How Guitar Amps Work
Ever wondered "How does a guitar amp work?" If you've plucked an electric guitar with the amp turned off, or not plugged in at all, you'll know how different the sound is. So what's going on when the amplifier gets involved?
To understand how guitar amps work, first let's think about electric vs. acoustic guitars. We're all familiar with the shape of an acoustic guitar, and especially its wide body and hollow interior. That interior acts as a resonating chamber, naturally amplifying the vibrations of the strings.
An electric guitar works differently. The pickups convert the vibrations into tiny electrical signals, which are transmitted to an external amplifier. The amp itself does the same job as the body of an acoustic guitar, boosting the volume of the strings. It just does it separately from the guitar itself, which is why purely electric guitars (including most beginner electric guitars) typically have a much flatter body with little to no hollow space inside.
Amplifier Stages
Inside a guitar amplifier you'll normally find three stages: the preamp, power amp and speaker.
Preamp
The preamp gives the signal shape and quality, and is the most 'visible' part of the amp. If your amplifier has knobs for gain, volume and EQ (including bass, middle and treble) then usually all of these control the preamp.
Power amp
After receiving the optimised signal, the power amp boosts the overall volume and sends a much more powerful version to the speaker. This stage gives the electric signal the voltage needed to physically drive the speaker to produce sound.
Speaker
Up to this point, no actual sound has been produced - the signal from the strings has been nothing but an electric current ever since passing through the pickups. The speaker, driven by this electric current, vibrates back and forth to turn it back into sound.
Preamp Controls
In practical terms there's little difference between preamp and amp controls - because the knobs on the front all adjust the first stage of processing the signal, the actual experience of adjusting the amp vs. preamp controls is the same.
However, it's good to know the difference between the preamp vs. power amp behind the scenes, and to remember that the amplifier gives you several ways to clean up and optimise the signal before you amplify it.
Gain
Gain should not be confused with volume. While the two are similar, the overall volume of the amp vs. preamp gain are two different things. You can think of the gain as adjusting the 'loudness' of the signal, the overdrive and clipping. It's part of what gives electric guitars their distinctive sound, especially in genres like rock and metal.
EQ
The equaliser or 'EQ' adjusts the bass, middle and treble of the signal. Want to hear more of the high frequencies? Turn up the treble. Want to rattle the walls? Crank the bass and hook up a powerful subwoofer. The EQ is arguably the biggest difference between the preamp vs. power amp, and an important element in how guitar amps work to give you the tone you want.
Power Amp Controls
The difference between preamp and amp output is all down to the power amp, which takes the preamp signal and boosts it to the desired volume or 'amplitude'. Different types of amplifiers do this in different ways, from old-style vacuum tubes/valves, to modern transistor amps.
Volume
Whereas the gain of the preamp affects the overdrive and distortion of the signal, the power amp's volume is strictly about the amplitude of the output. The valves in the power amp are much bigger, allowing it to take the preamp's output and make it much, much louder.
Effects
Some amps include one or more effects, such as reverb (which adds an echo to the signal), delays and pedal effects. These are typically added to the output signal after it leaves the preamp, and can bypass the preamp completely for a crisper, less processed sound. Effects are an advantage and a big reason for learning electric guitar!
Speaker Cabinets
It's common to see amplifiers with the speaker built in, especially on beginner guitar amps, but it's also not unusual for more professional, high-power amps to have separate speaker cabinets. These amps will typically have one box that contains the preamp and power amp, which then sends the signal via cables to one or more speakers. The separate, larger speakers allow for a louder volume of sound to be produced overall.
Power wattage
Speakers are given a power rating in watts. Generally speaking, the total power of your speakers added together should be greater than the maximum power output of your amplifier. That way, even at full volume, you know that your speakers can handle the noise level without suffering damage.
Impedance ohms
Impedance is the resistance of the electric circuit in the speaker, measured in ohms. It affects the amount of current drawn from the power amp and if the impedance is too low, you risk damaging your amp, blowing a fuse or triggering the amp's protection cut-off. The calculations become more complicated when you use multiple speakers, but overall if you keep your speaker impedance within the rated value for your amp, you should be OK.
Tone Setting
The way that I’ve been taught, which sounds like a fair and logical one, is to put everything at 12 o’clock, or right in the middle. However, keep the master volume relatively low to avoid being too loud suddenly. Then, go through each setting from gain to EQ one by one, adding or subtracting in increments of ‘1 hour’ at a time (one to two millimetres). This is to avoid drastic changes, and for your ears to adjust to newly added sonic frequencies. Tiny changes in the control knobs can create a significant difference in the overall tone as it emphasises or takes away certain frequencies.
Although there are general settings for different styles (for example, more mids for that mellow jazz guitar tone, or scooped mids for that saturated metal distortion), there are too many factors affecting the tone from the make of the guitar, the resonance of the wood, the type, brand, model of the amp, down to the details of the strings or plectrum! Therefore it is highly recommended that you spend the necessary time to familiarise yourself with each set up and to get a good tone every time - it is worth it!
FAQs
What should the gain on my amp be set at?
Remember gain is not about how loud your amp's output is, but is more about its 'loudness' in terms of tone. When setting up your amp, start with the gain quite low. Play some test tones or some familiar music and turn up the gain to a 'comfortable' level with no unwanted distortion.
What happens if the gain is too high?
Excess gain will introduce distortion. This can be a welcome overdrive effect, but can also add background noise such as buzzing or humming. Think Marty McFly cranking Doc Brown's gigantic amp in Back To The Future, and you've got the idea. This is why it's good to start with the gain quite low, and adjust it upwards until you get the loudness you want, without clipping or humming.
Does gain increase volume?
Strictly speaking, gain does not increase the volume of your amp. In practice, turning up the gain will sometimes give you louder output, but that's mainly the role of the power amp. Gain is more about shaping the signal, whereas volume is about boosting its overall amplitude before it's sent to the speakers.
What does the presence control do?
Fender describes the presence control as "boosts upper-mid and treble frequencies in a specific manner that makes the tone sound notably livelier and a little 'wilder' than normal". This control is part of the power amp, but it interacts with the EQ settings of the preamp, giving you another way to tweak the high end of the frequency range. Depending on the genre you play, this can help your audience to feel more 'in the moment' while still delivering richness in the bass and midtones.
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