An Explanation of the Inner Workings of Modern Amplifiers

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The design of audio amplifiers has changed quite a bit over the last few decades. As a result, modern audio amplifiers are less expensive to manufacture and take up less space. These advances have come at a critical time because home theater systems have become quite popular. These systems employ a number of loudspeakers to reproduce surround sound. For that purpose, more than two amplifiers are required.

Modern AV receivers typically feature at least seven amplifiers which are built into one unit. Due to the small size of modern Class-D amplifiers, all of the amplifiers can be incorporated into a single unit without the receiver becoming huge. In addition, modern amplifiers feature high efficiency. That means that only a small fraction of the wattage that is consumed by the amplifier is wasted and therefore only very few heat sinks are required. In addition, having less wasted energy means that there has to be less of a concern about ventilation. Obviously, with the amplifier drawing less wattage, the power supply itself can also become smaller.

The reason why modern amplifiers are more efficient is because the audio signal is amplified in a different way. While the input stage of the amplifier is quite similar to that of previous audio amplifiers, the main change happened with the power stage. The power stage is the final stage inside the amplifier where the signal is being delivered in the form that can drive the loudspeaker. New audio amplifiers typically employ a switching mechanism where the power stage is switch between the positive and negative voltage supply rails. This switching happens very fast and the width of each pulse is modulated by using a pulse width modulator. This modulating signal is being generated from the analog signal by using a special technology inside the input stage.

In order to form the output signal, this power stage is followed by a low-pass filter. In most cases, this lowpass filter simply consists of a single coil and a single capacitor. Such a low-pass filter office low complexity but achieves the job of removing unwanted switching frequencies. In most cases, the frequency of these switching components which have to be removed is at least 300 to 400 kHz. This is one magnitude higher than the highest audio signal component.