An amplifier is a device that increases the level of an electrical signal without modifying it in any other way.
Modern amplifiers have also been designed to accommodate switching and control functions in addition to carrying out electrical damping of the loud speaker.
A good amplifier should be able to select, control and equalise signals from any desired source and provide smooth and regulated power-supply outlets for running radio tuners, preamplifiers, tape or compact disc players and turntables.
Mono and stereo switching arrangements may also be a useful feature.
Before stereo, amplifiers were mainly a separate single channel fed from a radio tuner or a single channel preamplifier.
With stereo, two electrically separate amplifiers were used on one chassis sharing a common power-pack.
Many modern amplifiers are integrated and include a stereo tuner, control-amplifiers and power supplies.
The replacement of valves with transistors and the use of integrated circuits has enabled the amplifier to be greatly reduced in size, weight and heat dissipation.
The main part of modern amplifiers are taken up with the power supply, control, equalisation and switching functions.
Even though valves have long been superseded, they still have a great following and are easier to design and construct than transistorised amplifiers.
There are many potential problems associated with transistors such as the possibility of thermal-runaway (overheating) and origin-distortion type of non-linearity which may result in the inability to minimise the cross-over distortion in push-pull power output stages.
A transient overload or an output short-circuit may cause transistors to fail more readily than valves.
The introduction of silicon transistors with the development of circuits has helped eliminate many of the problems.