The relationship between the sampling frequency of digital power amplifier chip and the sound quality of power amplifier is explained with examples

    The sound quality of digital power amplifier has been plagued by many people, the bass is good, the treble is harsh, in fact, it is true that we have found this problem in the process of developing products. We return to the principle of the digital power amplifier: the audio signal (20~20K) through a PWM modulation, and then through a switching power amplifier circuit, the PWM signal amplification, and finally through the filter, the PWM signal filter out, so that there is a high-power audio signal can directly promote the speaker. This modulation process is the key to digital power amplifiers.

    Generally, the PWM frequency of several popular digital power amplifier schemes is working in the range of 300K~500k, and some bass runs even work at frequencies below 100K. The higher the working frequency, the more difficult it is to select the switch tube, if the speed of the switch is slow, it is easy to heat, want to reduce the heat, you need to turn the dead zone up, the dead zone is turned up, resulting in greater distortion. This is a dilemma. Therefore, the selection of extremely fast switching tubes is the first priority of digital power amplifiers.

    The sampling frequency of digital power amplifier directly determines the sound quality, which is an important phenomenon we found in the process of developing digital power amplifier. A simple example should give you a good idea of how this works.

    1. If a 20HZ low-frequency signal is input, it is equal to dividing a 20HZ low-frequency signal period into 15,000 sampling points, which is enough to perfectly express a positive baser waveform when output, and the bass can be well performed.

    2, if the input of a 1K IF signal, then he produces 300K/1K, that is, a period of 300 sampling points, this is still acceptable, but has begun to deteriorate.

    3, if the input of a 20K intermediate frequency signal, then only 300K/20K, that is, a period of 15 sampling points, has not been able to fully express a positive basalt, personally believe that this is the main reason for the deterioration of the treble ugly, let‘s see, in the end how high frequency can express the audio signal.

    PWM 20 250 500 1K 2K 5K 10K 15K 20K

    100K 5000 400 200 100 50 20 10 7 5

    300K 15000 1200 600 300 150 60 30 20 15

    500K 25000 2000 1000 500 250 100 50 33 25

    600K 30000 2400 1200 600 300 120 60 40 30

    1000K 50000 4000 2000 1000 500 200 100 67 50

    2000K 100000 8000 4000 2000 1000 400 200 133 100

    From the above table, it can be seen that if the PWM frequency is 100K input a 20K audio signal, he can only distinguish 5 signals in a 20K cycle, which is obviously not good, 100K can better express the 1K signal (there are 100 sampling points), Therefore, the digital power amplifier working at 100K can only be used as a subwoofer (20~250HZ). A 300K digital amplifier can only perfectly express the high pitch of 5K(with 60 sampling points). A 600K digital power amplifier can better express 10K audio.

    When the working frequency reaches 1~2M, in order to really reduce the distortion of the treble, reducing is not equal to perfect:) To pursue a higher frequency is the dream of every digital amplifier designer, but it must be based on more advanced devices (higher working frequency power tube).

     The lower the sampling frequency, the more serious the polyline of the high-frequency waveform, why some low frequency (400K) digital power amplifier distortion is so low. This is mainly in the distortion measurement method, the ordinary distortion measurement is the input 1K signal, after the output measurement of the harmonic generated by the 1K signal (2K 3K, 4K, 5K, etc.), 2K 4K is relatively high, it is even distortion (common distortion of electronic tubes), 3K5K is relatively high is odd distortion (common distortion of transistor circuits), That is to say, in fact, the nominal distortion only represents the distortion of 1KHZ, and cannot represent the distortion of other signal frequencies. As a result, the nominal distortion is very low, but the actual hearing is uncomfortable. We can go back to see which table above, more than 300K digital power amplifier for 1KHZ expression is more perfect. From this point of view, it also proves the feeling of everyone at ordinary times, why the digital amplifier treble is always uncomfortable. The key problem is that the fundamental frequency is not high enough.