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Amplifiers

 The first electric instrument amplifiers were not designed for use with electric guitars. The earliest examples appeared in the early 1930s when the introduction of electrolytic capacitors and rectifier tubes allowed for the production of economical built-in power supplies that could be plugged into wall sockets. Consequently portable vacuum tube amplification equipment was no longer dependent on heavy multiple battery packs for power. While guitar amplifiers from the beginning were used to amplify acoustic guitar, electronic amplification of guitar was first widely popularized by the 1930s and 1940s craze for Hawaiian music, which extensively employed the amplified lap steel Hawaiian guitar.

 Instrument amplifiers are designed for a different purpose than 'Hi-Fi' (high fidelity) stereo amplifiers used for radios and home stereo systems. Hi-fi home stereo amplifiers are designed to accurately reproduce the source sound signals from pre-recorded music, with as little harmonic distortion as possible. In contrast, an instrument amplifier is an electronic amplifier that converts the inaudible electric or electronic signal from musical instruments into sounds which can be heard by the performers and audience but unlike home "hi-fi" amplifiers or public address systems, instrument amplifiers are often designed to add additional tonal coloration to the original signal or emphasize (or de-emphasize) certain frequencies. The two exceptions are keyboard amplifiers and "acoustic" instrument amplifiers, which typically aim for a relatively flat frequency response.

 Guitar amplifiers were at first used with bass guitars and electronic keyboards where much more amplifier power is required to clearly reproduce low-frequency pitches produced by these instruments, especially at higher volumes. Reproducing low-frequency pitches also requires a woofer or sub-woofer capable of handling low frequencies and a speaker cabinet that is designed for low-frequency reproduction. Such speaker cabinets need to be larger and more sturdily built than speaker cabinets for mid-range or high-frequency sounds. Today instrument amplifiers are available for specific instruments, including the electric guitar, electric bass, electric keyboards, and acoustic instruments such as the mandolin and banjo. Some amplifiers are designed for specific styles of music, such as the Fender "traditional"-style "tweed" guitar amplifiers used by blues and country musicians, and the Marshall amplifiers used by hard rock and heavy metal bands.

Vox Combo Amp

 

 

Combination ("combo") amplifiers include a preamplifier, a power amplifier, tone controls, and one or more speakers in a cabinet, a housing usually made of plywood, particle board, or, less commonly, molded plastic. Instrument amplifiers for some instruments are also available without an integral speaker; these amplifiers have to be plugged into an external speaker cabinet. Combo amplifiers range from small practice amplifiers with one 6" or 8" speaker and a 10 to 15 watt amplifier, to mid-sized combo amps with a 12" speaker and a 50 watt amplifier (suitable for rehearsals or performances in small venues), to large combo amplifiers with four 12" speakers and 100 or more watts of power, which can be used for shows in large clubs or halls. Combo amps have at least one 1/4" input jack where the patch cord from the electric guitar can be plugged in. Other jacks may also be provided, such as an additional input jack, "send" and "return" jacks to create an effects loop (for connecting electronic effects such as compression, reverb, etc), an extension speaker jack (for connecting an additional speaker cabinet). Some smaller practice amps, stereo RCA jacks for connecting a CD player or other sound source and a 1/4" headphone jack so that the player can practice without disturbing neighbors or family members.

 

 

 

Instrument amplifiers may be based on thermionic ("tube" or "valve") or solid state (transistor) technology.

