In rock music and related genres, overdrive is a term used to describe the sound of an amplifier running at high volume, usually deliberately, to the point where distortion (clipping) is clearly audible in the output signal. This distortion may range from a slight added "growl" or "edge" with some increase in sustain, up to a thick, distorted fuzzy sound.
History
In the early history of instrument amplification, the effect was caused by the final amplification stage of a tube amplifier being driven beyond its linear range (literally, "over-driven"). This required the amplifier to be run at high volume, so amplifier manufacturers soon began adding an overdrive ability to the preamplifier stage. This allows the user to select any degree of distortion, whilst the overall volume may be controlled independently. Overdrive is predominantly used with an electric guitar. In the 1960s, bands such as the Beatles used overdrive with the bass guitar to create "fuzz bass." As well, keyboard players use overdrive with the Hammond Organ (played through a Leslie speaker) in the heavy metal bands from the 1960s and 1970s.
Types
The sound of an overdriven tube amplifier is usually considered more acceptable to the ear than the distortion which most hi-fi audio amplifiers create when overdriven. This is usually attributed to the fact that the transfer function of a typical class A amplifier topology is asymmetric. This leads to generation of even harmonics, which are perceived as more musical than the odd harmonics that a hi-fi amplifier will produce. In the 1970s and 1980s, most guitar amplifiers began including an overdrive circuit, to provide a real or simulated tube overdrive sound. As well, many musicians use separate effects units to obtain an overdrive sound, often in guitar pedal or bass effect pedal form which is separate from the amplifier.
