The main parameters of a power amplifier include: input sensitivity, harmonic distortion, signal-to-noise ratio, frequency response, damping factor, and conversion rate.
1．Input sensitivity：
It refers to the minimum input signal level required by the power amplifier. It is a necessary condition for amplifying the audio source signal to a sufficient extent to drive the subsequent power amplifier.
2．Harmonic distortion：
Harmonic distortion is a very important indicator of amplifiers. Harmonic distortion is a kind of nonlinear distortion, which is caused by the nonlinear characteristics of the amplifier in operation. The result of distortion is the generation of a new harmonic component, causing the sound to lose its original tone. In severe cases, the sound is broken and distorted. Harmonic distortion can also have odd numbers or even a second point. Odd harmonics can make people irritable, disgusted and easily perceptible.
Some amplifiers sound annoying, feel tiring, or are caused by greater distortion. The most significant impact on the amplifier is the degree of distortion. Generally, high fidelity requires harmonic distortion to be less than 0.05%, and the lower the better.
In addition to harmonic distortion, there are also intermodulation distortion, crossover distortion, cancellation distortion, transient distortion, phase distortion, etc. These are all the main reasons affecting the quality of amplifiers. The validity of the assessment should first be judged by its distortion. For instance, the total harmonic distortion of the Italian Sinfoni amplifier is less than 0.01%.
3．Signal-to-noise ratio：
The larger the value, the better. Generally, (S/n) is used, which is the ratio of signal-to-noise (ps) to noise power (pn) with signal-to-noise (S/n increase by 10Lgss/pn). As the signal-to-noise ratio and the input signal level increase, the signal-to-noise ratio gradually increases, but when the input signal level reaches a certain value, the signal-to-noise ratio remains basically unchanged.
According to the high-fidelity requirements, the signal-to-noise ratio should also reach over 90dB. Imported high-end amplifiers often reach as high as 110-120dB, and their performance can be imagined. Some signal-to-noise ratios are followed by the letter "A". A-weighting refers to the noise signal measured through A weighted network. Since people are relatively less sensitive to high-frequency and low-frequency band noise, there is such a method: signal-to-noise ratio.
Weighted noise more intuitively represents the state of the noise signal that people actually perceive. In conclusion, the higher the signal-to-noise ratio, the smaller the noise in the signal, the better the sound quality, and the clearer, cleaner and more reasonable the layers of music reproduction.
4．Frequency response：
In the early days, it was commonly known as power bandwidth, referring to the width of half of the rated power frequency band of the power amplifier when the harmonic distortion does not exceed the specified value. That is, the frequency band included between two frequency points with a drop of -3dB between the high and low ends is called power bandwidth.
5．Damping coefficient：
It mainly targets low frequencies and is a very important technical parameter that directly affects the quality of bass sound. It is well known that the larger the diameter of the speaker, the better the relative bass, but the greater the movement inertia of the sound pool. This inertia makes it difficult to synchronize with the movement of the audio signal.
Often, it shows that the sound turbidity is not clear, especially in the low frequencies of 100-400Hz, which is prone to cause sound coloration, making the human voice blurry and very unnatural. Some modified car subwoofers have strong low-frequency signal chatter and severe bass trailing. This is the sound color caused by audio inertia.
6．Conversion rate：
The conversion rate of an amplifier has a significant impact on the quality and performance of high-frequency reproduction. The faster the conversion rate, the better the high-frequency quality, and the more accurate the high-frequency information captured will be.
High-end amplifiers can achieve a conversion rate of 10 to several dozen V/us, while low and mid-range amplifiers are generally not marked. The value of this conversion rate is closely related to the design materials, but it should not be too high. If it is too high, it will cause an overshoot signal beyond 20KHz that the human ear cannot hear. This not only has no effect on improving the sound quality but also easily burns out the tweeter.
Key indicators of audio power amplifiers
Audio power amplifiers have been widely applied in fields such as cellular phones, portable devices, and audio equipment. In different application fields, the emphasis on the parameter indicators of audio power amplifiers will vary. For instance, in the field of mobile phones, the focus is on the ability to suppress radio frequency interference, while in audio equipment, more attention is paid to distortion and frequency response characteristics. Therefore, positioning the key technical indicators of audio power amplifiers based on market demand has become a highly challenging topic
The basic parameters of an audio power amplifier include static operating current (IDD), turn-off current (ISD), input offset voltage (Vos), total harmonic distortion plus noise (THD+N), output power (PO), and other indicators. In addition, parameters such as signal-to-noise ratio (SNR), power rejection ratio (PSRR), GAIN (GAIN), efficiency (η), and Noise (Noise) are also indispensable technical indicators for evaluating an audio power amplifier. Of course, parameters such as THDN, SNR, PSRR, and GAIN are all obtained at each fixed frequency, for instance, 1KHz as the excitation. Therefore, the sweep curves of these parameters can reflect the performance of the audio power amplifier throughout the entire audio range (20Hz-20KHz).