Tube Amplifiers

 Vacuum tubes (valves) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s, when semiconductors (transistors) started taking over for performance and economic reasons, including heat and weight reduction, and improved reliability. Some musicians believe that tube amplifiers produce a "warmer" or more "natural" sound than solid state units and tube amplifiers continue to be preferred by some professional musicians.
High-end tube instrument amplifiers have survived as one of few exceptions, because of the sound quality. Typically, one or more dual triodes are used in the preamplifier section in order to provide sufficient voltage gain to offset losses by tone controls and to drive the power amplifier section. The output tubes are often arranged in a class AB push-pull connection to improve efficiency; this requires another triode or dual triode to split the phase of the signal. The tubes of the power amplifier stage are almost always of the pentode or beam tetrode type (also known as "kinkless tetrodes", hence the KTxx nomenclature). Some high power models use paralleled pairs of output tubes (4 or more in total) in push-pull. Except for the light negative feedback from the secondary end of the output transformer to the driver stage, most amplifying stages work in "raw" open-loop mode. Some designs employ current feedback via un-bypassed cathode resistors.6L6 Power Tube12AX7 Preamp Tube

 

 

Since most tubes show "soft clipping" gain non-linearity, applying an input signal high enough to overdrive any stage tends to produce favorably natural distortion. Today, most vacuum tube amplifiers are based on the ECC83/12AX7/7025 (dual triode) tubes for the preamplifier and driver sections and the EL84/6BQ5 or EL34/6CA7/KT77 or 6L6/KT66 or 6V6 tubes for the power output section. Some use the KT88/6550 beam power tubes in the output stage. The differing codes for equivalent tubes generally reflect those used by the original European or U.S.A. based manufacturers. These tubes are now mainly manufactured in Russia, China and Eastern European countries. Some amplifiers, such as the Marshall Silver Jubilee, use solid state components in the pre-amp, most commonly diodes, to create distortion, a design feature known as diode clipping.

 

 

 

 

 Tube instrument amplifiers are often equipped with lower-grade transformers and simpler power regulation circuits than those of hi-fi amplifiers. They are usually not only for cost-saving reasons, but also are considered as a factor in sound creation. For example, a simple power regulation circuit's output tends to sag when there is a heavy load (that is, high output power) and vacuum tubes usually lose gain factors with lower power voltages. This results in a somewhat compressed sound which could be criticized as a "poor dynamic range" in case of hi-fi amplifiers, but could be desirable as "long sustain" of sounds on a guitar amplifier. Some tube guitar amplifiers use a rectifier tube instead of solid-state diodes specifically for this reason.

Tube Amp

 

 

Rear view of a tube (valve) bass guitar amplifier. Visible are glass output tubes, smaller pre-amp tubes, and both the power transformer and the output transformer.

 

Tube amps have the following disadvantages in comparison to solid-state amps. They are bulky and heavy, primarily due to the iron in power and output transformers. Solid-state amplifiers still require power transformers, but are usually direct-coupled and don't need output transformers. Glass tubes are fragile, and require more care and consideration when equipment is moved repeatedly. Tube performance can deteriorate slightly over time before eventual catastrophic failure. When tube vacuum is maintained at a high level, though, excellent performance and life is possible. They are prone to pick up mechanical noises (microphonic noise), although such electro-mechanical feedback from the loudspeaker to the tubes in combo amplifiers may contribute to sound creation. Tubes benefit from a heater warm-up period before the application of high tension anode voltages, this allows the tube cathodes to operate without damage and so prolongs tube life. This is also of particular importance for amplifiers with solid state rectifiers. Tube amplifiers produce more heat than solid state amplifiers, tubes need to attain a proper operating temperature, for this reason few manufacturers use fans for cooling tubes. If the temperature goes above this operating temperature, it may shorten the tubes' life span and lead to tonal inconsistencies.

 Tube amps have the following advantages over solid-state amps. Compared to semiconductors, tubes have a very low "drift" (of specs) over a wide range of operating conditions, specifically high heat/high power. Semiconductors are very heat-sensitive by comparison and this fact usually leads to compromises in solid-state amplifier designs. When a tube fails, it is replaceable. While solid state devices are also replaceable, it is usually a much more involved process (i.e., having the amplifier tested by a professional, removing the faulty component, and replacing it). For working musicians this is usually a huge problem by comparison to looking in the back of a tube amp at the tubes and simply replacing the faulty tube. In addition, tubes can easily be removed and tested, while transistors cannot. Tube amplifiers respond differently from transistor amplifiers when signal levels approach and reach the point of clipping. In a tube-powered amplifier, the transition from linear amplification to limiting is less abrupt than in a solid state unit, resulting in a less grating form of distortion at the onset of clipping. For this reason, some guitarists prefer the sound of an all-tube amplifier; the aesthetic properties of tube versus solid state amps, though, are a topic of debate in the guitarist community.

