Audio amplifier is an electronic circuit designed to amplify signals, which lie within the audio frequency spectrum. To ensure that these signals are amplified to sufficient power levels, the amplifiers are designed to respond to frequencies above and below the audio frequency, often from 10 to 100000 Hz. Both the transistors and electron tubes are used in audio frequency amplifiers for voltage or power amplification. It consists of an input voltage amplifier stage, whose gain can be varied to provide the desired output signal level, an impedance converter stage to adjust the output impedance of the amplifier to suit the load, which could be a loudspeaker, a pair of headphones or the cutting head in a vinyl disc manufacturing machine. When an audio amplifier is used to build up very week audio signals, it is generally referred to as preamplifier. The preamplifiers find their greatest application with weak audio signal sources, which includes microphones or magnetic tape and pick up. The internal impedance of these devices may be either high or low depending on their type.

Audio transformers are wide band transformers. It has two or more windings coupled by a magnetic field. It is available in two different types such as step-up or step-down transformer and unity 1:1 transformer. It is used to step up or step down a signal voltage and increase or decrease the impedance of a circuit. It converts the circuit from unbalanced to balance. It passes audio signals and block signals that are below or above the audio range of 20 - 20,000 Hz.