GUITAR AMPLIFIER

A guitar amplifier is an electronic amplifier designed for use with an electric or electronic musical instrument, such as an electric guitar.

Guitar amplifiers are manufactured in two main forms. The "combination" (or combo) amplifier contains the amplifier head and guitar speakers in a single unit. The amplifier head or "amp head" contains the electronic circuitry constituting the preamp, built-in effects processing, and power amplifier. In the other form, the amplifier head is separate from the speakers, and joined to them by cables. The separate' amplifier is called an amplifier head, and is commonly placed on top of one or more loudspeaker enclosures. A separate amplifier head placed atop a guitar speaker enclosure or guitar speaker cabinet forms an amplifier stack or "amp stack".

Some amplifiers used with electric guitars are solid state, because they are easy to repair, lighter, and less expensive. Many guitarists, particularly in the genres of blues and rock, prefer the sound of vacuum tube amplifiers despite their drawbacks such as heavy weight, the need to periodically replace tubes and need to re-bias the output tubes (every year or two with moderate use). Some companies design amplifiers that require no biasing as long as properly rated tubes are used. Some modern amplifiers use a mixture of tube and solid-state technologies. Since the advent of microprocessors and digital signal processing, "modeling amps" have been developed in the late 1990s, these can simulate the sounds of a variety of well-known tube amplifiers without always using vacuum tubes. Amplifiers with processors and software emulate the sound of a classic amps almost perfectly, but from the player's point of view the response of these amplifiers is not quite the same due to the digital element of modeling. In blind auditory tests tube amplifier fans may not be able to tell the difference, but most will always choose to play a tube amp because of its analog sensitivity.

A wide range of instrument amplifiers are now available, some general purpose and some designed for specific instruments and even for particular sounds. These include:

• "Traditional" guitar amplifiers, with a clean, warm sound, a sharp treble roll-off at 5 kHz or less and bass roll off at 60–100 Hz, and often built-in reverb and tremolo ("vibrato") units. These amplifiers, such as the Fender "Tweed"-style amps, are often used by traditional rock, blues, and country musicians.Traditional amps have more recently become popular with musicians in indie and alternative bands
• Hard rock-style guitar amplifiers, which often include a preamplification controls, tone filters, and distortion effects that provide the amplifier's characteristic tone. Users of these amplifiers use the amplifier's tone to add "drive", intensity, and "edge" to their guitar sound. Amplifiers of this type, such as Marshall amplifiers, are used in a range of genres, including hard rock, metal, and punk.
• Bass amplifiers, with extended bass response and tone controls optimized for bass guitars (or more rarely, for upright bass). Higher-end bass amplifiers sometimes include compressor or limiter features, which help to keep the amplifier from distorting at high volume levels, and an XLR DI output for patching the bass signal directly into a mixing board. Bass amplifiers are often provided with external metal heat sinks or fans to help keep the amplifier cool.
• Keyboard amplifiers, with very low distortion and extended, flat frequency response in both directions. Keyboard amplifiers often have a simple onboard mixer, so that keyboardists can control the tone and level of several keyboards.
• Acoustic amplifiers, similar in many ways to keyboard amplifiers but designed specifically to produce a "clean," transparent, "acoustic" sound when used with acoustic instruments with built-in transducer pickups and/or microphones. (Note that there was once also a brand of guitar and bass amplifier called Acoustic, still seen second-hand.)

In addition to a 1/4" input jack, some instrument amplifiers have an additional input jack for a microphone, which is easily identified because it will use a three-pin XLR connector. Phantom power is not often provided on general-use amps, restricting the choice of microphones for use with these inputs. However, for high-end acoustic amplifiers, phantom power is often provided, so that musicians can use condenser microphones.

info : wikipedia