Power Amplifier Indicators Guide

Power amplifier’s main technical indicators are as follows:   

1. Input Impedance: often represents the anti-interference ability of power amplifier, normally 5000-15000 Ω. The higher the value is, the stronger the anti-interference ability is;

2. Distortion: refers to the distortion degree of the input signal compared with the output signal. The smaller the numeral value, the better the quality, generally below 0.05%. Distortion is the alteration of replayed audio signal waveform. There are a number of causes, thus resulting in many types of waveform distortions, mainly harmonic distortion, intermodulation distortion, transient distortion.

3. Signal to Noise Ratio/SNR: It is defined as the ratio of signal power to the noise power, often expressed in dicebels.The greater the value, the cleaner the sounds.

4. Output Power: refers to the power that the power amplifier circuit delivers to the load. Current methods of measurement and evaluation to the output power is not unified. Please note that output power of power amplifier should be greater than RMS of the loudspeaker as much as possible.

   RMS: a root mean square related to average audio power. It means the maximum power output, strictly speaking, sine wave signal of the power amplifier working for a long time within a certain range of harmonic wave. The average power when harmonic distortion is 1% is known as the rated output power, maximum useful power, continuous power or distortionless power. Apparently, RMS is subject to the provisions of distortion degree.

   Maximum Power Output: Regardless of the degree of distortion, the output power of power amplifier circuit can be much higher than RMS. The maximum power output by power amplifier is known as the maximum output power. RMS and the maximum output power are different output powers based on two preconditions.

   Music Power Output: MPO is the abbreviation of Music Power Outputwhich refers to the output power of power amplifier circuit working in themusic signal, that is, the instantaneous maximum output power for music signal under the condition that the distortion shall not exceed the specified value.

Music power output can be used to evaluate the dynamic sound efficacy of the power amplifier, reflecting the sudden power output ability.

    Peak Music Power Output: PMPO is the maximum music output power, another dynamic indicator of power amplifier circuits. Regardless of distortion, the maximum music power output of power amplifier circuit is peak music power output.

Usually we can put them in this order: PMPO˃Music Power Output ˃ Maximum Power Output ˃RMS. Statistics in practice shows that PMPO is 5-8 times the value of RMS.

5. Frequency Response: reflects the amplifying ability of power amplifier for each frequency component of audio signal. Power amplifier’s frequency response range should not be less than the audible frequency range of the human ear. Ideally, operating frequency range of the main channel audio amplifier is 20-22kHz. The international stipulation for general audio power amplifier is 40-16 kHz ± 1.5dB.

6. Dynamic Range: describes the ratio of the amplitude of the loudest possible undistorted sine wave to the root mean square(rms) noise amplitude, say of a microphone or loudspeaker.Practically we use dB to denote two signal level gap. The dynamic range of high fidelity amplifier should be greater than 90 dB. Various noise of Nature forms background noise which has bad effect on listening to the audio signal. A good audio system shall neither cause overload distortion for strong signal nor overwhelmed by its own noise. Noise can be reduced, but not eliminated.

7. Output Impedance and Damping Factor

 Output Impedance: equivalent internal impedance that power amplifier output port and the load (loudspeaker) present is called the output impedance of the amplifier. Usually amplifiers and loudspeakers for household usage are 4 ohm to 8 ohm.

 Damping Factor: refers to the ability that power amplifier imposes electrical damping to the load (loudspeaker).The value of the damping factor between a particular loudspeaker and a particular amplifier describes the ability of the amplifier to control undesirable movement of the speaker cone near the resonant frequency of the speaker system. It is usually used in the context of low-frequency driver behavior, and especially so in the case of electrodynamic drivers, which use a magnetic motor to generate the forces which move the diaphragm.


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