An amplifier takes an input signal from a source, such as a laptop, turntable or CD player, and creates a larger copy of the original signal before it’s sent to the speakers.
It gets the power to do this from your mains electricity, which is sent directly to the power supply within the amplifier. Here, it’s converted from an alternating current to a direct one, which only flows in one direction, and is sent on to the transistor.
The transistor works like a valve, and determines the amount of current that flows through the circuit at any time. It bases that decision on the size of the input signal from the source. That means a large signal will cause the transistor to allow more current to flow, which in turn creates a greater amplification.
The volume control is the final bit of the puzzle, which decides how much of this current is passed through to the speakers.
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