Audio line noise suppression for home audio:(1) Audio line

Audio line noise suppression for smart phones(1) Problems Audio line noise suppression for smart phones (2) Countermeasures Audio line noise suppression for home audio (1) Audio line Audio line noise suppression for home audio (2) Power supply line

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Noise problems in home audio  
Audio line problems in home audio  
Audio distortion characteristics and
noise suppression performance in audio line noise filters
               

Noise suppression examples
 
Audio line filter item list
 

Noise problems in home audio


In the past, analog amplifiers were used in high output audio equipment to emphasize audio quality, but in recent years, digital amplifiers have improved in terms of audio quality and have become popular in home audio as well.

Due to the switching noise that occurs in digital amplifiers, noise that exceeds the EMC standards can be emitted into the space around the cable connected to the speaker. Noise suppression using capacitors and inductors is implemented to solve this noise problem.

* EMC standards: Noise regulation standards prescribed by various countries (examples include VCCI in Japan, the FCC in the United States, and the EN standards in Europe)

Murata is commercializing the NFZ series of audio line filters as noise filters that can suppress unwanted noise while maintaining audio quality. This section introduces noise problems in home audio and their solutions.


Audio line problems in home audio


Inductors are generally introduced as EMI suppression components to suppress noise in electronic equipment. 
To ensure high audio quality in home audio, low distortion noise suppression products are required for audio lines.
In some cases, noise suppression is also needed for the power supply line to satisfy EMC standards.
Power supply noise suppression  >


Suppression component: [1-2A class]
    NFZ_SM/SG series
NFZ_SD series(Under development as of November 2017)
     

  Suppression component: [5A class]
    NFZ2MSD series(Under development as of November 2017)、
NFZ32SW series
     

Figure 1 Home audio_Noise suppression product use cases by output class


<Related knowledge>
  What is audio distortion? >
Noise suppression comparison with conventional ferrite beads >


Audio distortion characteristics and noise suppression performance in audio line noise filters


<Audio distortion>*Basics of audio distortion
The audio distortion characteristics of common ferrite beads and the NFZ_SD audio line filter are shown in Figure 2.
While common ferrite beads exhibit high audio distortion, this does not occur in the NFZ_SD Series, which has impedance characteristics that do not change even when a large current is flowing. This makes it possible to achieve noise suppression while maintaining audio quality without a filter.

 

Figure 2 Audio characteristics (THD + N) of common ferrite beads and audio line filters


<Benefits of noise suppression>

In the case of home audio, the speakers output very high volumes compared to smart phones and other mobile devices, so a large current (signal) flows through the noise suppression components. The characteristics of noise suppression products that use ferrite materials change when a current is flowing, so caution is necessary when making a selection.

As an example, Figure 3 shows the changes in impedance characteristics of a noise suppression product when a current is flowing. The left side shows the characteristics of ferrite beads with a rated current of 5 A, while the right side shows the Murata NFZ_SD Series. The impedance characteristics of the NFZ_SD Series do not change even when a large current is flowing, so they are able to maintain noise suppression performance. Because the volume output from a speaker is not constant, it is necessary to select a filter with impedance characteristics that do not fluctuate for noise suppression.

In addition, because the noise occurring in a digital amplifier has a spectrum of up to several 100 MHz, suppression products with a high impedance in that band are effective. 

Figure 3 Frequency characteristics of common ferrite beads and audio line filters during DC superposition


As shown above, the NFZ_SD Series of products provide effective noise suppression while maintaining audio quality.



Noise suppression examples

<Suppression Example 1: Impact of the speaker cable length>

In recent years, various types of speakers ranging from conventional large speakers to portable types are being sold.

The length of the speaker cable changes according to the size of the audio equipment, and the noise level also changes accordingly.

As an example, Figure 4(a) shows the radiation noise level measurement results when the length of the speaker cable is changed. As the length of the cable increases, the noise level radiating from the cable also increases, so noise suppression is essential to clear noise regulations.

Figure 4(b) shows the results after applying noise suppression. Noise regulations can be cleared by using the NFZ_SD audio line filter in the speaker line. 

Figure 4 Block diagram of smart phone audio circuits


<Suppression example 2: Comparison with common ferrite beads>

Figure 5 shows the results of measuring radiation noise when noise suppression is implemented in the Class D amplifier of a home audio device.

The Murata NFZ_SD audio line filter and common ferrite beads were used as suppression components.

While the NFZ_SD audio line filter exhibits significant noise suppression effects even when a large current is flowing, the noise suppression of common ferrite beads is insufficient when the current value is large (2.4 A), which results in noise radiation that exceeds the standard value.

This is due to the superposition characteristics of the component. The NFZ_SD Series is able to maintain effective noise suppression even when a large current is flowing, so it can be said to be effective for radiation noise suppression in the audio lines of home audio equipment.

Figure 5 Radiation noise measurement results for a Class D amplifier



Murata Icon X Audio line filter item list

As explained above, it is important to avoid degrading audio quality while satisfying the target characteristics for noise level and reception sensitivity in audio line noise and isolation countermeasures.

  

In order to satisfy both requirements, Murata offers the NFZ and LQW series of audio line noise filters.

Adopting these audio line countermeasure components enables manufacturers to design smart phones equipped with miniature, high-quality audio circuits.


NFZ series

For high output audio (1 - 5 A class) *Under development as of November 2017

Product name
Impedance
at100MHz(Ω)
DC resistance
(Ω)
Rated current
(A)
Product size
(max. height)
(mm)
Max
NFZ2MSD100SN10
10±25%
18 5.2
2.0x1.6
(1.2)
NFZ2MSD150SN10
15±25%
23 4.4
NFZ2MSD200SN10
20±25%
28 3.9
NFZ2MSD300SN10
30±25%
33 3.7
NFZ2MSD600SN10
60±25%
54
2.7
NFZ2MSD900SN10
90±25%
95 2
NFZ2MSD131SN10
130±25%
144 1.5
For medium output (1 - 2 A class)
Product name
Impedance
at1MHz(Ω)
 
Impedance
at100MHz(Ω)
DC resistance
(Ω)
Rated current
(A)
Product size
(max. height)
(mm)
typ. Max
NFZ32SW301HN10
3.2±30%
 300 0.036
2.55
3.2x2.5
(1.55)
NFZ32SW901HN10
6.8±30%
 900 0.054
2.05
Product name
Impedance
at100MHz(Ω)
DC resistance(Ω)
Rated current(A)
Product size
(max. height)
(mm)
Max
85℃
125℃
NFZ18SM121SN10
120±25%
0.14 1.25 1.1
1.6x0.8
(0.95)
NFZ18SM251SN10
250±25%
0.19
1.1 1.0
NFZ18SM501SN10
500±25%
0.25 0.95 0.85
NFZ18SM701SN10 700±25%
0.29 0.8 0.8
Product name
Impedance
at100MHz(Ω)
DC resistance
(Ω)
Rated current
(A)
Product size
(max. height)
(mm)
typ. Max
NFZ2MSM101SN10
100±25%
0.018 4
2.0x1.6
(1.0)
NFZ2MSM181SN10
180±25%
0.025 3.4
NFZ2MSM301SN10
300±25%
0.03 3.1
NFZ2MSM601SN10
600±25%
0.046 2.5





Audio line noise suppression for smart phones(1) Problems Audio line noise suppression for smart phones (2) Countermeasures Audio line noise suppression for home audio (1) Audio line Audio line noise suppression for home audio (2) Power supply line