Performance index of power amplifier

There are many performance indicators for power amplifiers, including output power, frequency response, distortion, signal-to-noise ratio, output impedance, damping coefficient, etc., among which three indicators are mainly output power, frequency response, and distortion.

    1. Output power Output power refers to the power delivered by the amplifier to the load, with watts (W) as the basic unit. With a certain amount of amplifier and load, the output power is determined by the input signal. In the past, people used the rated output power to measure the output power. Now, because the pursuit of high fidelity is different from the evaluation of sound quality, and the measurement methods used are different, there are many names of power names that should be paid attention to.

    (1) Rated output power (RMS). The rated output power refers to the maximum power output by the power amplifier within a certain harmonic distortion index. It should be noted that the load and harmonic distortion indicators of the power amplifier are different, and the rated output power is also different. Generally, the harmonic distortion indicators are 1% and 10%. Since the output power is related to the input signal, for convenience of measurement, a continuous sine wave is generally used as the measurement signal to measure the output power of the audio equipment. Normally, input a sine signal with a frequency of 1000 Hz to the power amplifier during measurement, and measure the effective value V of the voltage on the equal resistance load resistance. 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 gradually increases, the power amplifier starts 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 magnitude of distortion, input a sufficiently large signal to the power amplifier and adjust the volume and tone potentiometer 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 kinds of power commonly used in early audio product manuals in my country. Usually the maximum output power is twice the rated power. However, there is such a situation during playback. When two power amplifiers with similar maximum useful power and speaker sensitivity are used to audition a symphony program, when a piece of music has passed from a low ebb, there may be a sudden sudden percussion sound. One power amplifier can give quite a lot of power in an instant, giving people a sense of strength, but the other power amplifier is lacking in confidence. In order to mark the ability of the power amplifier to burst output power in such a moment, in addition to measuring the above-mentioned maximum useful power and maximum output power, it is necessary to measure the music output power and peak output power of the power amplifier. In order to fully reflect the output capacity of the power amplifier.

    (3) Music output power (MPO). Music output power (Music Power Output) refers to the output power of the power amplifier when working with music signals, that is, the instantaneous maximum output power of the power 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, and 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 power amplifier can output when the volume and tone potentiometer of the power amplifier are adjusted to the maximum without distortion. The peak music power not only reflects the performance of the power amplifier, but also reflects the power supply capability of the DC power supply of the power amplifier. Generally speaking, the above-mentioned output power of a certain power amplifier has the following relationship: peak music output power>music output power>maximum output power>rated output power. Generally, the peak music output power is 8-10 times the rated output power, but there is no consensus.

    2. Frequency response Frequency response refers to the ability of a power amplifier to evenly amplify each frequency component of an audio signal. Frequency response can generally be divided into amplitude frequency response and phase frequency response. The amplitude frequency response characterizes the working frequency range of the power amplifier and the degree of uniformity and unevenness within the working frequency range. The so-called operating frequency range refers to the frequency range between the upper limit frequency and the lower limit frequency where the output signal level of the amplitude frequency response drops 3dB relative to the 1000Hz signal level. In the working frequency range, it is a measure of whether the frequency response curve is flat, or the unevenness is generally expressed in dB. For example, the working frequency range of a certain power amplifier and its unevenness are expressed as: 20Hz-20kHz, +-1dB. Phase frequency response refers to the mutual phase relationship between the output signal of the power amplifier and the frequency of the original signal, that is, whether there is phase distortion. In general, phase distortion is not very important to power amplifiers, because the human ear is not very sensitive to phase distortion. Therefore, the frequency response of the general power amplifier refers to the amplitude frequency response. At present, the operating frequency range of general power amplifiers is 20Hz-20kHz.

    3. Distortion refers to the undue change in the waveform of the reproduced audio signal. Distortions include harmonic distortion, intermodulation distortion, crossover distortion, clipping distortion, phase distortion and transient distortion.

