I. What is distortion
    In the field of audio, distortion (commonly referred to as "distortion") refers to the phenomenon where the output signal of an audio system differs from the input signal in terms of waveform, frequency, amplitude, etc., causing the sound to deviate from its original state.
    It is one of the core issues affecting sound quality. According to the causes and manifestations, it can be divided into many types. The following are the main classifications and characteristics:
II. Common distortions in audio equipment
    1- Distortion
    Any abnormal deformation of the output waveform in a sound system compared to the input waveform is considered distortion.
    2- Harmonic distortion: When nonlinear components of audio equipment (such as power amplifiers and speakers) process signals, they generate harmonic components that do not exist in the original signal (with frequencies being integer multiples of the original signal).
    Low-order harmonics (such as the 2nd and 3rd harmonics) may make the sound more "full" (like the warm feeling of some tube amplifiers).
    High-order harmonics (above the 5th order) can cause the sound to be harsh and rough, and damage the purity.
    5- Intermodulation distortion: When a device processes two or more signals of different frequencies simultaneously, new frequencies (intermodulation products) are generated due to nonlinear characteristics. These additional frequencies will interfere with the original signal, causing the sound to be mixed and the sense of layering to be blurred (for example, when instruments play together, the timbre "claps").
    6- Clipping distortion: When the input signal strength exceeds the processing limit of the equipment (such as insufficient power of the power amplifier or excessive volume), the signal waveform is "flattened", resulting in shrill noise and distortion. This is commonly seen in the "popping sound" when the volume is turned up too high.
    7- Transient distortion: The device fails to respond promptly to rapidly changing signals (such as drumbeats and string plucking), unable to accurately reproduce the instantaneous changes of the signals, resulting in a sluggish sound and a lack of explosive power (for instance, drumbeats sound "soft" rather than "crisp").
    The above-mentioned distortions are mainly caused by factors such as the circuit design of the equipment, the quality of components, and power matching. In severe cases, they can significantly damage the listening experience. Therefore, in the design and debugging of audio systems, controlling the degree of distortion is the key to improving sound quality.

III. How to reduce the distortion of audio equipment
    To reduce distortion (including both explicit and implicit distortion) at the initial stage of audio design, it is necessary to start from key links such as core component selection, circuit topology design, and signal path optimization, and specifically address fundamental issues such as nonlinearity, signal interference, and load matching. This can be carried out from the following six core directions:
    1. Prioritize the selection of core components with low nonlinear characteristics
    The nonlinearity of components is the main source of distortion (especially harmonic distortion and intermodulation distortion). When designing, it is necessary to give priority to screening devices with "better linear performance".
    ① Amplification element:
    In the field of transistor operation: Select an electron tube with a smoother transconductance curve and a wider linear range (such as some classic side thermal transistors), and determine the optimal operating point through precise design (avoid the electron tube operating in the nonlinear section of the cut-off or saturation region) to reduce excessive even harmonics caused by the "imbalance" of the gate control electron flow.
    ② Transistor/chip scenarios: Select low-distortion operational amplifiers (such as Hi-End audio operational amplifiers) and field-effect transistors (FETs), taking advantage of their high input impedance close to that of vacuum tubes and more linear transfer characteristics to reduce odd harmonics and intermodulation products.
    ③ Passive components:
    Capacitors: Prioritize the use of audio-specific capacitors (such as polypropylene capacitors and Teflon capacitors) to avoid phase distortion caused by the "capacitive reactance varying with frequency" of ordinary electrolytic capacitors.
    Resistance: Select metal film resistors with low temperature coefficients (instead of carbon film resistors) to reduce resistance value fluctuations caused by temperature changes and avoid nonlinearity in signal voltage division/current limiting.
    Inductors/transformers: Audio transformers (such as tube amplifier output transformers) should use cores with high magnetic permeability and low loss (such as silicon steel sheets, permalloy), and optimize the winding process (such as layer-by-layer and segmented winding) to reduce nonlinear distortion caused by magnetic saturation and phase distortion caused by leakage inductance.
    2. Optimize the circuit topology to suppress nonlinear distortion
    The rationality of the circuit structure directly determines the linearity of the signal amplification process and should be designed from the perspective of "reducing the nonlinear superposition of the signal".
    1- Adopt a low-distortion topology:
    Power amplification stage: The tube amplifier preferentially uses "push-pull amplification" (symmetrically amplifying the positive and negative half-cycle signals through two vacuum tubes to cancel out part of the nonlinearity), or "single-ended Class A" (keeping the vacuum tubes always operating in the most linear Class A range to avoid crossover distortion between Class B and Class A/B).
    Pre-amplification: Avoid complex multi-stage amplification (where nonlinearity is superimposed at each stage), and prioritize the use of a "co-emitter - co-set" combined topology (balancing gain and linearity), or a "differential amplification circuit" (utilizing symmetry to counteract temperature drift and even nonlinear interference).

