Bass instrument amplifier:

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

Bass instrument amplification for the bass guitar, double bass and similar instruments is distinct from other types of amplification systems due to the particular challenges associated with low-frequency sound reproduction. This distinction affects the design of the loudspeakers, the cabinet, and the preamplifier and amplifier. Speaker cabinets for bass instrument amplification usually use larger loudspeakers (or more loudspeakers) than the cabinets used for other instruments. This is due mainly to the fact that loudspeakers become less efficient at lower frequencies (depending on the size of the driver) and because the human ear requires greater sound pressure levels at lower frequency to hear equal loudness; the result is that more power is needed to reproduce low frequencies at any given level of perceived volume. Speakers used for bass instrument amplification must be sturdier to handle the higher power levels.

Bass instrument speaker cabinets are typically more rigidly constructed and heavily braced than cabinets for non-bass instrument amplification, and bass cabinets usually include tuned bass reflex ports or vents which are cut into the cabinet. Bass instrument amplifiers' preamplifier sections have equalization controls that are designed for bass instruments, and extend down to 40 hertz or even below. As well, bass instrument amplifiers are more likely to be designed with cooling fans than regular guitar amplifiers, due to the high power demands of bass instrument amplification, and bass amplifiers are more commonly equipped with compressor or limiter circuitry which protects the speakers from damage from sudden loud notes.

History

1950s-1960s

When the Fender company invented the first widely-produced electric bass guitar in the early 1950s, they also developed a bass amplifier, the Fender Bassman amplifier. The Fender Bassman was a 50-watt tube amplifier with four 10" speakers. The Ampeg Bassamp Company, founded in 1949, also produced bass amplifiers that were widely used by electric bass guitarists in the 1950s and 1960s.

The first bass amplifier offered by Ampeg was an 18 watt model with a single 12" speaker and a rear ventilation port called the Super 800. In 1951, a 20 watt version with a 15" speaker was put on the market. In 1960, they introduced the B-15 Portaflex, a flip-top 25 watt bass amplifier with a single 15" speaker and in the late 1960s, the 300 watt Super Valve Technology (SVT) amplifier head which was intended for large performance venues. The SVT was intended for use with one or two speaker cabinets containing eight 10" speakers.

In the mid-1960s, the bassist for The Who, John Entwistle, was one of the first major players to make use of Marshall stacks. At a time when most bands used 50-100w amps with single cabinets, Entwistle used twin Stacks with new experimental prototype 200w amps. This, in turn, also had a strong influence on the band's contemporaries at the time, with Cream and the Jimi Hendrix Experience both following suit. Entwistle also experimented throughout his career with "bi-amplification," where the high and low registers of the bass sound are sent to separate amplifiers and speakers. This allows for more control over the tone, because each register can then be modified (e.g., in terms of tone, added overdrive, etc) individually. The Versatone Pan-O-Flex amplifier used a different approach to bi-amplification, with separate amplifier sections for bass and treble but a single 12-inch speaker. The Versatone was used by well-known bassists such as Jack Casady and Carol Kaye.

1970s-2000s

As PA systems have improved, horn-loaded "bass bins" and subwoofers were added and were often well-equipped to amplify directly fed five-string bass guitar and keyboard 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.

A MesaBoogie bass amplifier "head"; note the graphic equalizer sliders on the right side

As a result of the improvements to PA systems and monitor systems, bass players in the 2000s no longer need to have huge, powerful amplifier systems; instead, contemporary bass amps featuring preamp-out jacks can be patched to the PA. 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. As a result, in many large venues much of the onstage sound reaching the musicians now comes from the monitor speakers, not from the instrument amplifiers. Stacks of huge speaker cabinets and amplifiers are still used in concerts in some genres of music, especially heavy metal, but they tend to be used more for the visual effect than for sound reproduction.

Types

Different types of equipment are used to amplify the electric bass and other bass instruments, depending on the performance setting and style of music, and the type of tone or sound desired by the bassist. For rehearsals, recording sessions, or small club performances, electric and upright bass players will typically use a "combo" amplifier. The "combo" amplifier, was given this nickname because it combines an amplifier and a speaker in a single cabinet.

A bass rack from a professional bass player's touring setup. The bass amplifier is the lowest chassis in the rack; above it are a power conditioner, a wireless receiver, and several pre-amp devices.

For larger venues such as large clubs and outdoor music festivals, or for music genres which use bass instruments with an extended low range (e.g., metal) bass players will often use a more powerful amplifier (300 to 1000 watts) and separate speaker cabinets in various combinations.

Separate bass amplifiers, often called 'heads' or 'amp heads' are usually integrated units, in which the preamplifier and power amplifier are combined in a single unit. More rarely, bass players may use separate preamplifer/power amplifier setups, in which one or more preamplifiers are used to drive one or more power amplifiers.

Amplifier technology

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

Tube Amplifiers

Vacuum tubes were the dominant active electronic components in bass amplifiers manufactured until the early 1970s, and tubes continue to be used for higher-end units. Tube amplifiers for bass almost always use class AB₁ topology for efficiency reasons. Many bass players believe that tube amplifiers produce a "warmer" or more "natural" sound than solid state amps and that they have a greater level of perceived loudness for a given amount of amplifier power.

Even though tube amplifiers produce more heat than solid state amplifiers, few manufacturers of tube amplifiers include cooling fans in the amplifiers' chassis. While tube amplifiers do need to attain a proper operating temperature, if the temperature goes above this operating temperature, it may shorten the tubes' lifespan and lead to tonal inconsistencies.^[1]

Solid State Amplifiers

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 bass 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 small amplifier in this picture is an example of a bass "practice amp"; its low wattage and low volume make it mostly suited to individual practice.

