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 multiplies the amplitudes of all frequencies by the same factor is said to be linear. Departures from linearity lead to various types of distortions.

The operational details of amplifiers are buried in the field of electronics, but for audio purposes it is usually safe to say that current commercial audio amplifiers are so good that a normally operating amplifier is seldom the limitation on the fidelity of a sound reproduction system. One must be sure that the amplifier can provide enough power to drive the existing loudspeakers, but otherwise amplifiers are typically one of the most trouble-free elements of a sound system.

Amplifier Distortion

The amplitudes of all frequencies 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.

In the diagram, the input is a single frequency (pure sine wave), but the output waveform is clipped by the amplifier. The result is that harmonic frequencies not present in the original signal are produced at the output (harmonic distortion). This harmonic distortion contains only odd harmonics if the clipping is symmetrical. For example, a geometrical square wave has only odd harmonics, and as a signal is clipped, it approaches a square wave rather than a sine wave.

The frequency spectrum at right is that measured at the output of a particular amplifier driven above its rated power. The spectrum has a larger amount of odd harmonic than even harmonic output, but the fact that even harmonics are present suggests that the distortion was not symmetrical with respect to the waveform.

An amplifier can be said to be linear if the output voltage is strictly proportional to the input signal. Any nonlinearity, such as that arising from the semiconductor devices themselves, will give rise to harmonic distortion. Such defects in the performance of the devices can be minimized by using negative feedback in the circuit so long as the output is not overdriven to the point of clipping.

Plots of frequency spectra such as those illustrated here can be important diagnostic and research tools. Converting a signal from a plot as a function of time to a plot as a function of frequency is called Fourier analysis, and a common display is the Fast Fourier Transform or FFT of the signal.

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.