There are many performance indicators for power amplifiers, including output power, frequency response, distortion, signal-to-noise ratio, output impedance, damping factor, etc. Among them, output power, frequency response, and distortion are the main indicators.
1. Output power Output power refers to the power delivered to the load by the power amplifier, with watts (W) as the basic unit. When the power amplifier has a certain amplification capacity and load, the output power is determined by the input signal. In the past, people used rated output power to measure output power. Now, due to the pursuit of high fidelity and different evaluations of sound quality, different measurement methods are used, resulting in many power names, which should be paid attention to.
(1) Rated output power (RMS). Rated output power refers to the maximum power output of the power amplifier within a certain harmonic distortion index. It should be noted that the rated output power of the power amplifier varies with the load and harmonic distortion index. The harmonic distortion indexes usually specified are 1% and 10%. Since the output power is related to the input signal, for the convenience of measurement, a continuous sine wave is generally used as the measurement signal to measure the output power of the audio equipment. Usually, a sine signal with a frequency of 1000Hz is input to the power amplifier during measurement, and the effective value of the voltage V on the equal-resistance load resistor is measured. At this time, the output power P of the power amplifier can be expressed as P=V2/RL, where RL is the impedance of the speaker. The output power obtained in this way is actually the average power. When the volume is gradually turned up, the power amplifier begins to overload, the waveform is clipped, and the harmonic distortion increases. The average power when the harmonic distortion is 10% is called the rated output power, also known as the maximum useful power or undistorted power.
(2) Maximum output power. In the above case, regardless of the size of the distortion, the power amplifier is input with a sufficiently large signal, and the volume and tone potentiometers are adjusted to the maximum. The maximum power that the power amplifier can output is called the maximum output power. Rated output power and maximum output power are two types of power commonly used in the manuals of early audio products in my country. Usually, the maximum output power is twice the rated power. However, when playing music, there is such a situation. When two amplifiers with similar maximum useful power and speaker sensitivity are listening to a symphony program, when a piece of music passes from the low point and suddenly a sudden percussion instrument sound comes, one amplifier may be able to give a considerable power in an instant, giving people a sense of strength, while the other amplifier seems to lack confidence. In order to mark the ability of the amplifier to output such instantaneous burst power, in addition to measuring the maximum useful power and maximum output power mentioned above, it is necessary to measure the music output power and peak output power of the amplifier. Only in this way can the output capacity of the amplifier be fully reflected.
(3) Music output power (MPO). Music output power refers to the output power of the amplifier when it works on the music signal, that is, the instantaneous maximum output power of the amplifier to the music signal under the condition that the output distortion does not exceed the specified value. There is no unified measurement standard for output power (MPO) and peak music output power (PMPO) in the world. Foreign manufacturers generally have their own measurement methods. Usually, the music output power is 4 times the rated power. (4) Peak music output power (PMPO). It usually refers to the maximum music power that the amplifier can output when the volume and tone potentiometers of the amplifier are adjusted to the maximum without considering distortion. Peak music power not only reflects the performance of the amplifier, but also reflects the power supply capacity of the DC power supply of the amplifier. Generally speaking, the above-mentioned output powers of a certain amplifier have the following relationship: peak music output power > music output power > maximum output power > rated output power. Usually, the peak music output power is 8-10 times the rated output power, but there is no unified conclusion. 2. Frequency response Frequency response refers to the ability of the power amplifier to uniformly amplify the frequency components of the audio signal. Frequency response can generally be divided into amplitude frequency response and phase frequency response. Amplitude frequency response characterizes the operating frequency range of the amplifier and whether the amplitude within the operating frequency range is uniform or uneven. The so-called operating frequency range refers to the frequency range between the upper and lower limits of the amplitude frequency response where the output signal level drops 3dB relative to the 1000Hz signal level. Within the operating frequency range, the measurement of whether the frequency response curve is flat, or unevenness, is generally expressed in dB. For example, the operating frequency range and unevenness of a certain power amplifier are expressed as: 20Hz-20kHz, +-1dB. Phase frequency response refers to the phase relationship between the output signal of the power amplifier and the frequency in the original signal, that is, whether phase distortion occurs. Usually, phase distortion is not very important for power amplifiers, because the human ear is not very sensitive to phase distortion. Therefore, the frequency response of general power amplifiers refers to the amplitude frequency response. At present, the operating frequency range of general power amplifiers is 20Hz-20kHz.
