A circuit or a device that increases the power of a signal. Preamplifier : A small signal amplifier that brings a signal or signals up to a common standard which is then further amplified, by far the most common version of this is the control amplifier but in modern usage the 2 are usually synonymous, but lots of specialised pre-amplification devices exist, a Head Amplifier and Phono Preamps are a good examples of this. Control amplifier : A device that acts as the control unit for an audio system, compromising of a number of pre-amplification circuits that interface units with specific output characteristics to a common standard and also allows the users to control which signal is sent to the amplifier and at what strength (i.e. volume). A control amplifier often also allows the user to perform householding tasks such as equalisation or other filtering, re-routing signals to a recording device and so on. In modern times all incoming and outgoing signals are line level which has given birth to a variant of the control amplifier called a line stage in which the device only features an input selector switch and a preamplifier section with volume control. Power amplifier : An amplifier that amplifies a signal to a degree that it can be used to drive a loudspeaker with, a power amplifier is almost invariably has a set amount of amplification so the loudness is controlled by the incoming signal which is typically a control amplifier or a similar preamp such as a mixer. Integrated amplifier : 1) A device that combines the functions of a control and power amplifier into a single unit. 2) A power amplifier built into a device that typically does not feature it such as a mixer or a loudspeaker, that case the device is often referred to as "Active" Passive pre/control amplifier/line stage Not an amplifier at all but a passive functional replacement for a control amplifier, basically you get a number of switches that allow you to choose between inputs and an attenuator, usually a variable resistor, a bank of switched resistors or a transformer, that allows you to control the strength of the signal that is delivered to the power amplifier and thus the volume coming out of the speakers. This sort of a design takes advantage of the fact that almost all modern audio and AV devices feature line level outputs and thus do not need amplification or equalisation prior to being sent to a power amp. There also exist minimal devises that have just an attenuator and no switching capabilities, these are useful only for systems with only one source component.

This definition is in context to Audio Technology. See more contextual defintions for Amplifier.