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by Lorelei Shannon

What Does a Guitar Amp Do?

Everyone knows that guitars and amps go together – just like rock & roll and loud! We've all seen musicians onstage, with their guitars plugged into amps or stacks of amps. But why did he/she choose that particular amp – and what does an amp do other than make your guitar sound louder?

A guitar amp has two main functions. First, it allows you to “shape” the signal going from your guitar into the amp. Second, it takes that custom-tailored signal and drives it through the power amp, which then boosts the signal level and sends it to the speaker, so you can get face-melting volume (or at least you should be heard at the back of the room).

When you begin shopping for the ideal amplifier, you're sure to have lots of questions. This article will help provide some basic information, and along the way we’ll explain some of the terminology you'll need to understand in your quest for the ultimate tone.

How does a guitar amp work?

Although they may come with lots of bells and whistles, amplifiers are just electronic devices that modify and amplify sound signals. Instrument amplifiers feature 3 main components: the preamp, power amp, and speaker.

Guitar amplifiers work like this:

  1. You plug your guitar into the amp, play a note and the signal from your guitar flows through the cable and into the preamp
  2. You can adjust the gain, as well as other signal qualities, such as the treble or bass to change the distortion or “shape” of the sound
  3. The preamp then sends the modified signal to the power amp, which amplifies it in preparation to drive the speaker
  4. Using the master volume control, you adjust the amount of power that is sent to the speaker

You could have a combo system that contains a preamp, power amp and speaker all in the same cabinet, as shown below:

Combo amplifier





Or, you might have a component system that consists of the preamp and power amp in the same cabinet (an amp head), connected to a separate speaker cabinet, as shown below:

Amp head with separate speaker





Finally, you could have a system with 3 component, each in separate cabinets, as below:

separate amplifier components

Half Stacks and Full Stacks

These are just configurations of amp heads and speaker cabinets.
This system is called a “half-stack” because it has only 1 speaker cabinet:

a half stack

A Bugera half stack

This system is called a “full stack” because it has 2 speaker cabinets:

a full stack

A Bugera full stack

Are there different kinds of amps?

There are two main types of amplifiers – tube amps and solid-state amps. People usually choose tube amps for the unique kind of distortion you can get with vacuum tubes. Solid-state amps are lighter and less prone to overheating, but you may need effects pedals to get the same or similar sounds as a tube amp.

Amplifier Terminology

Amp Head or Head – contains only the preamplifier and power amplifier and must be connected to a separate speaker cabinet or “cab” to generate sound.

Cabinet, Cab – is the box that contains the amplifier, speaker, or combination of the two.

Channel(s) – are independent preamp “voicings”. Multi-channel guitar amps may have 2 or more channels, such as a "clean" channel, or a "dirty/crunch" channel. Each channel typically has its own set of controls to adjust the tonal characteristics of the input signal.

Clean Tone – is the pure sound of the instrument unaffected by any type of distortion.

Clipping – is a type of distortion that occurs when the input signal exceeds the normal limit of the device or stage it is driving. (Example, the preamp signal is too high, pushing the power amplifier into unwanted distortion.)

Combo Amp – combines the preamp, power amp and speaker(s) into one complete unit.

Compressor – is an electronic circuit that can be used to “smooth-out” the overall dynamics of the sound. Compressors are sometimes built into amplifiers, but typically available as external devices, such in the form of effects units or “stompboxes”. A compressor can be used to:

  • Make all the notes or strings sound as if they are the same volume
  • Reduce the dynamic range of a signal. The “dynamic range” is the difference between the loudest and softest parts of a signal.

By reducing the difference between loud and soft, compression can also increase the average sound level of a signal and make it seem louder, commonly referred to as a “psychoacoustic” effect. A TV commercial often sounds loud in comparison to the program, because the commercial’s audio has been compressed. The actual TV volume setting doesn’t change, but the commercial’s average level is higher, so it is perceived to be louder.

Compression can be used to help a note ring longer (“sustain”) than usual, and the compressor’s action can also make the guitar sound punchier and be perceived as more out front. This effect is often used to produce super-clean tones, which are very evident in the shimmery ‘80s and modern-country guitar styles.

Connectors – are the electrical plugs (male) on cables, and jacks (female) on your instrument and amplifier (typically 1/4” in diameter) that feed the signal to the amplifier. If you are using an amp head/speaker combination, there is also an output jack on the head to send the signal to the speaker cabinet input jack using a speaker cable. Most guitar connectors are 2-circuit plugs, also referred to as TS (Tip/Sleeve) connectors.

Crunch – can best be described as a distorted tone created by setting the gain on your amp much higher than needed for a clean sound. This sound is commonly applied heavy metal rhythm-guitar tracks.

Distortion – the audio effect that occurs when signal peaks are compressed or overtones are added.

Preamp distortion is created when you adjust the gain, FX, and/or equalizer controls on your amp
Power amp distortion is introduced as the power amp approaches its maximum output level

Pedals, also referred to as stompboxes, are common tools for creating artificial distortion:
Fuzz pedals (or “fuzzboxes”) clip a sound wave (typically a sine wave) until it is nearly square, resulting in a heavily distorted or "fuzzy" sound

Overdrive pedals are designed to create distortion similar to the type you get from a tube amplifier
Distortion pedals clip sound wave peaks, creating flattened sound waves with a “harsher” or “grittier” sound

EQ, Equalizer – is an electronic circuit that adjusts the balance between the various frequency components of the preamp signal. Treble, midrange and bass are types of equalizer controls.

