page_title: Guitar amplifier
section_title: Power output
hierarchical_section_title: Guitar amplifier / Distortion, power, and volume / Power output
caption_attribution_description: Guitar Amp Setting
context_page_description: A guitar amplifier is an electronic device or system that strengthens the weak electrical signal from a pickup on an electric guitar, bass guitar, or acoustic guitar so that it can produce sound through one or more loudspeakers, which are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier circuits, requiring the use of a separate speaker cabinet–or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. There is a wide range of sizes and power ratings for guitar amplifiers, from small, lightweight "practice amplifiers" with a single 6" speaker and a 10 watt amp to heavy combo amps with four 10” or four 12" speakers and a powerful 100 watt amplifier, which are loud enough to use in a nightclub or bar performance.
  Guitar amplifiers can also modify the instrument's tone by emphasizing or de-emphasizing certain frequencies, using equalizer controls, which function the same way as the bass and treble knobs on a home hi-fi stereo, and by adding electronic effects; distortion and reverb are commonly available as built-in features.
context_section_description: The relationship between power output in watts and perceived volume is not immediately obvious. The human ear perceives a 5-watt amplifier as half as loud as a 50-watt amplifier (a tenfold increase in power), and a half-watt amplifier is a quarter as loud as a 50-watt amp. Doubling the output power of an amplifier results in a "just noticeable" increase in volume, so a 100-watt amplifier is only just noticeably louder than a 50-watt amplifier. Such generalizations are also subject to the human ear's tendency to behave as a natural compressor at high volumes.
  For electric guitar amplifiers, there is often a distinction between "practice" or "recording studio" guitar amps, with output power ratings of less than one watt to 20 watts, and "performance" or "stage" amps of 30 watts or higher. Traditionally, these have been fixed-power amplifiers, with some models having a half-power switch to slightly reduce the listening volume while preserving power-tube distortion.
  Power attenuation can be used with either low-power or high-power amplifiers, resulting in variable-power amplifiers. A high-power amplifier with power attenuation can produce power-tube distortion through a range of listening volumes, but with a decrease in high power distortion. Other technologies, such as dual rectifiers and the sag circuit—which should not be confused with attenuation—allow high power amplifiers to produce low power volume while preserving high power distortion.
  Speaker efficiency is also a major factor affecting a tube amplifier's maximum volume. 
  For bass instruments, higher-power amplifiers are needed to reproduce low-frequency sounds. While an electric guitarist would be able to play at a small club with a 50-watt amplifier, a bass player performing in the same venue would probably need an amplifier with 200 or more watts.
caption: A Marshall JCM 900's knobs for equalization, gain, reverb and volume.
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