The output transistors of solid state amplifiers can be passively cooled by using metal fins called heatsinks to radiate away the heat. For high-wattage amplifiers (over 800 watts), a fan is often used to move air across internal heatsinks.^[2]

Since transistor bass amplifiers used for large venues need to produce a high output, this usually means that bass amplifiers are very heavy. Most powerful transistorized bass amplifiers are Class AB amplifiers, which need heavy transformers of copper wiring and large metal heat sinks for cooling. However, Class D amplifiers (also called "switching amplifiers") are more efficient than conventional Class-AB amplifiers, and so are lighter in weight and smaller. The Acoustic Image Focus head, for example, produces 800 watts of power and weighs 2.2 kilos. Class-D amplifiers use MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) rather than 'ordinary' (bipolar) transistors and they generate a pulse-width modulated signal which is filtered before it reaches the speaker.^[3]

Loudspeakers

The bassist for Danish grunge band performing with a bass stack that includes an amplifier "head", a 4x10" cabinet, and a 15" cabinet

The requirement to reproduce low frequencies at high sound pressure levels means that most loudspeakers used for bass guitar amplification are designed around large diameter, heavy-duty drivers, with 10", 12" and 15" being most common.

Bassists who want powerful low end may use a subwoofer cabinet designed for a PA system; because subwoofers can only produce frequencies up to about 150 or 200 Hz, a subwoofer cabinet would have to be paired with a full range speaker to obtain the full tonal range of an electric bass or upright bass.

Cabinet Design

In this 2007 photo of The Police's singer-bassist Sting, several 8x10" Ampeg cabinets can be seen on the left side of the picture.

Most bass speaker cabinets employ a vented bass-reflex design, which use a port or vent, while others have sealed cabinets. Some cabinets use a transmission-line design which is similar to bass-reflex, and some very large cabinets use horn-loading of the woofers.

Bass reflex speaker enclosures, which have a vent or port cut into the cabinet, are widely used for bass cabinets.

Music store display showing a variety of bass "combo" amplifiers and speaker cabinets.

Tweeters

A 150-watt "combo" bass amp with a horn-loaded tweeter (in the top right of the speaker cabinet)

In the 2000s, high frequency tweeters, typically horn-loaded, are included in some bass instrument speaker cabinets. Vox's 1960s-era "Super Beatle" amplifier was an early enclosure that used horn tweeters. During the late 1960s Acoustic's 260 Series guitar amp used a treble horn in the dual 15" 261 guitar enclosure, and Kustom's nearly 5 foot tall 2J + 1H guitar enclosure used two 15" speakers and a 15" diameter treble horn. Horn-equipped cabinets were not available for bass players until much later.

In the early 1980s, some performers began using two-way or three-way cabinets that used 15" woofers, a vented midrange driver and a horn/driver, with a crossover directing the signal to the appropriate driver. Folded horn bass guitar rigs have remained more the exception than the rule due to their size and weight. As well, since the 1990s, most clubs have PA systems with subwoofers that can handle the low range of the bass guitar. Extended range designs with tweeters were more the exception than the rule until the 1990s. The more common use of tweeters in traditional bass guitar amps in the 1990s helped bassists to use effects and perform more soloistic playing styles, which emphasize the higher range of the instrument.

One problem with adding a tweeter to a bass speaker cabinet is that the driver may be damaged by the overdriven amplifier tone that is popular in some musical genres, such as hard rock and heavy metal. Horns and speakers in the same cabinet are sometimes wired separately, so that they can be driven by separate amplifiers. Biamplified systems and separately-wired cabinets produced by manufacturers such as Gallien-Krueger and Carvin allow bassists to send an overdriven sound to the speaker, and a crisp high sound to the horn, which prevents this problem.

Amplifying the double bass

A jazz bassist performing on an upright bass, using an amplifier and speaker to augment the instrument's natural volume

Double bass players performing in traditional blues, rockabilly, jazz, folk, and bluegrass often blend the sounds picked up by a piezoelectric transducer with the sounds picked up by a small condenser microphone mounted on the bridge. The microphone picks up the resonance coming from the body and the sounds of the strings being plucked, bowed, or slapped against. The two sound signals are blended using a simple mixer and then routed to the amplifier.

Double bass players playing in genres where a louder amplified tone (emphasizing the fundamental frequencies) is desired for the bass may be more likely to face the problem of feedback. Feedback for double bass generally manifests itself as a sharp, sudden high-volume "howling" sound which can damage the loudspeakers. Instruments with laminated (plywood) tops are less prone to feedback problems than those with solid wood tops. When acoustic instruments with resonant bodies are amplified with microphones and piezoelectric transducer pickups, the common approach used for amplified double basses, they are prone to have feedback problems.

Preamplification and effects

The basic sound of the amplified electric bass or double bass can be modified by electronic bass effects. Preamplifiers, "compression", limiting, and equalization are the most widely-used effects for bass.

A range of other effects are used in various genres. "Wah-wah" and "synth" bass effects are associated with funk music. As well, since the 1960s and 1970's, bands have experimented with "fuzz bass" where the bass is distorted either by overdriving the amp or by using a distortion unit. Octave-generating effects, which generate an octave below the pitch being played are also used by bass players.

Bass instrument amplifier

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