Ⅳ Suppress intermodulation distortion:
    The core of intermodulation distortion is "the interaction of multi-frequency signals in nonlinear components", and when designing, it is necessary to:
    ① Limit the gain of single-stage amplification (to prevent the single-stage gain from being too high and causing the signal to enter the nonlinear region);
    ② Add a "frequency compensation network" (such as an RC compensation circuit) to the signal path to reduce mutual interference between signals of different frequencies;
    ③ Adopt "current feedback type operational amplifier" (compared with voltage feedback type, it has lower intermodulation distortion).
    3. Ensure the stability of the power supply system and reduce the distortion introduced by the power supply
    Fluctuations in the power supply (such as ripple and voltage drop) can directly cause the operating point of the amplification element to drift, resulting in "power supply noise distortion" (a type of latent distortion). During the design process, it is necessary to focus on optimizing
    ① Power supply filtering and voltage stabilization:
    By adopting "multi-stage filtering" (such as π -type filtering composed of capacitors and inductors, and choke filtering), the AC ripple after rectification of the mains power supply is significantly reduced (the goal is to control the ripple voltage below the mV level).
    The key amplification stage (such as the preamplifier stage and power stage) is separately equipped with a voltage regulating circuit (such as a linear regulated power supply LDO, rather than a switching power supply - the high-frequency noise of a switching power supply is prone to introduce hidden distortion), ensuring the stability of the supply voltage at each stage and avoiding voltage fluctuations caused by load changes (such as dynamic signal increase).
    Power isolation
    The preamplifier, power stage, and digital circuits (such as the control section) are powered by independent power windings or isolation transformers to prevent power noise from different modules from interfering with each other (for example, large current fluctuations in the power stage will not affect the small signal amplification in the preamplifier).
    4. Optimize the signal path to reduce distortion during transmission
    If the signal encounters impedance mismatch or parasitic parameter interference during transmission, reflection, attenuation or phase shift will occur, resulting in hidden distortion. When designing, it is necessary to:
    ①- Impedance Matching design
    Ensure that the impedance of the signal source, transmission line and load is consistent (for example, for audio signals, the commonly used matching is "high input impedance - low output impedance" : the output impedance of the front stage < 1kΩ, and the input impedance of the back stage > 10kΩ), to avoid waveform distortion caused by signal reflection.
    The impedance of the power amplifier stage is strictly matched with that of the speaker (for example, a 4Ω power amplifier is connected to a 4Ω speaker), reducing power loss and intermodulation distortion caused by impedance mismatch.
    ② Shorten the signal path and shield
    Try to shorten the transmission distance of the small signals in the preamplifier as much as possible (small signals are prone to interference), and use shielded wires (such as coaxial cables with metal mesh) for the signal lines to avoid hidden distortion introduced by external electromagnetic interference (such as power supply ripple, radio frequency noise).
   When laying out the circuit board, strictly separate the strong current area (power supply, power stage) from the weak current area (preamplifier, signal input) to reduce the coupling interference caused by parasitic capacitance and parasitic inductance.
    5 . Control the load characteristics to avoid distortion caused by load
    The "nonlinear impedance" (impedance varies with frequency) of loads such as speakers will in reverse affect the power amplifier, causing nonlinear distortion in the output of the power amplifier. When designing, it is necessary to:
    ①- Adaptation of power amplifier to load:
    When designing a power amplifier, reserve sufficient "damping factor" (damping factor = power amplifier output impedance/load impedance. Generally, the damping factor of Hi-Fi power amplifiers needs to be greater than 20) to ensure that the power amplifier can stably control the movement of the speaker diaphragm and avoid distortion caused by fluctuations in load impedance.
    For the frequency response curve of the speaker, an "impedance compensation circuit" is added to the output end of the power amplifier or the crossover to reduce the amplitude of the load impedance variation with frequency.
    ②- Avoid overload distortion:
    When designing the "dynamic range" (the ratio of the maximum undistorted output power to noise) of a power amplifier, sufficient margin should be reserved (usually 3-6dB higher than the actual maximum volume requirement in use) to prevent large dynamic signals (such as drumbeats and vocal climaxes in music) from causing the power amplifier to enter the saturation zone and resulting in overload distortion.
    6. Utilize negative feedback technology to correct nonlinear distortion
    Negative feedback is a classic method to reduce distortion. By "reverse feedback" a part of the output signal to the input end, it offends the nonlinear errors generated during the amplification process:
    ①- Reasonably design negative feedback:
   Adopt "voltage series negative feedback" (increase input impedance, reduce output impedance, and minimize the impact of load) or "current parallel negative feedback" (enhance load-carrying capacity), and the feedback depth needs to be balanced - too deep negative feedback may introduce phase distortion, while too shallow negative feedback cannot effectively suppress nonlinearity.
    To address the phase shift that is prone to occur in high-frequency signals, a "phase compensation capacitor" is added to the feedback loop to prevent self-excitation or distortion caused by phase differences in the high-frequency band.
    ②- Avoid negative feedback failure:
    Ensure that the sampling point of the feedback signal (such as the output end of the power amplifier) is far from the interference source, and that the impedance matching of the feedback path is good to avoid the feedback signal being contaminated and causing the distortion correction to fail.