What does an amplifier do?

Things I Mean to Know #3

8 min readNov 16, 2018

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I’m pretty big into music. I play drums, I play piano, and I even use a website that keeps track of the music I listen to because new music is basically my oxygen. So why is it that I have no clue what amplifiers do, let alone how they work? (Also, what are these lightbulbs on top of some of them?) This has been itching me for long enough, it’s time to figure this out!

Original image source: Wikimedia commons

So… what’s an amplifier?

An amplifier, as it turns out, doesn’t just relate to music or sound. It’s a general electrical apparatus that amplifies any kind of electrical signal. (I’ll go out on a limb here and guess that that’s why it’s called an ‘amplifier’.) So what does it mean for a signal to be amplified?

Let’s start by imagining that you’re hitting one string on a guitar. The string will start vibrating, and this vibration will move the air around it. This moving air we call sound. (Because sound is really just air moving around at different speeds.) Specifically, the string will create ripples in the air, much like ripples in a pond when you throw a stone in the water. We can visualize these ripples so that we can say something about how they sound. For example:

A simple sine wave (made with the help of Toby Thain’s PostScript code).

The image above is a very simple sound wave, also known as a sine wave. You can imagine this as a fraction-of-a-second snapshot of our guitar tone; a few ripples of air. The time it takes for one ripple to happen (from up to down and back up again) is called the frequency. And a higher frequency means a higher-pitched sound. In other words: the more waves per second, the higher the tone is that we hear. If you would play a higher note on top of your previous note, it could look something like this:

Two simple sine waves (made with the help of Toby Thain’s PostScript code).

However, amplifiers generally leave the frequency of a sound alone. What they care about is the height of the ups and downs of the wave. This is called the amplitude, and this is what an amplifier increases. The greater the difference between these ups and downs, the louder the sound will be. So what if you would play your first note again, but this time you pluck the string much harder?

The frequency of the blue note didn’t change, but the amplitude did! And that, in a nutshell, is what an amplifier does: it increases the amplitude of a sound signal.

Let’s look at an example of how this works. Imagine that you play some notes on your guitar, but this time you plug it into an amplifier. Your guitar will produce a weak signal, and the amplifier will take it in and make it stronger.

In the image below, you can see your guitar signal (blue) and the amplified output signal (red).

The input (blue) and output (red) of a simple amplifier. This particular amplifier has a so-called **gain** of 3, meaning that every point on the input line is moved three times further from the point 0, making it three times stronger. (image source: Wikimedia Commons)

As you can see, the amplifier makes the low points lower, the high points higher, and so it makes the signal as a whole stronger.

But why do we need this amplification in the first place?

Well, you can imagine that a big concert speaker needs a very strong signal to produce the 100+ decibels necessary to make crowds of people dance, jump or headbang. After all, sound is just moving air, and to move air you’ll need speakers that physically move.

A beautiful illustration of the fact that speakers move, by The Slow Mo Guys

The amplifier is the piece of equipment that turns the low-power signal from a turntable, electric guitar or microphone into a high-power signal able to carry the sound through the loudspeakers and across vast festival grounds.

An audio amplifier can be a separate unit, which is popular among audiophiles who like to have granular control over every single aspect of their music. However, most modern music installations — as well as phones, mp3-players, televisions, laptops, etc. — have an amplifier built-in. The speakers that musicians use for their instruments (sometimes referred to simply as ‘amplifiers’ or ‘amps’) usually combine an amplifier with a loudspeaker into a single case.

So… That’s it? Amplifiers make sound louder?

Well, yes. But that’s not as easy as you may think. Nowadays, it’s easy to take for granted that every device can make sound louder on demand (preferably with a smartphone). But that this is possible is all thanks to built-in amplifiers in the systems we interact with every day.

And that doesn’t mean that amplifying is straightforward, either. Lots of factors, like gain (the power of the output), noise (unwanted extra sounds) and distortion (a fuzzy effect caused by pushing the amp past its limits), determine the character and quality of an amplifier.

One other important factor is frequency response. This is the measure of how much an amplifier reacts to high and low frequencies. If, for example, a particular amplifier makes low sounds much louder than high sounds, then the frequency response of that amplifier is skewed. In other words: it misrepresents the input.