Frequency – is the periodic vibration of a sound. Low-frequency sounds vibrate slowly and are perceived as low-pitched to the human ear. High-frequency signals vibrate quickly and are perceived as high-pitched. 2,000 Hz is faster than 20 Hz, making 2,000 Hz sound significantly higher in pitch. 2,000 Hz means the sound is made up of 2,000 repeating sine waves per second. More on frequency

FX, Effects – Some amplifiers have controls that can add special enhancement to the sound of your guitar. Reverb, echo, delay and chorus are a few types of effects. You can also use pedals or rack-mountable signal processors if your amp does not have this function.

Full stack – An amp head and two speaker cabinets.

Gain – the ratio between the magnitude of input and output signals. But what does that actually mean?

Although they're closely related, gain is not the same thing as volume. When you turn up the gain (sometimes labeled as “drive”) on your amplifier, you're controlling the strength of the signal sent through the preamp stage—the signal from your guitar to the amplifier.

Adjusting gain may effect overall volume, but gain is a tone control, not a volume control. Adjusting the gain sets the distortion level in your tone, entirely apart from how loud or soft you set the master volume.

Setting the gain high produces a lot of distortion, or a “dirty” tone. Low gain produces a “clean” tone. When you've set the tone, you can set the master volume as softly or as loudly as you want.

In amps with no gain control, adjusting the master volume usually adjusts both volume and gain.

Head – In a guitar stack, the “head” is the preamp and power amp in one cabinet. The speakers are in a separate cabinet in this arrangement.

Headroom – The range in amplitude for a signal, between nominal and peak, before the signal starts to distort. In other words, how loudly your amp can play before the signal starts to distort.
In older amps (built before the 1970's) where gain and master volume were the same, the higher an amplifier was rated for wattage, the louder you had to run the amp to create distortion. A 5-watt amp would distort more easily than a 100-watt amp. That is no longer the case for modern amps with preamp distortion controls.

Impedance – Impedance or Ohm rating is a measure of the load that a circuit places on an amplifier. If you are running an amp head and separate speakers, you should match the impedance of the speakers to the impedance rating of the amplifier. See Ohms.

Limiter – A form of compressor used to avoid exceeding a signal's maximum current. A limiter is used mainly for putting a ceiling on the peaks (top) of the waveform, to prevent spikes. The troughs (bottom) of the signal may be left alone.

Ohms (symbol: Ω) – Units of electrical resistance. If you are running an amp head and separate speakers, you should match the Ohm rating (impedance) of the speakers to the Ohm rating of the amplifier. A tube amp will usually have an impedance selector switch or multiple speaker outputs, so you can select the ohm rating that matches the speaker cabinet. See impedance.

Learn more about ohms
Ohms law calculator (calculate ohms to watts)

Overdrive – Occurs in a tube amplifier when the input gain (the sound signal) exceeds the capacity that the preamp tube(s) can handle. The smooth sound wave form that goes into the device gets “clipped,” because the device’s input capacity is less than what is being driven through it. You can get the same effect in a solid-state amp using preamp controls or effects pedals. See distortion.

Overtones – Any frequency that is higher than the basic frequency of a sound signal. Some preamp controls allow you to add overtones.

Pickup – A magnet on an electric guitar that picks up the vibration of the strings. This is the signal that is sent to the amplifier via 1/4" TS cable. (See connector.) Most guitars have multiple pickups. A combination of pickups is called a pickup configuration.

Preamplifier (preamp) – The section of the amp that prepares the initial signal sent to the amp for further amplification or processing. You can adjust the gain and EQ of the preamp signal before you set the master volume.

Power amplifier (power amp) – The section of the amp that that takes the preamp signal and boosts its power to drive the speaker(s).

Resonance – See subharmonic.

Solid-state amplifier – A solid-state or transistor amp uses solid-state electronics, like diodes and transistors, to amplify the preamp signal.

Stompbox – Another name for an effects pedal.

Subharmonic – A subharmonic or undertone is any frequency that is lower than the basic frequency of a sound signal. Synonym for “resonance.”

Transistor amplifier – See solid-state amplifier.

TS jack – A port in an amplifier where you can plug in a connector. TS stands for “tip/sleeve,” which describes the poles of the connector.

Tube amplifier – An amplifier that uses vacuum tubes (also called “valves”) to amplify the preamp signal. Some musicians find tube amps to be more flexible and effective with electric guitar signals. A tube amp has small tubes in the preamp section that develop the tone and prepare it to go through the power amp, and larger tubes in the power amp section that amplify the signal by pushing significant wattage to the speakers.

Undertone – See subharmonic.

A vacuum tube

A vacuum tube

Vacuum tube – A device that controls electrical current through a vacuum, inside a sealed glass tube. Vacuum tubes control the preamp signal in “tube amps” or “valve amps.”

Valve – Another name for a vacuum tube.

Valve amplifier – Another name for a tube amplifier.

Watt, Wattage – An amount of electrical power expressed in Watts or Kilowatts. The higher the wattage output on your amplifier, the more powerful the amplifier is.S

XLR connector – A type of electrical connector found on audio, video, and stage lighting equipment. XLR connectors have a circular opening, and have 3-7 pins.

XLR connectors

XLR connectors

Behringer acoustic guitar amplifiers
Behringer electric guitar amplifiers