Audio power amplifier distortion refers to the phenomenon where the waveform of the reproduced audio signal is altered, typically categorized into two main types: electrical distortion and acoustic distortion. Electrical distortion occurs during the amplification process, when the signal current is distorted. Acoustic distortion happens when the signal passes through a speaker, and the speaker fails to accurately reproduce the sound.
Regardless of whether it's electrical or acoustic distortion, both can be further classified based on their nature—frequency distortion and nonlinear distortion. Linear distortion involves changes in the amplitude and phase relationships between frequency components, but no new frequencies are introduced, only waveform distortion. On the
other hand, nonlinear distortion generates new frequency components or modulation products from the original signal. These unwanted components often result in harsh, rough, or distorted sound. Examples include harmonic distortion (THD), intermodulation distortion (IMD), and others. In this discussion, we'll focus on harmonic distortion, intermodulation distortion, transient intermodulation distortion (TIM), and AC interface distortion (IHM).
Harmonic distortion arises due to nonlinearities in the power amplifier. This distortion introduces additional harmonic components that mix with the original signal, leading to waveform distortion. The total harmonic distortion (THD) measures the ratio of the root mean square (RMS) value of all harmonics in the output signal to the RMS value of the fundamental frequency. Lower THD values indicate better performance. High-fidelity amplifiers typically have THD below 0.05%, while high-quality models may go as low as 0.01%. Professional-grade amplifiers usually maintain THD under 0.03%. Human ears generally find it difficult to distinguish distortions below 0.1%.
Interestingly, even with similar THD measurements, different amplifiers can sound very different. This is partly due to the type of harmonics each produces. For instance, solid-state amplifiers tend to generate more even harmonics, which are less noticeable to the human ear, while tube amplifiers often produce odd harmonics, which can create a harsher sound. This difference contributes to the distinct tonal characteristics of various amplifier types.
To reduce harmonic distortion, several approaches can be used. These include applying negative feedback, selecting components with good linearity, improving circuit consistency, using Class A amplification, and enhancing power supply stability and filtering.
Intermodulation distortion occurs when two or more signals interact within an amplifier or speaker, producing new frequency components such as sum and difference frequencies. This distortion is often measured by mixing two signals at a specific ratio (e.g., 4:1) and analyzing the resulting nonlinear components. Intermodulation distortion is particularly noticeable because the added frequencies are not part of the original signal, making them sound unnatural and harsh. Hi-Fi amplifiers aim for intermodulation distortion below 0.1% to ensure clarity and definition.
Reducing intermodulation distortion involves methods like limiting the amplifier’s bandwidth, adding high-pass filters, and choosing linear components. These techniques help minimize unwanted frequency interactions and improve overall sound quality.
Transient distortion, also known as transient intermodulation distortion (TIM), measures how well an amplifier responds to sudden changes in the audio signal. It is crucial for preserving the dynamic and spatial aspects of music. Causes include excessive reactive components in the circuit and the inability of the speaker to keep up with rapid signal changes. TIM can lead to a loss of detail and clarity in the sound.
Improving transient response involves optimizing feedback levels, selecting fast components, and reducing loop negative feedback. Additionally, increasing the operating current of the output stage and using complementary circuits can enhance the amplifier’s ability to handle transients without distortion.
Another form of transient distortion is caused by a low slew rate (SR). Slew rate refers to the maximum rate at which the amplifier can change its output voltage. A higher SR allows the amplifier to respond more quickly to rapid signal changes, ensuring accurate reproduction of complex audio signals. High SR values are essential for maintaining clarity and detail, especially in genres like pop and rock.
AC interface distortion results from back-EMF generated by the speaker being fed back into the system. To mitigate this, designers may reduce the number of stages, adjust bias currents, choose suitable speakers, and use high-quality power supplies with proper filtering.
Other nonlinear distortions, such as crossover and clipping distortion, can also affect sound quality. Crossover distortion occurs in push-pull amplifiers due to non-linear behavior at low signal levels, while clipping distortion happens when the amplifier cannot handle large signals, causing saturation and loss of fidelity.
By addressing these distortions through careful design and component selection, audio systems can deliver clearer, more natural, and more enjoyable listening experiences.
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