Buying Guide:Instrument Amps (Quick Guide)
An amplifier boosts the tiny electronic signal that comes from your guitar, bass, or keyboard then drives a speaker to project the music into the room. The most common type of musical instrument amplifier is called a combo amp. This is a single box which contains the amplifier and a speaker. You simply plug the combo amp into the wall, plug your instrument into the amp, and play. The other type of amp is called a head. An amp head contains only the components that amplify the electrical signal, boosting it to drive an external speaker. The special speaker or speakers come in a box called a speaker cab.
All amplifiers have two circuits or stages—the preamp and the power amp. The preamp boosts the signal from the instrument to a point where other electronics—such as EQ, effects loops, etc.—have something to work with. The power amp builds on the signal from the preamp to generate the real wattage that drives the speakers. By "pushing" or "overdriving" the preamp tubes or circuits, distortion can be produced which increases the sustain of a note played on a guitar or bass. This sustain is desirable in many types of music, especially rock.
An amp's power output is measured in watts, but different amp types and amp/speaker combinations produce different volume at the same wattage. Generally speaking, for a guitar or keyboard amp 1 to 30 watts is considered a small amp, 30-60 is midsized, and over 60 is large. Bass amps require more wattage to drive larger speakers.
Amps which use tubes produce a slightly "warmer" tone preferred by many players while solid state or transistor-based amps are more reliable, generally more affordable, and maintenance free. Many guitar amps have two or more channels to make it easy to switch between distorted and clean sounds. Many amps have built-in special sound effects as well.