 

Solid State Amplifiers

 Most low-end guitar amplifiers currently produced are based on Semiconductor (solid state) circuits, and some designs incorporate tubes in the pre-amp stage for their subjectively warmer overdrive sound. Tubes create warm overdrive sounds because instead of cutting the peaked signal off, they more or less pull the peaked audio information back (like natural compression) which creates a fuzzy overdrive sound. While this is a desirable attribute in many cases, the tube's characteristic will "color" all the sounds at any volume, unlike solid state. However, solid state in general have the quickest response time, perhaps even more so than modeling amps.

 High-end solid state amplifiers are less common, since many professional guitarists tend to favor vacuum tubes. Some jazz guitarists, however, tend to favor the "colder" sound of solid-state amplifiers, preferring not to color the sound of their guitar with the tube distortion and compression so popular with rock, blues, and metal musicians. Solid-state amplifiers vary in output power, functionality, size, price, and sound quality in a wide range, from practice amplifiers to professional models. Some purist or inexpensive amplifiers have only volume and tone controls.

 By the 1960s and 1970s, semiconductor transistor-based amplifiers began to become more popular because they are less expensive, lighter-weight, and require less maintenance. In some cases, tube and solid state technologies are used together in amplifiers. A common setup is the use of a tube preamplifier with a solid state power amplifier. There are also an increasing range of products that use digital signal processing and digital modeling technology to simulate many different combinations of amp and cabinets.

 The output transistors of solid state amplifiers can be passively cooled by using metal fins called heat sinks to radiate away the heat. For high-wattage amplifiers (over 800 watts), a fan is often used to move air across internal heat sinks.

 

Modeling amplifiersLine 6 Modeling Amp

 

Modeling amplifiers simulate the sound of well-known guitar amps, cabinets, and effects, as well as simulating the way traditional speaker cabinets sound when captured with different types of microphones. This is usually achieved through digital processing, although there are analog modeling amps as well, such as the Tech 21 Sansamp. Modeling technology offers several advantages over traditional amplification. A modeling amp typically is capable of a wide range of tones and effects, and offers cabinet simulation, so it can be recorded without a microphone. Most modeling amps digitize the input signal and use a DSP, a dedicated microprocessor, to process the signal with digital computation. Some modeling amps incorporate vacuum tubes, digital processing, and some form of power attenuation. The Pod series from Line 6 combines amp, cab, and effect models into a small unit without its own speaker cabinet.

 

 

 

Amplifier Configuration and Arrangements

Fender Stack

Marshall StackIn the case of electric guitars, an amplifier stack consisting of a head atop one cabinet is commonly called a half stack, while a head atop two cabinets is referred to as a full stack. The cabinet which the head sits on often has an angled top in front, while the lower cabinet of a full stack has a straight front. The first version of the Marshall stack was an amp head on an 8x12 cabinet, meaning a single speaker cabinet containing eight 12" guitar speakers. After six of these cabinets were made, the cabinet arrangement was changed to an amp head on two 4x12 cabinets, meaning four 12" speakers, to enable transporting the amp rig.

In heavy metal bands, the term "double stack" or "full stack" is sometimes used to refer to two stacks, with a second amplifier head serving as a slave to the first and four speaker cabinets in total. Another name for the "Head & Cab" that comes from the 1960s and 1970s is "Piggyback". Vox amp stacks could be put on a tilt-able frame with casters. Fender heads could be attached to the cab and had "Tilt-Back" legs, like those used on larger Fender combo amps. Typically, a guitar amp'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.