Some people want an amplifier with a frequency response that’s as linear as possible, so that every frequency is equally represented in the output. However, if you like the feeling of a deep bass tickling your eardrum, you might want to go for an amplifier that emphasizes low frequencies. Like the amplifier in a pair of Monster Beats Tour headphones:

The frequency response of the Monster Beats Tour headphones. (Source: InnerFidelity)

Okay, so how does an amp work?

Let’s take a look at the basic steps any amplifier will take:

First, it takes in two inputs: 1) a strong electrical current from a power outlet, and 2) a weak signal from your guitar, microphone or electronic triangle.
Then, it uses the signal to regulate the high-power current. Imagine that the input acts as a gate that lets the current pass exactly according to the input signal. This step is crucial, because it turns the current into a stronger version of the input signal.
The new signal is sent out to a speaker.

Aside from this core process, your average amplifier will do a lot of transforming and filtering and checking, all to make sure that the output sound is accurate and pretty.

So what about the lamps?

Time to get historical! Amplifiers haven’t stayed the same since their invention in 1912. For decades, the amplification was done by vacuum tubes. These are physical glass tubes with three components:

a heater element at the bottom called a cathode;
a plate at the top called an anode;
a grid in the middle which can block or allow particles.

A simplified vacuum tube, from a 1943 video about electronics.

Now let’s look at how a tube amplifier carries out the three steps from above.

First, the cathode heats up; it starts glowing red and it fires off electrons — tiny electrical particles. These electrons will want to flow to the top, because we also charge the anode positively. Since electrons are negatively charged, they are attracted by anything positive. So this makes for a continuous stream of electrons from the cathode to the anode; the electrical current from step 1.

This is where the grid in the middle comes into play. It is negatively charged, and thus repels the electrons and prevents them from reaching the anode. This grid, however, is connected to the input signal from your guitar. And although it acts as a gate to block the stream of electrons, it will allow some particles to pass through if it gets a signal from the guitar. Just imagine playing a note on your guitar. It will send a signal to the amplifier, to the gate in the tube, which tells the gate to let through some electrons. What’s more: the stream of electrons is so fierce, that for every electron in the input signal, the gate will let several electrons through. So the stream of electrons turns into the input signal, except stronger. This is the amplification of step 2.

This new signal is then picked up by the anode, which channels it further to the next component in the system. Most amplifiers have multiple tubes for multiple stages of amplification, so the signal is either sent to the grid of the next tube, or out of the amplifier and to the speaker. This is step 3.

A vacuum tube from the 1960’s. (Source: Wikimedia Commons)

As it turns out, these old tube amplifiers did a pretty great job! The sound on some of these amps was phenomenal, as the electronics magazine Wireless World wrote about the Williamson amplifier in 1947:

No distortion can be detected, even when the amplifier is reproducing organ music including pedal notes of the 20c/s order [that’s 20 hertz, a very low frequency], which reach the threshold of maximum output. […] Tests using a direct microphone circuit with noises such as jingling keys reveal extraordinary realism. The amplifier can be described as virtually perfect for sound-reproducing channels of the highest fidelity.

In the 1970’s, most amplifiers ditched their bulky vacuum tubes and switched to transistors. These are — usually tiny — electronic devices that allowed for the same kind of amplification as vacuum tubes. And it’s easy to see why people switched: transistors are smaller, lighter and more energy-efficient.

Some people say, though, that this efficiency comes at a price. They say that the sound of a tube amp is warmer and richer than that of a transistor amp. Much like the debate about vinyl and mp3's, this is a discussion that is far from settled among audiophiles.

Some musicians prefer tube amps simply because it gives them a natural way to create overdrive: when you push a tube amp too far, it cuts off the peaks and valleys of the sound waves, creating a gritty effect. Artists like Chuck Berry would use this effect to their advantage, intriguing the youth and terrifying parents with the edgy sound.

Of course, these effects can now also easily be replicated with pedals. But even if their vintage perks have been overtaken by more sophisticated technology, amplifiers are still an irreplaceable part of our everyday lives.