An instrument amplifier is an electronic amplifier designed for use with an electric or electronic musical instrument, such as an electric guitar.
Most Common Forms
Instrument amplifiers come in two main forms. The combination (or combo) amplifier contains both the amplifier and loudspeakers in a single unit. In the other form, the amplifier is separate from the loudspeakers, 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, the amplifier head and loudspeaker enclosures together forming an amplifier stack.
In the case of electric guitars, an amplifier stack consisting of a head and one cabinet is commonly called a half stack, while a head and two cabinets is referred to as a full stack. A head and two cabinets may also be called a double stack or just a stack, depending on what is most common in the particular musical style; A retro heavy metal guitarist would likely just use the term stack, the two cabinets being understood, while a jazz guitarist might use the term double stack, the two cabinets being the exception in this genre. By a double stack, the heavy metal guitarist might well mean two stacks, with a second amplifier head serving as a slave to the first and four cabinets in all. Another name for the "Head & Cab" that comes from the 60's and 70's is "Piggyback". Vox amp stacks could be put on a tiltable frame with casters. Fender heads could be attached to the cab and had "Tilt-Back" legs like some larger Fender combos had.
While most amplifiers that are used with electric guitars are solid state, some purists prefer the sound of vacuum tubes. Some modern amps use a mixture of both technologies, with 1960s vintage vacuum tubes next to integrated circuits. With the advent of microprocessors, there have been new "modelling" amps that don't use vacuum tubes and can simulate a variety of vintage amps. As of 2005, these modelling amps still account for a small minority of amp sales.
The first instrument amplifiers were guitar amplifiers designed for use with electric guitars. Traditional guitar amplifiers provided a great deal of treble boost but had poor high treble and bass response. Some better models also provided effects such as spring reverb and/or an electronic tremolo unit (for information about a debate over nomenclature, see also vibrato unit, electric guitar, tremolo).
In the 1960s guitarists experimented with distortion produced by deliberately overloading (or overdriving) their amplifiers. The Kinks guitarist Dave Davies produced early distortion effects by connecting the output of one amplifier into the input of another, an abuse which the designers could never have imagined (but see Maton). Later, many guitar amplifiers were provided with distortion controls, and fuzz boxes and other effects pedals were engineered to safely and reliably produce these sounds. Today distortion is an accepted part of nearly all styles of electric guitar playing.
Guitar amplifiers were at first used with limited success with bass guitars and electronic keyboards, but it was quickly recognized that other instruments had different requirements than the electric guitar. Much more amplifier power is required to clearly reproduce low-frequency pitches produced by bass guitars and electronic keyboards, especially at high volumes. Reproducing low-frequency pitches also requires a woofer or subwoofer speaker capable of handling low frequencies and a speaker cabinet that is designed for low-frequency output. Speaker cabinets for low-frequency sound reproduction need to be larger and more sturdily built than speaker cabinets for mid-range or high-frequency sounds.
A wide range of instrument amplifiers is 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 "vibrato" units. These amplifiers, such as the Fender "Tweed"-style amps, are often used by traditional rock, blues, and country musicians.
- 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 optimised 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.)
Some amplifiers are designed to fill more than one of these roles, and may have multiple inputs. In addition, for electric guitar amps, there is often a distinction between "practice" amps, which tend to have ratings of 20 watts or less, and "performance" amps, which are generally 50 watts or higher. For bass instruments, higher-wattage amplifiers are needed to reproduce low-frequency sounds. While an electric guitarist would be able to play at a small club with a 50 watt amplifier, a bass player performing in the same venue would probably need an amplifier with 200 or more watts.
Some also have a microphone input, 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 condensor microphones.
Less Common Forms
Typically, an instrument amplifier's preamplifier section provides sufficient gain so that an instrument can be connected directly to its input, and sufficient power to connect loudspeakers directly to its output, both without requiring extra amplification. But other forms are possible.
Another arrangement, often used for public address amplifier systems, is to provide two stages of amplification in separate units. First a preamplifier or mixer is used to boost the instrument output, normally to line level, and perhaps to mix signals from several instruments. The output from this preamplifier is then connected to the input of a power amplifier, which powers the loudspeakers.
Performing musicians that use the "two-stage" approach (as opposed to an amplifier with an integrated preamplifer and power amplifier) often want to custom-design a combination of equipment that best suits their musical or technical needs, and gives them more tonal and technical options. Some musicians require preamplifiers that include specific features. Acoustic performers sometimes require preamplifiers with "notch" filters (to prevent feedback), reverb, an XLR DI output, or parametric equalization. Hard rock, metal, or punk performers may desire a preamplifier with a range of distortion effects. As well, some musicians have specific power amplifier requirements, such as low-noise design, very high wattage, the inclusion of limiter features to prevent distortion and speaker damage, or biamp-capable operation.
With the "two-stage" approach, the preamplifier and power amplifier are often mounted together in a rack case. This case may be either free-standing or placed on top of a loudspeaker cabinet. If many rack-mounted effects are used, the rack may be a large unit on wheels. Some touring players need several racks of effects units to reproduce on stage the sounds they have produced in the studio.
On the other extreme, if a small rack case containing both preamplifier and power amplifier is placed on top of a loudspeaker, the distinction between this arrangement and a traditional amplifier head begins to blur. Another variation is to combine the power amplifier with the loudspeakers cabinet, which is then called a powered speaker, and to use these with a separate preamplifier, sometimes combined into a pedal board.
Preamplifiers are also used to connect very low-output or high-impedance instruments to instrument amplifiers. When piezoelectric transducers are used on upright bass or other acoustic instruments, the signal coming directly from the transducer is often too weak and it does not have the correct impedance for direct connection to an instrument amplifier. Fishman brand preamplifiers are often used with acoustic instruments to resolve these problems.
(Courtesy of Wikipedia)
All guitarists should be aware of the amps made by the major manufacturers. The best guitar amp in our opinion is the Fender Twin Reverb Amp (pictured above) which is available from our store to the left. Just click on the manufacturers links below to visit their informative web sites.