 

 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 preamplifier 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 pre-amps that include specific features. Acoustic performers sometimes require pre-amps 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. At the other extreme, if a small rack case containing both pre-amp and power amp is placed on top of a guitar speaker cabinet, the distinction between a rack and a traditional amp head begins to blur. Another variation is to combine the power amplifier into the speaker cabinet, an arrangement called a powered speaker, and use these with a separate pre-amp, sometimes combined into an effects pedal board or floor pre-amp/processor.

 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. Small, battery-powered pre-amps are often used with acoustic instruments to resolve these problems.

Power output

 For electric guitar amplifiers, there is often a distinction between "practice" or "recording studio" guitar amps, which tend to have output power ratings of 20 watts down to a small fraction of a watt, and "performance" amps, which are generally 50 watts or higher. Traditionally, these have been fixed-power amplifiers, with a few models having a half-power switch to slightly reduce the listening volume while preserving power-tube distortion. The relationship between perceived volume and power output is not immediately obvious. A 5-watt amplifier is perceived to be half as loud as a 50-watt amplifier (a tenfold increase in power), and a half-watt amplifier is a quarter as loud as a 50-watt amp. Doubling the power of an amplifier results in a "just noticeable" increase in volume, so a 100-watt amplifier is held to be only just noticeably louder than a 50-watt amplifier. Such generalizations are also subject to the human ear's tendency to behave as a natural compressor at high volumes.

 Power attenuation is being built into both low-power and high-power amplifiers, resulting in variable-power amplifiers. A high-power amplifier with flexible power attenuation built-in can produce power-tube distortion through the widest range of listening volumes. Speaker efficiency is also a major factor affecting a tube amplifier's maximum volume. For bass instruments, higher-power 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. 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 amplifier can be perceived by the listener to be as loud as a 100-watt solid state amplifier, particularly when both are driven into maximum distortion.

Distortion and volume

 Distortion is a feature available on many guitar amplifiers that is not typically found on keyboard or bass guitar amplifiers. Tube guitar amplifiers can produce distortion through pre-distortion equalization, pre-amp tube distortion, post-distortion EQ, power-tube distortion, tube rectifier compression, output transformer distortion, guitar speaker distortion, and guitar speaker and cabinet frequency response. Distortion sound or "texture" from guitar amplifiers is further shaped or processed through the frequency response and distortion factors in the microphones (their response, placement, and multi-microphone comb filtering effects), microphone pre-amps, mixer channel equalization, and compression. Additionally, the basic sound produced by the guitar amplifier can be changed and shaped by adding distortion and/or equalization effect pedals before the amp's input jack, in the effects loop just before the tube power amp, or after the power tubes.

Power-tube distortion

 Power-tube distortion is required for amp sounds in some genres. In a standard master-volume guitar amp, as the amp's final or master volume is increased, more power tube distortion is produced. This is slow-onset distortion. As such a tube (or valve) power amp rated at 50 watts produces 50 clean watts or, when pushed harder, twice as many distorted watts. To make the amount of power-tube distortion independent from the volume heard at the guitar speaker, several approaches are used:

 The "power soak" approach places the attenuation between the power tubes and the guitar speaker. This pushes the power tubes to full power and then drains away the unwanted excess power by sending it to a mostly resistive dummy load and allowing only a portion through to the guitar speaker. Some power soak products include the Rockman Power Soak, the THD Hot Plate, the Weber MASS, and the Marshall Power Brake. The power-supply approach places the attenuation in the power supply and power amp; it runs the power tubes at a lower plate voltage and lets the full resulting power through to the guitar speaker. This prevents wearing-out the tubes or blowing the output transformer, and obviates purchasing and transporting a separate, bulky power attenuator. Some power supply attenuation circuits include London Power's Power Scaling and Mojave's Power Dampening circuit.

 The Maven Peal Sag Circuit is the only patented power-supply that applies nonlinear analysis to power-supply design instead of the tradition attenuation approach. As a result, the Sag Circuit offers power tube distortion and response control at reasonable volumes, along with other features such as a built-in power conditioner and zero power supply hum. Unlike other attenuation approaches, the Sag Circuit adds power amp distortion and response control to transistor amps as well as tube-based amps.