3. Distortion Distortion refers to the inappropriate changes in the waveform of the reproduced audio signal. Distortion includes harmonic distortion, intermodulation distortion, crossover distortion, clipping distortion, phase distortion and transient distortion. (1) Harmonic distortion. Harmonic distortion is caused by nonlinear elements in the power amplifier. This nonlinearity will cause the audio signal to generate many new harmonic components. The distortion is expressed as a percentage of the effective value of all harmonics in the output signal to the effective value of the fundamental voltage. The smaller the harmonic distortion, the better. Harmonic distortion is related to frequency. Usually around 1000Hz, the amount of harmonic distortion is small, and at the high and low ends of the frequency response, the amount of harmonic distortion is large. Harmonic distortion is also related to the output power of the power amplifier. When it approaches the rated maximum output power, the harmonic distortion increases sharply. At present, the total harmonic distortion of high-quality amplifiers in the entire audio range is generally less than 0.1%; the harmonic distortion values of excellent power amplifiers are mostly between 0.03% and 0.05%.
(2) Intermodulation distortion. When the power amplifier inputs two or more frequency signals at the same time, due to the nonlinearity of the amplifier, sum and difference frequency signals between the frequencies and harmonics will be generated at the output. For example, when a 200Hz signal and a 600Hz signal are added together, two weak intermodulation distortion signals of 400Hz (difference signal) and 800Hz (sum signal) are generated. Since intermodulation signals have no similarities with natural signals, they are easily detected by people. They can be heard at relatively small intermodulation distortions, which is annoying. Therefore, reducing intermodulation distortion is one of the keys to improving the sound quality of audio.
(3) Crossover distortion and clipping distortion. Crossover distortion, also known as crossover distortion, is caused by the initial conduction nonlinearity of the power amplifier tube of the Class B push-pull amplifier of the power amplifier. It is also one of the causes of intermodulation distortion. Clipping distortion is a nonlinear distortion caused by the clipping of the signal when the power amplifier tube is saturated, and the output signal amplitude cannot be further increased. Clipping distortion will make the sound fuzzy and jittery. Clipping distortion cannot be eliminated. You can only be careful not to push the amplifier to its full power limit when listening to music.
(4) Transient distortion and transient intermodulation distortion. Transient distortion is also called transient response, which refers to the ability of the power amplifier to follow transient signals. When a transient signal is added to the amplifier, if the amplifier has a poor transient response, the output of the amplifier cannot keep up with the changes in the transient signal, resulting in transient distortion. The transient response of the power amplifier is mainly determined by the frequency range of the amplifier, which is one of the main reasons why high-fidelity amplifiers have a very wide frequency range. Transient intermodulation distortion is an important technical indicator in the modern audio field. Since power amplifiers often add large-loop deep negative feedback, and generally add phase lag compensation capacitors, when a transient signal is input, the output end cannot immediately reach the maximum value, so that the input stage cannot obtain the required negative feedback voltage and transient overload occurs, generating a lot of new intermodulation distortion. Since these distortions are generated in the transient state, they are called transient intermodulation distortion. Transient intermodulation distortion is an important reason why transistor amplifier circuits and integrated amplifier circuits produce the so-called "transistor sound", making their sound quality inferior to that of tube amplifiers.
4. Signal-to-noise ratio The signal-to-noise ratio refers to the decibel value of the ratio of the various noise levels (such as AC noise and white noise) output by the amplifier to the output signal level. The higher the decibel value of the signal-to-noise ratio, the smaller the noise of the amplifier and the better the performance. It is generally required to be above 50dB, and the signal-to-noise ratio of a high-quality amplifier is greater than 72dB.
5. Output impedance and damping coefficient The equivalent internal impedance presented by the output end of the amplifier to the load (speaker) is called output impedance, and the damping coefficient refers to the magnitude of the electrical damping of the amplifier to the speaker. Since the output impedance of the amplifier circuit is in parallel with the speaker, it is equivalent to connecting a very small resistor in parallel at both ends of the speaker voice coil, which will damp the inertial oscillation of the speaker paper plate. The smaller the output impedance of the amplifier, the greater the damping of the speaker, so the damping coefficient is often used to describe the damping degree of the amplifier circuit to the speaker. The damping coefficient is defined as the ratio of the speaker impedance to the amplifier output impedance (including the speaker cable resistance). That is, the smaller the output impedance of the amplifier, the larger the damping coefficient DF, indicating that the amplifier has a stronger braking ability (i.e., damping ability) to prevent the speaker from free oscillation. However, the damping coefficient is not the larger the better. From the listening experience, if the damping coefficient is too large (overdamped), the sound will be dry; and if the damping coefficient is too small (underdamped or insufficiently damped), the bass will become turbid and the distortion will increase due to the long oscillation tail. Generally speaking, for civilian amplifiers, the damping coefficient should be 15-100. For professional amplifiers, the damping coefficient should be 200-400 or higher.
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