Power Output
For electric guitar amplifiers, there is often a distinction between "practice" or "recording studio" guitar amps, which tend to have output power ratings of 20 watts down to a small fraction of a watt, and "performance" amps, which are generally 50 watts or higher. Traditionally, these have been fixed-power amplifiers, with a few models having a half-power switch to slightly reduce the listening volume while preserving power-tube distortion. The relationship between perceived volume and power output is not immediately obvious. A 5-watt amplifier is perceived to be 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 power of an amplifier results in a "just noticeable" increase in volume, so a 100-watt amplifier is held to be 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.
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 wide range of listening volumes. 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.
Read more about this topic: Guitar Amplifier, Distortion, Power, and Volume
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