 In the pre-amped dummy load approach, the tube power amp drives a mostly resistive dummy load rather than a guitar speaker. The dummy load is typically the internal dummy load inside a power attenuator, with the load selector switched all the way to internal load rather than a mix of internal load and guitar speaker. A line-level signal is tapped from the dummy load, optional signal processing (such as equalization and reverb) is applied, and then the signal is sent through the final amplifier, typically solid-state run only in the linear region, which finally drives a guitar speaker. The guitar speaker contributes complex physical transducer dynamics at a quiet listening level controlled by the final amp, or even more complex dynamics if the speaker is pushed into distortion.

 In the isolation box approach, the guitar amplifier is used with a guitar speaker in a separate cabinet. A soundproofed isolation cabinet, isolation box, isolation booth, or isolation room can be used. The guitar speaker and microphones are placed inside a soundproofed space separate from the control room or listening area, such as in a double-walled isolation room in a different floor of a house. A speaker cable runs from the tube guitar amplifier in the control room out to the miked guitar speaker cabinet out in the separate isolated area. The microphone cables then run back from the isolated area back into the control room. Professional recording studios often use this approach to obtain power-tube distortion independently of the listening volume.

Volume controls

 A variety of labels are used for level attenuation potentiometers in a guitar amplifier and other guitar equipment. Electric guitars and basses have a volume control to attenuate whichever pickup is selected. There may be two volume controls in parallel to mix the signal levels from the neck and bridge pickups. Rolling back the guitar's volume control also changes the pickup's equalization or frequency response, which can provide pre-distortion equalization.

 The simplest distortion effect pedals have a volume and a distortion control. The volume control is a potentiometer at the output jack of the distortion pedal. The distortion control affects the driver amplitude prior to the clipping stages, so it affects the amount of clipping much more than it affects the output level. However, some distortion controls also change the amount of bass or treble. More complex distortion effect pedals may also add effects such as delay, additional harmonics, or octave generation. The simplest guitar amplifiers have only a volume control. Most have at least a gain control and a master volume control. The gain control is equivalent to the distortion control on a distortion pedal, and similarly may have a side-effect of changing the proportion of bass and treble sent to the next stage.

 A simple amplifier's tone controls typically include passive bass and treble controls. In some cases, a midrange control is provided. The amplifier's master volume control restricts the amount of signal permitted through to the driver stage and the power amplifier. When using a power attenuator with a tube amplifier, the master volume no longer acts as the master volume control. Instead, the power attenuator's attenuation control controls the power delivered to the speaker, and the amplifier's master volume control determines the amount of power-tube distortion. Power-supply based power attenuation is controlled by a knob on the tube power amp, variously labeled "Wattage", "Power", "Scale", "Power Scale", or "Power Dampening".

 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 transducer driver similar to a loudspeaker at one end and an electro-magnetic pickup and preamplifier stage at the other end that picks up the long sustaining spring vibration, which is then mixed with the original signal. Some guitar amplifiers have a tremolo control. An internal oscillator generates a low frequency continuous signal that modulates the input signal's amplitude, producing a tremolo effect.Fender Tweed

 

 Musicians often run sound-sources other than guitars through guitar amps. For live performances, synthesizers and drum machines or keyboards are often put through guitar amps to create a richer sound than can be obtained by patching the direct-outs right into the PA system. Guitar amplifiers can add tonal coloration, roll off unwanted high frequencies, and add overdrive or distortion. Deep Purple's Jon Lord played his Hammond Organ through a distorted Marshall amp to create a sound more suitable for heavy rock. String instruments and vocals are also put through guitar amps to add distortion effects. Some blues harp players also use guitar amps to create a warmer overdrive sound for their harmonica playing; 1950s-style "Tweed" amps are often used for this purpose, such as Fender Bassman combo amps.

