Audio power amplifiers are the core element between your source and speakers. They are required in order to drive speakers which have a very low impedance with a high voltage swing. If you were to connect speakers directly to a source then the source most likely would take some damage. Therefore an amplifier not only boosts the signal sufficiently to achieve the desired volume on a set of speakers but also it serves as the protection for your source. The way audio amplifiers work is changed quite a bit in the last decade. I’m going to explain how amplifiers work and what has changed in the last few years.
Nowadays the bulk of audio amplifiers works by using digital modulation. The core element of these switch mode amplifiers is a pair of complementary transistors. These transistors are switched on and off repeatedly. Even if the audio input signal doesn’t change, these transistors are switched constantly. Once there is an audio signal applied to the input of the amplifier, the signal which is modulating the power stage changes. This a change in time of the switching signal is either analog or digital. The frequency of the switching signal also might change depending on the technology.
Obviously, connecting the output of this power stage to a long speaker wire would cause a lot of electromagnetic interference. In addition, there’s a lot of energy located at high frequencies. Therefore, the signal coming from the power stage is usually filtered by a first-order low-pass filter. Following this filter, there still is usually some residual noise. However, this noise is not harmful to the speaker. Also, the frequency of this noise is similar to the switching frequency of the modulating signal. Speakers are not able to reproduce such high frequencies and thus simply ignore this signal.
However, the switching itself is not totally linear. That means that due to the working principle of switch mode amplifiers, there will be a certain amount of distortion. In order to reduce distortion, some amplifier manufacturers have implemented circuits which take the output signal and feedback to the input for comparison. This comparison will show how much distortion excess. By using a suitable circuit, this error signal is then used to compensate for the distortion. However, since the signal is being fed back to the input, there’s a chance for instability of the amplifier. Therefore, designers of such circuits have to be very careful.
Nonetheless, switch mode audio amplifiers have almost entirely replaced analog amplifiers. The main reasons are the higher power efficiency as well as the small size.