An amplifier head
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
Two combo amplifiers
A full bass stack
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. Most professional
guitarists or experienced players use Tubes however, despite the inconveniences. 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 not only sounded
very nice, but an abuse that 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. It must also be noted that peak output of tube amplifiers is heard as being up to three times louder than similar rated solid
state guitar amps. For example, a 30 watt tube amp can be perceived by the listener to be as loud as a 100 watt solid state amp.
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.
Though instrument amplifiers share with Hi-Fi (high fidelity) stereo amplifiers the commonality of amplifying music audio signals, they
differ in design; Hi-Fi amplifiers ultimately aim at reproducing the source sound signals at very high fidelity, whereas instrument
amplifiers aim at creating sound. Because of this, distortions or dissimilarities between the input and output signals are not only
acceptable but also preferred as long as the final output sound is favorable to the player. In fact, many instrument amplifiers today
deliberately add distortions by aggressively processing the input signal.
Until 1960s or so, high output power amplifiers were preferred especially for large concerts, but, as large scale music public address
equipment that even amplifies instrument amplifer output becomes common, high power is no longer a must for high-end instrument amplifiers
today. Modern instrument amplifiers concentrate more on sound creation than public address. This separation also helps concert stage mangement
especially with various attractions and omnibus players.
Vacuum tube amplifiers
Vacuum tubes were by far the dominant active electornic components in most amplifier applications until 1960s when semiconductor transistors
started taking over for performance and economical reasons. High-end vacuum tube instrument amplifiers have survived as one of few exceptions
for many players like their warm sounds.
A typical tube amplifier circuit is quite simple. Two triodes work in the preamplifier section for gaining the enough signal level to drive
the power output section as well as implementing tone controls. Another two triodes drive pentode power tube in a push-pull connection in
the output stage. Some high power models use paralled output tubes (4 or more in total). Except for the light NFB from the secondary end of
the output transformer to the driver stage, all amplifying stages work in "raw" mode. Since most tubes show "soft clipping" gain non-linearity,
applying overdrving input signal tends to produce favorably natural distortion. Today, most vaccuum tube amplifiers are based on the ECC83/12AX7
(dual triode) tubes for the preamplifier and driver sections and the EL84/6L6 (pentode) tube for the power output section. Some use the KT88
power tubes. These tubes are mainly manufactured in Russia, China and Eastern European countries.
Tube instrument amplifiers are often equipped with lower grade-looking tranformers and simpler power regulation circuits than those of Hi-Fi
amplifiers. They are usually not only for cost-saving reasons, but also are considered for sound creation. For example, a simple power regulation
circuit's output tends to sag when there is a heavy load (i.e. high output power) and vacuum tubes usually lose gain factors with lower power
voltages; this results in somewhat compressed sound volume and it could be criticised as a "poor dynamic range" in case of Hi-Fi amplifiers, but
it can be favorably accepted as "good compression" or "long sustain" of sounds on a guitar amplifier. Some tube guitar amplifiers use a rectifier
tube possibly for this reason.
Some models have a "spring reverb" unit that simulates the reverberation of an echoic ambient. A reverb unit usually consists of one or more coil
springs driven by the preamplifier section using a similar driver for a loudspeaker at one end and a electro-magnetic pickup and a preamplifier
stage at the other end that picks up the long sustaining spring vibration that is then mixed with the original signal.
A few amplifiers have a tremolo control. An internal oscillator generates a low frequency continuous signal that modulates the input signal's
amplitude simulating a tremolo effect.
Tube amplifiers have disadvantages that provide some reason for replacement by semiconductor technology.
- Bulky and heavy (power and output transformers)
- Generates much heat
- Components wear faster (because of heat and high voltage). High voltage also attracts dust that may cause electic leakage or even a fire.
- Tubes wear (like light bulbs)
- Prone to pick up mechanical noises (microphonic noise) (but some say the acoustic feedback path from the loudspeaker to the tubes in a
combo amplifier also contributes to sound creation)
Solid state amplifiers
The vast majority of instrument amplifiers produced these days are based on semiconductor (solid state) circuits. Many high-end (high quality)
products, however, continue to employ varying degrees of vacuum tube technology. Solid state products vary in output power, functionality, size,
price, and sound quality in a wide range from practice amplifiers to professional models. Many budget-oriented entry level models have a simple
volume level control and simple tone control knobs, but some, notably guitar amplifiers, have a "gain" control knob for adding distortion.
Many medium range guitar amplifiers sold today have a mode control switch for selecting sound of multiple famous vintage amplifiers to simulate.
Most modeling amplifiers digitize the input signal and use a DSP, a dedicated microprocessor, to process the signal with digital computation.
Amplifier configuration and set up
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.
Some major instrument amplifier manufacturers (alphabetical)
- Alembic preamplifiers and filters
- Ashdown Engineering
- Bad Cat
- B.C. Rich
- Carvin A&I
- Hughes & Kettner
- Laney Amplification
- Line 6
- Randall Amplifiers
- Tech 21
- Tone King
- Trace Elliot
- Victoria Amplification
- Ultrasound Amplifiers
This article is licensed under the GNU Free Document License
It uses material from the Wikipedia article - Instrument amplifier