Recording engineers occasionally run pre-recorded parts through miked guitar amps, a process called re-amping. When a guitar part is recorded "dry" (e.g., without effects or distortion), straight into the mixing board for a recording, this gives the producer and mixing engineer much more flexibility to create new re-mixes or new tones from the recording. If a guitar player records an electric guitar part that is run through a chorus pedal and a distortion pedal, there is little that can be done at the "mix-down" stage to change the sound of this recording, beyond "tweaking" the equalization and modifying the level. Since re-mixing or mix-down can take place weeks, months, or even years after the original recording session, it may be impossible to have the guitarist come in to re-record a new part. If the dry guitar sound is recorded, though, the mixing engineers can add any effects they want to the signal and then re-play it through a miked guitar amplifier which is being recorded. The effects, amplifiers, cabinets, and miking processes can be changed to any combination. When a dry guitar has been recorded, it can be a useful tool for "updating" an older recording. For example, if a band wants to re-release a 1980s-era album on which the guitar has a very dated 1980s sound, with heavy flanging and artificial-sounding electronic distortion, the band can update the guitar sound by re-amping the dry signal and using 2000s-era effects.

Mixing guitar amp signals with other signals is also done by some musicians. Chris Squire of Yes produced his bass guitar sound by playing through a guitar amplifier with its bass turned down, treble turned up, and volume turned up well into distortion; the miked guitar speaker signal was then mixed with a direct-input (DI) signal, a technique that has been used for processing synth keyboards as well. A bass guitar can also be played through a bass amp in parallel with a distorted guitar amp by using a DI box; the bass amp provides the low frequencies, while the guitar amp--which is not capable of reproducing the lowest frequencies of the bass guitar--emphasizes the upper harmonics of the instrument's tone.

 

Bass

Hartke Bass AmpBass amplifiers are designed for bass guitars or more rarely, for upright bass. They differ from amplifiers for the regular electric guitar in several respects. They have extended bass response and tone controls optimized for bass instruments, which produce pitches of 40 Hz, in the case of a standard four-string electric bass, or even lower for five- or six-string electric basses.

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 or PA systems. Larger, more powerful bass amplifiers (300 or more watts) are often provided with external metal heat sinks or fans to help keep the amplifier cool.

A 2 x 10" bass speaker cabinet stacked on top of a 15" cabinet, with separate bass amplifier "head" unit

Speaker cabinets designed for bass instrument amplification usually use larger loudspeakers (or more loudspeakers, in the case of the popular 4 X 10" cabinets, which contain four 10" speakers) than the cabinets used for other instruments, so that they can move the larger amounts of air needed to reproduce low frequencies. While the largest speakers commonly used for regular electric guitar are 12" speakers, electric bass speaker cabinets often use 15" speakers. Bass players who play styles of music that require an extended low-range response, such as death metal, sometimes use speaker cabinets with 18" speakers.

The speakers used for bass instrument amplification tend to be more heavy-duty than speakers used for regular electric guitar, and the speaker cabinets are typically more rigidly constructed and heavily braced, to prevent unwanted buzzes and rattles. Bass cabinets often include bass reflex ports or openings in the cabinet, which improve the bass response, especially at high volumes.

Keyboard

This type of amplifier is used to amplify a range of electric and electronic keyboards, such as synthesizers, Hammond organ-style keyboards, stage pianos and electric pianos. Since keyboard instruments contain a wide frequency range, from very low bass notes to extremely high pitches, keyboard amplifiers are often provided with a large woofer speaker to handle the low notes and a horn (or tweeter) for the high notes.Behringer Keyboard Amp

 

Keyboard amplifiers intended for general use for a range of keyboard applications usually have very low distortion and extended, flat frequency response in both directions. The exception to this rule is keyboard amplifiers designed for the Hammond organ, such as the vintage Leslie speaker cabinet and modern recreations, which have a tube amplifier which is often turned up to add a warm, "growling" overdrive to the organ sound.