    (1) Harmonic distortion. Harmonic distortion is caused by non-linear components in the power amplifier. This non-linearity will cause the audio signal to produce many new harmonic components. The distortion is expressed as a percentage of the ratio of the effective value of all harmonics in the output signal to the effective value of the fundamental voltage. The lower the harmonic distortion, the better. Harmonic distortion is related to frequency. Usually around 1000 Hz, the amount of harmonic distortion is small, and the amount of harmonic distortion is large at the high and low ends of the frequency response. Harmonic distortion is also related to the output power of the power amplifier. When it is close to 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, the sum frequency and difference frequency signals of each frequency and harmonic frequencies will be generated at the output end. For example, the sum of a 200Hz signal and a 600Hz signal will produce two weak intermodulation distortion signals of 400Hz (difference signal) and 800Hz (sum signal). Since the intermodulation signal has no similarity with the natural signal, it is easy to be noticed, and it can be heard when the intermodulation distortion is relatively small, which is annoying. Therefore, reducing intermodulation distortion is one of the keys to improving audio quality.

    (3) Cross distortion and clipping distortion. Crossover distortion, also known as crossover distortion, is caused by the non-linearity of the initial conduction of the Class B push-pull amplifier power amplifier tube of the power amplifier, and it is also one of the causes of intermodulation distortion. Clipping distortion is a nonlinear distortion caused by the signal being clipped when the power amplifier tube is saturated, and the output signal amplitude cannot be further increased. Clipping distortion can make the sound blurry and jittery. Clipping distortion cannot be eliminated, only when listening to music, be careful not to make the amplifier reach its full power limit.

    (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 transient response of the amplifier is poor, the output of the amplifier cannot keep up with the change of 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 the high-fidelity amplifier has a wide frequency range. Transient intermodulation distortion is an important technical indicator in the modern audio field. Because power amplifiers often add large loop deep negative feedback, and generally add phase lag compensation capacitors, when transient signals are input, the output terminal cannot reach the maximum value immediately, so that the input stage cannot get the due negative feedback. The transient overload of the feedback voltage causes a lot of new intermodulation distortion. Since these distortions are generated in transient states, they are called transient intermodulation distortion. Transient intermodulation distortion is an important reason why transistor power amplifier circuits and integrated amplifier circuits produce the so-called "transistor sound", which makes the sound quality inferior to tube power amplifiers.

    4. Signal-to-noise ratio The signal-to-noise ratio refers to the ratio of the level of various noises (such as hum and white noise) output by the power amplifier to the level of the output signal in decibels. The higher the decibel value of the signal-to-noise ratio, the smaller the noise of the power amplifier and the better the performance. It is generally required to be above 50dB, and the signal-to-noise ratio of high-quality power amplifiers is greater than 72dB.

    5. Output impedance and damping coefficient The equivalent internal impedance presented by the output of the power amplifier to the load (speaker) is called the output impedance, and the damping coefficient refers to the size of the resistance of the power amplifier to the speaker. Since the output impedance of the power amplifier circuit is connected in parallel with the speakers, it is equivalent to connecting a small resistor in parallel at both ends of the speaker voice coil, which will dampen the inertial oscillation of the speaker tray. The smaller the output impedance of the power amplifier, the greater the damping of the speaker, so the damping coefficient is often used to describe the damping degree of the power amplifier circuit to the speaker. The damping coefficient is defined as the ratio of the speaker impedance to the output impedance of the power amplifier (including the speaker wire resistance). It can be seen that the smaller the output impedance of the power amplifier, the larger the damping coefficient DF, which means that the power amplifier prevents the speaker from being able to oscillate freely. ) The stronger. But the damping coefficient is not as large as possible. From the sense of hearing, if the damping coefficient is too large (being over-damped), the sound will be dry; while the damping coefficient is too small (being under-damped or insufficiently damped), and the tailing will be more Long, it will make the bass become muddy and unclear, and distortion will increase. Generally speaking, for civil power amplifiers, the damping coefficient should be 15-100. For professional power amplifiers, the damping coefficient should be 200-400 or higher.