The purpose of an amplifier is to receive a small electrical signal and enlarge or amplify it. In the case of a pre-amplifier the signal must be amplified enough to be accepted by a power amplifier. In the case of a power amplifier, the signal must be enlarged much more, enough to power a loudspeaker. Although amplifiers appear to be a mysterious ‘black box’, the basic operating principles are relatively simple. Simply stated, an amplifier receives an input signal from a source (CD player or other source) and creates an enlarged replica of the original smaller signal. The power required to do this comes from the 110-volt wall receptacle. So, an amplifier has three basic connections: an input from the source, an output to the speakers and a source of power from the 110-volt wall receptacle.
The power from the 110-volts is sent to the section of the amplifier known as the power supply where it is converted from alternating current to direct current. Direct current is like the power found in a battery - electrons, or electricity flows in one direction only (alternating current flows in both directions). From the ‘battery’ or power supply the electrical current is sent to a variable resistor, also known as a transistor. The transistor is essentially a valve, like a water valve, that varies the amount of current flowing through the circuit based on the input signal from the source. A signal from the input source causes the transistor to reduce or lower its resistance and allowing current to flow. The amount of current allowed to flow is based on the size of the signal from the input source. A large signal causes more current to flow and results in more amplification than the smaller signal. The frequency of the input signal also determines how quickly the transistor operates. For example, a 100Hz tone from the input source causes the transistor to open and close 100 times per second and a 1,000Hz tone from the input source causes the transistor to open and close 1,000 times per second. So, the transistor controls level (or amplitude) and frequency of the electrical current sent to the speaker, like a valve, and this is how it achieves its amplifying action.
Add a potentiometer, also known as a volume control to the system and you have an amplifier. The volume control allows the user to control the amount of current that goes to the speakers and thus the volume level. There are different types and designs of amplifiers, but essentially they all operate in this manner.