Key indicators
Audio power amplifiers can be classified into many types based on their functions. Whether they are single-channel, stereo, driving headphones or driving speakers, the indicators they focus on in the common application fields are similar. At present, the Audio analyzer of Audio Precision can automatically complete the tests of most parameters and has become an evaluation criterion in the industry.
Quantifiable indicators
Total Harmonic Distortion plus Noise (THD+N
THD+N is the abbreviation of "Total Harmonic Distortion + Noise" in English, which is translated into Chinese as "Total harmonic Distortion plus Noise". THD+N technology is extremely attractive because any component in the output except for the pure measurement signal will cause the measurement index to decline. Low THD+N measurement results not only indicate low harmonic distortion, but also suggest that humming sounds, interference signals, and wideband white noise are lower than (or equal to) the measured values. THD+N has been widely applied in audio testing.
For users of audio equipment and high-end mobile phones, THD+N reflects the distortion of audio power amplifiers and is a very important indicator. To comprehensively examine the performance of the audio power amplifier throughout the entire frequency range, the sweep curve shown in Figure 1 is also very important. By making appropriate adjustments to the peripheral circuits based on the characteristics of the audio amplifier and speakers, a satisfactory tone can be achieved.

Output power (PO
Output power refers to the output capacity of an audio power amplifier on a load when it meets a certain degree of distortion (THD+N) under a specified voltage. It should be noted that when comparing this parameter, attention should be paid to the differences in test conditions, especially for Class D power amplifiers. Because different loads (speakers are inductive loads), different filters, and different distortion requirements will have a significant impact on the test results.
For users of low-end mobile phones and audio equipment, audio amplifiers with high output power are more attractive. Therefore, it is required that the output power be as high as possible without distortion. Many chip suppliers directly use output power as the title of their specification sheets to increase their selling points.

Power supply rejection ratio（PSRR）
When it comes to the application of audio amplifiers in mobile phones, the parameter PSRR cannot be overlooked. PSRR (Power supply rejecTIon raTIo) is the ability of the output of an audio amplifier to suppress power supply ripple.

In TDMA and GSM mobile phones, the most serious power supply voltage noise comes from RF-level on and off. The switching frequency of GSM phones is 217Hz. When the RF power amplifier is turned on, a high current is obtained from the power supply, and at this time, the power supply drops by up to 500mV. An audio amplifier with poor PSRR will generate harmonic "click-click" noise greater than 217Hz in the speaker. Figure 2 shows the suppression capability of an amplifier with a PSRR of 60dB for GSM signals.

Relatively speaking, mobile phone users pay more attention to the PSRR at 217Hz, as this parameter directly affects the call quality when using a hands-free device. If not handled properly, even the most beautiful timbre mixed with interfering sounds can be unbearable. Of course, this indicator is not omnipotent, as radio frequency interference not only occurs in the power supply but also the noise coupled to the input and output ends is a factor that needs to be carefully considered.
Unquantified indicators
The parameter indicators mentioned above have all been quantified and can be automatically tested by an audio analyzer. However, these indicators cannot cover all application requirements. There are still many phenomena that occur in different application environments but cannot be measured by a unified standard.
Pop Click
Pop ClicK is the noise generated by the audio transient signal in the audio amplifier or speaker during the process of turning it on or off. Maxin proposed using KCP to measure the size of Pop Click. However, the commonly used method at present still takes the auditory effect in the actual environment as the final evaluation criterion.

Radio frequency suppression capability
The three major noise sources of audio power amplifiers are: power supply noise, input coupling noise and output coupling noise. The ways of radio frequency interference are further divided into conduction and spatial radiation. Therefore, the RF suppression capability of audio power amplifiers is difficult to describe with fixed indicators. Take mobile phones as an example. Different power amplifiers can all work normally in well-designed mobile phones. Only in systems with severe radio frequency interference can chips with strong suppression capabilities stand out