What Are Stereo Amplifiers and How Do They Work?

A cable being plugged into the back of a receiver/amplifier
It's worth knowing a bit of how your stereo amplifier works. Kirby Hamilton/Getty Images

It's easy enough to purchase new/replacement stereo components and hook it all up for fantastic results. But have you thought about what makes all of it tick? Stereo amplifiers can be a critical element for best audio performance.

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. An amplifier receives an input signal from a source (mobile device, turntable, CD/DVD/media player, etc.) and creates an enlarged replica of the original smaller signal. The power required to do this comes from the 110-volt wall receptacle. Amplifiers have three basic connections: an input from the source, an output to the speakers, and a source of power from the 110-volt wall socket.

The power from the 110-volts is sent to the section of the amplifier – known as the power supply – where it is converted from an alternating current to a direct current. Direct current is like the power found in batteries; electrons (or electricity) flows only in one direction. 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 (think 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, thereby 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, resulting in greater amplification of the smaller signal. The frequency of the input signal also determines how quickly the transistor operates. For example, a 100 Hz tone from the input source causes the transistor to open and close 100 times per second. A 1,000 Hz 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, just like a valve. This is how it achieves the amplifying action.

Add a potentiometer – also known as a volume control – to the system and you have an amplifier. The potentiometer allows the user to control the amount of current that goes to the speakers, which directly affects the overall volume level. Although there are different types and designs of amplifiers, they all operate in this similar manner.