Can the LC filter on the output of a Class D amplifier be removed?

wack_007

Can the LC filter on the output of a Class D amplifier be removed? [Copy link]

Background: According to data and actual tests, the EMI noise of a Class D amplifier is different at different powers. When <10W, the low-frequency noise of CE and RE is very low. And after removing the output LC filter (removing C for overcurrent protection), the noise of CE and RE is still not high, but PWM harmonics will obviously appear. At this time, there is no difference in the sound. However, many data mention that LC is used to remove the PWM carrier so that its waveform meets the audible range of the human ear. So, is LC just a filtering device, or is it actually integrated with Class D to meet functional requirements? Can LC be removed? Will it affect the sound quality and efficiency? How much will it affect?

timberabc

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maychang

The PWM frequency of the Class D amplifier is much higher than the upper limit of audio frequency, which is inaudible to the human ear. In addition, the vibration of the speaker cone cannot keep up with the PWM frequency. In this sense, the speaker itself is a low-pass filter. Therefore, if the LC filter at the output of the Class D amplifier is removed, the human ear will not be able to hear any difference.

maychang

However, it is recommended to retain the LC low-pass filter circuit at the output of the Class D amplifier because of its impact on EMI.

wack_007

maychang published on 2018-8-17 10:04 However, it is recommended to keep the LC low-pass filter circuit at the output of the Class D amplifier, because the filter circuit has an impact on EMI.

SPKs all have L parameters, which are basically at the uH level, so the load seems to be able to meet the conversion needs. Generally, Class D chip tests are done at full power EMC, and at this time, the CE RE noise is relatively large. However, in actual use, for different applications, it may only be half of the maximum power or even the minimum. At this time, from the perspective of EMC alone, it seems unnecessary to use LC. And doesn’t +LC have an impact on efficiency THD+N, etc.?

maychang

wack_007 posted on 2018-8-17 11:02 SPKs all have L parameters, which are basically at the uH level, so the load seems to be able to meet the conversion needs. And the general Class D chip tests are full...

And doesn't +LC have an impact on efficiency, THD+N, etc.? I'm not sure about this, it seems to have a very small impact on efficiency and THD.

topwon

1. Not all Class D amplifiers support filter-free design, which is related to the topology of the amplifier. 2. Generally, only low-power designs consider omitting filters, because EMI testing may be more troublesome. For example, mobile phones, MP3s, small speakers, etc. have a power of 1~2W and are cost-sensitive products. Some references: http://www.doc88.com/p-0962682878088.html.