The basic task of audio amplifiers is to increase a small signal into a bigger one without altering the sound. Indeed, it is demanding, as musical sounds generally contain several frequencies, to which, must be intensified by similar factor to prevent waveform changes, and the quality of sound. Amplifier that multiplies the magnitude of all frequencies with same factor is considered as linear. Departure from linearity yields to different distortions.
Distortion is the undesired change in audio signals between the input and output. To prevent such, amplitudes of the frequencies must be intensified by same factor. If the peaks are clipped of a certain waveform, it will result to harmonic distortion, the most common among the types of amplifier distortion. While another common specification in hi-fidelity amplifier is called total harmonic distortion. Such type of distortion is lesser than 1% or 0.5% from a range of 20 Hz to 20,000 Hz high quality amplifiers.
An amplifier is considered linear if its output voltage is proportional to its input signal. Non-linearity, on the other hand, pave the way for the occurrence of a harmonic distortion. Defect such as this can be eliminated with the use of negative feedback within the circuit itself, considering the output isn't overdriven to be clipped.
Intermodulation distortion is another form of amplifier distortion. It occurs when there are varied frequencies within the signal aiming to produce a sum and different frequencies. And transient distortion happens when the amplifier components are not able to handle the change rate of signal, like that to a rapid percussive attack. TIM or transient intermodulation distortion also exists specifically among integrated circuits. These circuits rely on feedbacks for linearity, and a delay in feedback causes intermodulation distortion.
Gener
ally, intermodulation distortion is troublesome in music reproduction since it generates frequencies that are not present within the original form of the music. Hence, it is very noticeable. Although harmonic distortion is similarly serious, but the musical sound already has the harmonics as part of the sound's harmonic content. Thus, it is tolerable to a certain degree, better than an intermodulation distortion.
Amplifier distortions certainly affect the overall quality of the music, as they alter the general sound. Distortion is brought upon by various factors such as compression, non-linear behaviors, clipping, aliasing, modulating, power supply insufficiency, and mixing phenomena.
Clipping is a waveform distortion that takes place when the amplifier is overly driven, like in an attempt to aggrandize the current or voltage way beyond its power capacity. Driving underpowered amplifiers to clipping is the common reason for loudspeaker failures. Take note that it is not highly possible to reestablish clipped signals back to their original state due to the fact that part of it was lost. Therefore, avoid clipping as much as possible. But in case, it cannot be totally prevented, repair is the option. The goal of which is to create a plausible replacement in lieu of the clipped part.
Several methods may be applied to achieve partial restoration of a clipped signal. As soon as a clipped portion is made known, then partial recovery may be attempted. One method to do this is called extrapolation or interpolation. While the result may not be that close to its original, at least the quality will be improved. Other method to correct the problem also include the so-called CuteStudio Declip, which directly copies the signal from a stereo channel to other channel, since usually one channel is clipped. Then there are also software solutions that deal with clipping issues like Adobe Audition, Audacity LADSPA, Sony's Sound Forge, Nero Wave Editor, and also the Clipfix from Nyquist plugin.
