Combo amps (and amps in general) come in a variety of sizes and shapes, but certain functions are common to most models. These are input sockets, individual channel volume controls, and tone controls, plus a master volume control.

There are two basic stages in producing a sound through a combo amp: the preamp, which controls the input volume and tone, and the power amp, which controls the overall volume. Most of the tone colors are created during the preamp stage.

Input Socket

The input socket takes the signal from the guitar and sends it through the cord that connects the guitar to the amp via a jack socket. If there is more than one input socket, other instruments, such as a second guitar, or a microphone can be fed into the amp at the same time through these other input sockets.

Volume and Tone Controls

Channel input volume controls allows the player to adjust the volume on the amp. It usually boosts the signal from the guitar and passes that signal along to the tone controls.

Channel tone controls can often be as simple as bass and treble controls. More sophisticated models such as the Mesa Boogie can feature a full-blown graphic equalizer. Basically, the controls split the signal into two or more bands, allowing for precise programming of the sound.

Output volume is the final stage of the process. The signal from the pre-amp passes through the power amplifier and is controlled by a master volume knob that controls the signal to the loudspeaker. The master volume controls the volume of the total output to the speaker, regardless of how many input channels are being used.

Other Controls

More sophisticated models have controls that affect distortion, reverb, and tremolo. Speakers come in a wide range of sizes and can be linked together to produce different kinds and volume of sound. Basically, a loudspeaker is the opposite of a microphone. When a string creates a disturbance in the magnetic field around the pickup, the final adjusted signal (having passed through preamp and power amp) is passed to a voice coil, connected to a large diaphragm, often made out of cardboard or some other responsive and flexible material. The coil receives the signal, transmutes that into a magnetic field of its own, and causes the diaphragm and cone to vibrate. The vibration disturbs the surrounding air, and recreates the sound waves that were originally generated by striking the guitar string.

Most guitar amp loudspeakers are rated by their impedance factor, which varies from 8 ohms to 16 ohms. (Impedance is an electrical term. Measured in ohms, it is the total opposition to the flow of the alternating current in a circuit.) It's important that amp output and speaker impedance are matched up. Too high a rating and the overall volume of the combo will be reduced. Standard twelve-inch speakers can be connected together in pairs or quads (though ten-inch and fifteen-inch are also common now), although the way they are connected will affect the overall sound output.