from the graph, the newly developed low-distortion ferrite material used in the MAF series helps to achieve excellent THD+N characteristics.
The effect of suppressing the radiation noise of the Class D amplifier using MAF1608F is shown in Figure 8. The MAF1608F is designed to have a high impedance value in the frequency band of 100 to 400 MHz, so it can exert an excellent noise suppression effect in this frequency band.