Amplifiers

The task of an audio amplifier is to take a small signal and make it bigger without making any other changes in it. This is a demanding task, because

a musical sound usually contains several frequencies, all of which must be amplified by the same factor to avoid changing the waveform and hence the quality of the sound. An amplifier which amplifies all frequencies and amplitudes by the same factor is said to be linear. Departures from linearity lead to various types of distortions.

Amplifier Distortion

All frequencies and all amplitudes within an amplifier's operating range must be amplified by the same factor to avoid distortion. An amplifier which satisfies this requirement is said to be perfectly linear. If the peaks of the waveform are clipped, this gives rise to what is called harmonic distortion. Another type of distortion is intermodulation distortion, which occurs when different frequencies in the signal mix to produce sum and difference frequencies which didn't exist in the signal. Transient distortion occurs when amplifier components cannot handle the rate of change of the signal, for example in rapid percussive attacks. There is also transient intermodulation distortion (TIM) to which modern integrated circuits are susceptible. Such circuits depend upon feedback for their linearity, but time delays in the feedback can cause intermodulation distortion on fast transients in the signal.

Harmonic Distortion

A common type of amplifier distortion is called harmonic distortion. It can arise if any component in the amplifer clips the peaks of the waveform. A common specification for high fidelity amplifiers is the total harmonic distortion. This distortion may be less than 1%, or even less than 0.5% from 20-20,000 Hz for high quality amplifiers.

Intermodulation Distortion

Non-linearity in amplifier components causes mixing of frequency components to form components at sum and difference frequencies. This intermodulation distortion is particularly troublesome in the reproduction of music because it generates frequencies which were not present in the original music and are thus very noticeable. Harmonic distortion may also be serious, but at least the musical sound probably already had these harmonics present as part of the harmonic content of the sound, so it can be tolerated to a greater degree than intermodulation distortion.