Most electric guitars are completely passive. That is, they consume no power, and you don't have to plug them into a power supply. (Some do have "active" electronics powered by an onboard battery.) The vibration of the strings produces a signal in the pickup coil. That bare, unamplified signal is what comes out of the guitar and into the amp.
The amp's job is to take the guitar's signal and make it audible by boosting it enough to drive a speaker. The fascinating thing about an electric guitar amp is that the amp is actually a part of the instrument.
The role of an electric guitar amp is completely different from the amplifier in a stereo system. A stereo amp is meant to be transparent -- its job is to reproduce and amplify sound with as little distortion as possible. With an electric guitar amp, musicians often seek distortion as well as the option of a "clean" sound. Distortion results when the signal in an amp's circuitry is too powerful for that circuitry. The distortion is actually a part of the desired sound, and many amps are designed so that guitarists can control the level of distortion.
Musicians may also take advantage of feedback loops between the amp and the guitar. If the sound coming out of the amp and speaker is loud enough, it can cause the guitar's strings to vibrate. The musician can hit a note with the guitar, and the amp will cause that string to continue vibrating indefinitely. Both of these concepts -- amp distortion and feedback -- are unique to the electric guitar.
You can hear the effects of distortion in these three examples:
A typical amp has at least three parts:
- A pre-amp
- A power amplifier
- A speaker
Some amps also include effects and reverb circuits between the pre-amp and the power amplifier.
The job of the pre-amp is to boost the guitar's signal enough so that it can actually drive the power amplifier stage. Because an electric guitar is passive, its signal does not have enough power to drive the power amp directly.
One of the interesting things about many electric guitar amplifiers is the use of vacuum tubes. Vacuum tubes have distortion patterns and characteristics that are known and loved by many musicians. These musicians seek out tube amps with specific tubes and specific amplifier circuits (for example, Class A versus Class AB amplifiers) to get the exact sound they are looking for.
In the next section, we'll review the history of the electric guitar.