When playing an acoustic guitar, the vibration of the strings transferred to the soundboard - the guitar's top - produces the sound we hear. All this takes place within the guitar itself.

With an electric guitar, the instrument is still used to create the basic sound, but an amplifier is added to carry the electrical signal from the guitar (see information on "Pick-ups") to speakers in order to produce the louder (and altered) sounds we hear. The amplifier and speakers take the place of the acoustic's soundboard, using its components to transmit the energy (the sound) we hear. Because they are such an important part of playing an electric guitar, the amplifier and speakers are the electric guitar's "other half."

Guitar, Bass, and Keyboard Amps

Unlike amps made for basses or keyboards, the electric guitar amplifier has an equalizer centered around the frequency range of the guitar. The speakers for a guitar amp are also designed to provide better distortion in the guitar's range. The guitar amp's pre-amp has more controls - treble, middle, bass, gain (or pre), reverb, graphic equalizers, etc.

Bass amps and keyboard amps are designed to produce a cleaner, undistorted signal. Because basses are tuned an octave lower than guitars and therefore require more power to project their lower frequencies, bass amplifiers need about twice as much power to compete with the sounds produced by a guitar. The speakers in these amps are made to withstand lower frequencies produced by basses and keyboards - frequencies that have the potential to destroy a guitar speaker. The equalization for the amps is centered around the most usable range for basses and keyboards and normally comes with only one volume control.

Acoustic guitar amplifiers are also designed with more durable speakers to help replicate the unique qualities of the acoustic instrument. A very useful feature - called a notch filter - can be used to help eliminate feedback. Acoustic guitar amps often contain an input jack that can be used for a microphone, a popular feature with solo acoustic guitarist/singers.

Parts of the Amplifier - Amplifiers have three distinct parts, Speaker Cabinet, Pre-amplifier, and Power Amplifier. These parts can be purchased as individual components or in one convenient package known as a "Combo Amp."

The Speaker Cabinet Average speaker sizes for an electric instrument are 8", 10", 12", 15" or 18". Large speakers are capable of producing deeper tones. Small speakers produce a quick response - a player who wants to sound "clean" while playing very fast will often choose smaller speaker sizes. On a combo amp, the speaker cabinet is the enclosure that houses the speaker, the power amplifier, and the pre-amplifier. With larger, non-combo amplifiers, the speaker cabinet contains only the speaker(s) while the power and pre-amplifiers are combined into a unit commonly referred to as the "head."

The Pre-amplifier (pre-amp) receives the signal from the electric instrument and alters the sound through the use of equalization and other effects - the effects available depend on the model amplifier you choose. The pre-amp is easy to identify because it contains all of the controls (except one) on the front of the amplifier - treble, middle, bass, gain (or pre), reverb, graphic equalizers, etc.

The Power Amp is controlled by the Volume Control knob on the front of the amplifier (it may also be called Master, Post, Level, or Master Volume). The power amp is the "meat and potatoes" of any amplification system - it determines how loud the overall sound will be. The "loudness" of an amplifier is measured in Watts - the higher the wattage, the louder the amp. Practice guitar amplifiers range from 10 to 30 watts. Guitar stage amps usually begin at 25 watts and go up to 130 watts.

The Combo Amplifier is the most common amplifier used by beginners and professionals today. It is made up of a speaker cabinet, a power amplifier and a pre-amplifier.

Smaller combo amps often have an open cabinet back creating a louder, more resonant sound. Larger combo amp speaker cabinets are usually sealed, providing more focused projection and better bass response. Combo amps are not the most powerful amps a player can use, but they work well in most practice, session and stage situations.

Stage amps are used to create the player's tone, usually angled toward the band to be used as stage monitors. The sound engineer takes that stage sound and "mixes" it - boosting and balancing - then sending the altered sound through the sound reinforcement (P.A. gear) to the audience.

Originally, amplifiers were powered by vacuum tubes or "valves." The size and type of tube affects the total volume and tone the amp can produce. Tube amps generally produce a warmer, fuller tone than solid state amplifiers. They require more maintenance than solid state amps because the tubes need periodic replacement.

In the never-ending search for efficiency, amp manufacturers replaced vacuum tubes with transistors. This change created a less expensive, solid state amp. Many guitarists felt that the solid state amp didn't "react" to their playing in the same way as the tube amp had; to them the new amps sounded "lifeless." As a result, amplifier manufacturers have worked for years to make an amp capable of producing a better, more tube-like sound.

Today, modern solid state amps have a much more sensitive feel and response. Although more expensive, tube amps are still available today for the discriminating player. Many companies also offer hybrid amps, combining a tube preamp with a solid-state power amp.

Amplifier Tone and Effects

As guitar players began "turning up the volume," amplifiers started clipping and distorting their sounds. At first this was undesirable, but more and more guitarists liked the sound "distortions" and manufacturers looked for ways to make their amps reliably "distort" the guitar's sound. Most modern guitar amps feature this "distortion" as an option by offering two volume controls - The Pre or Gain Control, and the Post or Master Volume Control.

The Pre or Gain Control determines the output volume of the pre-amp. The Post or Master Volume control of the power amp determines the overall output of the amplified signal. By turning the Gain control to a higher level than the Master Volume, the signal becomes "overdriven." This "overdrive" results in the distorted sound.

Equalization (or EQ) is a filter or tone control technique designed to shape sound. It is used to boost specific frequencies, or to remove (cut) or de-emphasize unwanted frequencies. The EQ can be adjusted on most amps with the treble, middle, and bass controls. The treble controls the high end of the frequency spectrum and the bass controls the low end.

Higher priced amplifiers offer a Graphic EQ consisting of five to nine or more sliders - each slider representing a frequency center - allowing the user to boost or cut the signal at these pre-set frequencies. This gives the player a greater ability to control tone than with the usual three controls of treble, middle and bass.

Reverb Spring reverb is a popular sound effect originally developed to make electronic organs richer sounding. Manufacturers began installing it in guitar amps during the late 1950s and early 1960s where it gained popularity as a favorite sound effect used by "surf" bands. Reverb enhances the sound of the guitar, causing it to sound like it's being played in a large room or hall.

Bass Pickups

Like the Electric Guitar, the Electric Bass relies on an electromagnetic transducer commonly referred to as a "pickup" to create its distinctive sound. The pickup converts the vibration of the strings into electrical energy. This energy may be amplified, distorted or changed in a number of ways and then converted back into sound waves by the amplifier. The two most common pickups for electric bass are called Single Coil and Split Coil.

Single Coil - Traditionally this pickup is really just an electron magnet. It is composed of a magnetic object called a pole-piece, with a thin copper wire wrapped around it many times. The most common bass single coil pickup has two pole-pieces per string seated in a "bobbin" which holds them in place on either side of the string. Copper wire is wrapped around the bobbin, and then wired to the volume and tone control knobs on the bass. These, in turn, are wired to the bass' output jack.

Split Coil - This pick-up consists of two bobbins, one for the top two strings, the other for the bottom two. The set of pole-pieces in each bobbin is then wired with opposite polarities. This technique helps eliminate any interfering hum caused by AM radios, florescent lights, microwave ovens, television sets etc. which might also be picked up.

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