How a Guitar Amplifier Works

By Mark
Published on

We’ve covered a few topics on this site such as how to tune your guitar, how to choose guitar strings, and even how an acoustic guitar works.  

Today we’re taking a look at how a guitar amplifier works.  I’ve done some extensive research and have compiled the best information into one article for you to learn how an amp works.  The sources are below the article.

Here’s what we’re going to cover in this article:

Parts of a Guitar Amplifier

Where it all starts: The Guitar Pickups

Preamp

Power Amp

Speaker and Speaker Cabinet

Master Volume Design

Effects Loops (FX Loops)

Multi-channel Guitar Amplifiers

Parts of a Guitar Amplifier

Let’s talk about a very simple guitar setup.  You have a Fender Stratocaster plugged straight into an amplifier without any distortion, wah, reverb or other pedals. How does the amp work?

A guitar amplifier has three basic parts: 

    • The preamp: this section takes the signal from the guitar pickups, shapes and optimizes it.
    • The power amp: takes the signal crafted from the preamp and makes it strong enough to play through a speaker
    • The speaker: the speaker is a transducer.  It will take the electric wave from the power amp and transform it into sound through the speaker

The preamps and the speakers come in two separate components: 

  • The amp head which stores the preamp and the power amp
  • The speaker cabinet which houses the speakers
Fender Bassman - Example of an Amp Stack, Head and speaker cabinet.
Source

Instead of having the head and the cabinet in two different boxes, there are combo amps that combine the amp head and speaker in one box.  This provides guitar players with the functionality of regular amps but with convenience and portability.  

Before we dive into the specifics of each component, let’s talk about where the signal starts: the guitar pickups.

Where it all starts: The Guitar Pickups

On an acoustic guitar, we learned that energy transfers from the vibrating strings and into the top soundboard.  The soundboard vibrates the air molecules inside the guitar and creates sound waves.  The sound waves bounce off the interior of the guitar and out the soundhole to create the music that you hear and enjoy.  This process is different with an electric guitar.

The process begins with the vibration of the strings but instead of the use of a soundboard, electric guitars use pickups.  These guitar pickups are copper wires wrapped around a magnet.  Placing the pickups underneath the strings creates a magnetic field. Plucking or strumming the strings causes a disturbance in the magnetic field and induces a tiny current in the coil of the pickups.  The current travels through the cable and into the preamp.

Preamp

The preamp is the first stage in a guitar amplifier.  This takes the weak electrical signal from the guitar pickups and shapes it to be used later.  This is done by passing the signal through gain stages.  

Before the 1970’s amplifier technology used vacuum tubes for the preamp stage. Vacuum tubes are created similarly to a light bulb.  The tube controls the current in a high vacuum between two electrodes.  As with light bulbs, they need time to heat up before they are ready to use.  These tubes were fragile because of the glass and required maintenance and replacement.  Tube amps are also inefficient as an estimated 50% of the energy used is wasted in heat.

Current technology employs the use of transistors in solid-state amps which are cheaper and more efficient.  

The preamp may also contain tone control knobs to shape the sound in the form of treble, middle, bass knobs.  

Once the signal is shaped and optimized it passes on to the power amp.

Power Amp

With the signal coming from the preamp, it is still not powerful enough to push through a speaker.  The job of the power amp is to duplicate the initial signal in the form of an electrical wave, making it many times stronger; strong enough to play through a speaker.

Similar to the preamp, this component of the amplifier employs the use of transits or vacuum tubes.  

Speaker and Speaker Cabinet

Once the signal is strong enough, it can be played through a speaker.  The speaker is a transducer; this means that it can transform one form of energy into another form.  In this case, the speaker takes the electrical wave and transforms it into sound.  

Depending on what the amp is being used for, the size of the speakers will vary.  For many guitar players starting out, a small practice amp will include speakers that are 4” to 8” in diameter.  For guitar players that need their amps to fill a large venue, the speakers will be around 12” in diameter.

The construction of the speaker cabinet will also contribute to how a guitar amp sounds.  Larger cabinets constructed around the speaker will have more bass and midrange response.  Smaller cabinets will have more midrange.  Amplifiers with a closed back will have more bass present in the sound.  

These are the basic components of the electric guitar amplifier: preamp, power amp, and speaker.  But with time comes more evolutions and innovations with amplifiers.  We’ll go over some of them here:

  • Master volume design
  • Effects Loops (FX Loops)
  • Multi-channel amplifiers

Master Volume Design

Amplifiers through the middle of the century saw very few changes.  Jazz guitar players sought the purest tone of the electric guitar and achieved this through the innovation of negative feedback in the 1930s.  

But there was a change in the way that guitar players were using amps towards the 1970s.  Guitar players would turn their volumes high.  High enough to overdrive the amp.  This warm, crunchy sound would be the next big thing in guitar playing.  This overdriven sound would be the staple of rock music for the next 50 years.   

Amp designers would find a way to introduce a second volume control.  This second volume control would be placed between the preamp and the power amp.  The first volume is now known as gain and is used to overdrive the amp.  The second volume would become the master volume control.  

The addition of the overdrive and master volume would be a great step forward but it wouldn’t be without its shortcomings.

Effects Loops (FX Loops)

With higher gain comes compression as a natural byproduct of overdriven sounds.  This compression would cause modulation effects such as reverbs and delay pedals to become unusable.  

Amp designers would eventually recognize this and introduce the effect loop before the power amp.  This would enable the use of delay and reverb effects with the high gain amps.  But now what about clean sounds?

Effects loop on Line 6 Flextone
Source

Multi-channel Guitar Amplifiers

The previous versions of amplifiers would be optimized for high gain sounds.  But guitar players wanted access to both clean and high gain sounds.  

The solutions of the 1980s were scarce.  One of them was to have two amps at the same time: one for clean and another for high gain.  Another solution was to reset the one amp between songs to go from high gain to clean.  These solutions weren’t good enough.  

The solution was to build two preamps into the amp. One preamp for clean sounds and the other for high gain sounds.  Guitar players can use a footswitch between the two preamps to choose between the clean and gain channels.  

Final Thoughts

We’ve covered the basic parts of the guitar amplifier: the preamp, the power amp, and the speaker.  We’ve learned how each of the parts does their job.  We’ve also covered some of the more important innovations with guitar amps.  

Sources:

Roland: Inside a guitar amp – part 1

Rock Universe: How a Guitar Amp Works 

Leave a Comment