Unlike bass amplifiers and electric guitar amplifiers, keyboard amplifiers are rarely used in the "amplifier head" and separate speaker cabinets configuration. Instead, most keyboard amplifiers are "combo" amplifiers that integrate the amplifier, tone controls, and speaker into a single wooden cabinet. Another unusual aspect of keyboard amplifiers is that they are often designed with a "wedge" shape, as used with monitor speakers. This allows the cabinet to be rocked back so that it will project sound upwards at a roughly 45' angle, which is more suitable for a seated keyboardist.

Keyboard amplifiers often have a simple onboard mixer, so that keyboardists can control the tone and level of several keyboards. In some genres, such as progressive rock, for example, keyboardists may perform with several synthesizers, electric pianos, and electro-mechanical keyboards.

A small amp capable of mixing the inputs from two keyboards. This amplifier would be suitable for at-home practice, but it would not be loud enough for a rehearsal or performance.

Acoustic

These amplifiers are designed to be used with acoustic instruments such as violin ("fiddle"), mandolin, and acoustic guitar, especially for the way these instruments are used in relatively quiet genres such as folk and bluegrass. They are similar in many ways to keyboard amplifiers, in that they have a relatively flat frequency response, and they are usually designed so that neither the power amplifier nor the speakers will introduce additional coloration.

Acoustic Amp

 

To produce this relatively "clean" sound, these amplifiers often have very powerful amplifiers (providing from 400 to 800 watts RMS), to provide additional "headroom" and prevent unwanted distortion. Since an 800 watt amplifier built with standard Class AB technology would be very heavy, some acoustic amplifier manufacturers use lightweight Class D amplifiers, which are also called "switching amplifiers."

Acoustic amplifiers are designed to produce a "clean", transparent, "acoustic" sound when used with acoustic instruments with built-in transducer pickups and/or microphones. The amplifiers often come with a simple mixer, so that the signals from a pickup and microphone can be blended. Since the early 2000s, it has become increasingly common for acoustic amplifiers to be provided with a range of digital effects, such as reverb and compression. As well, these amplifiers often contain feedback-suppressing devices, such as notch filters or parametric equalizers.

 

Size and power rating

 In the 1960s and 1970s, large, heavy, high output power amplifiers were preferred for instrument amplifiers, especially for large concerts, because public address systems were generally only used to amplify the vocals. Moreover, in the 1960s, PA systems typically did not use monitor speaker systems to amplify the music for the onstage musicians. Instead, the musicians were expected to have instrument amplifiers that were powerful enough to provide amplification for the stage and audience. In late 1960s and early 1970s rock concerts, bands often used large stacks of speaker cabinets powered by heavy tube amplifiers such as the Super Valve Technology (SVT) amplifier, which was often used with eight 10" speakers.

 However, over subsequent decades, PA systems were substantially improved, and different approaches such as horn-loaded "bass bins" (in the 1980s) and sub-woofers (1990s and 2000s) were used to amplify bass frequencies. As well, in the 1980s and 1990s, monitor systems were substantially improved, which allowed sound engineers to provide onstage musicians with a loud, clear, and full-range reproduction of their instruments' sound.

As a result of the improvements to PA systems and monitor systems, musicians in the 2000s no longer need to have huge, powerful amplifier systems; a small combo amplifier patched into the PA will suffice. In the 2000s, virtually all of the sound reaching the audience in large venues comes from the PA system. As well, in the 2000s onstage instrument amplifiers are more likely to be kept at a low volume, because high volume levels onstage makes it harder to control the sound mix and produce a clean sound. In many large venues much of the onstage sound reaching the musicians now comes from the monitor speakers, not from the instrument amplifiers. While stacks of huge speaker cabinets and amplifiers are still used in concerts (especially in heavy metal), this is often mainly for aesthetics or to create a more authentic tone. The switch to smaller instrument amplifiers makes it easier for musicians to transport their equipment to performances. As well, it makes concert stage management easier at large clubs and festivals where several bands are performing in sequence, because the bands can be moved on and off the stage more quickly.

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Amplifier Schematics - Nice Site for schematics to some of the more popular amplifiers.

 

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