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Most musical instruments and their amplification systems, such as those used for public address (PA), recording and hi-fi are designed to reproduce sound as accurately as possible. Electric guitar and its amplification is completely different: every item in the guitar signal chain is designed to modify the raw guitar signal in some way. Here's a short list of the key elements that combine to produce an electric guitar sound:
Most of these topics are covered in my guitar pages, which I have split broadly into:
Guitar Design - a good book!
Other than guitar pickups and wiring, I don't claim to be expert in details of how to design a great guitar. For this, I strongly recommend a book by Leonardo Lospennato called Electric Guitar & Bass Design. It explains clearly and accurately each aspect of electric guitar design with their different options and impacts on the finished product.
It might be surprising to non-guitarists that just about every item in the guitar signal chain is designed and intended to modify the signal in some desirable way. Let's look at each part of the chain and how it modifies the natural guitar sound. Here's a diagram of a typical guitar signal chain with an explanation for each item.
Your tone starts with the guitar. With so many different guitar types available, it's not surprising there are so many different guitar tones.
Surely the guitar cable should connect the guitar without modifying tone? Actually, no. All cables have a small amount of capacitance which rolls off some extreme highs. These tones are sometimes referred to as "ice-pick" tones and can be quite objectionable, particularly with single coil pickups. The same effect cannot be achieved with a lower treble setting, because those controls operate at a lower frequency.
Some guitarists use a radio system instead of a guitar cable for more performing freedom. It's interesting that the best digital radio systems are now capable of very accurate reproduction and often include settings to simulate treble loss from a real cable.
Effects are obviously intended to modify the guitar signal and you can read more about them on my effects pages. Typical effects between the guitar and amplifier are:
This is the first stage contained inside the amplifier. It's designed to prepare the signal for the final stage of the amplifier: the power amplifier that drives the speaker. The preamplifier has 3 important roles:
Some effects are ideally placed after the preamp, particularly if preamp overdrive is used. Some players feed the effected signals into a separate clean amp instead of sending it back into the same amplifier. This arrangement is called a wet/dry setup, where the effects (wet) remain relatively clean, while the original (dry) signal can be heavily overdriven. For basic live work, players will just use one amp. Some amps don't even have a send/return loop, so these effects can only be placed between guitar and amp. There are no rules for what should be placed pre or post preamp, but common post effects are:
This is the critical component in the chain for generating the "feel" that enables different electric guitar playing styles and music genres. There are many design choices in power amps that make significant differences to overdrive character and dynamic response. Two contrasting designs are Fender Twins that hold their clean power to near maximum power, then move into a compressed but still relatively clean zone. On the other hand, a typical Marshall design starts to break up gently well before maximum power, giving a really wide "sweet spot".
There's more than just choices of valve types, class of operation (class A, or AB usually) and power ratings: one of the key design decisions is if or how negative feedback is used. Class A designs are often used without negative feedback, giving them a somewhat "loose" and tonally rich tone. Class AB designs need at least a small amount of negative feedback, which gives them a tighter and punchier tone. Presence controls are common on amps with negative feedback. Presence reduces negative feedback at treble frequencies, which boosts overall treble as well as giving the treble some of the "no feedback" characteristics. Some amps include a resonance control that does the opposite: boosts bass by reducing bass in the negative feedback, which also gives a "looser" feel to low notes.
Yes, even the power supply is intended to modify guitar sound! It needs to be limited so that the overdriven power amplifier cannot deliver excessive power that could damage components in the power amplifier and the attached speakers. The way the power supply responds to excessive power requirements affects the "feel" of the amplifier. The key variations are how much the power supply drops when excessive power is required, how long it takes to drop, and how long it takes to recover. Amplifiers with noticeable power supply effects are described as having "sag" or "bloom".
One side-effect of a power supply sagging is that regulation also decreases, which can inject hum into the overdriven signal. The hum can interact with notes being played, causing additional inter-modulation notes, often called "ghost" notes. Most players find like this side-effect undesirable, but some purists will say it's an integral part of the amplifier's tone.
Speakers & Cabinets
An overdriven power amp delivers a significant amount of unwanted high frequency energy, so good guitar speakers are not good at reproducing very high frequency response. Guitar cabinets are not acoustically padded, and are in fact intended to have uneven frequency responses in conjunction with speaker irregularities. Open back designs have a good room spread, but can suffer from a "flabby" bass response because speaker excursion is not damped. Typical guitar speaker and cabinet characteristics that modify tone include:
Room & Microphones
The playing environment affects the final sound with reverberation effects as well as some frequency resonances. Just like orchestras have favourite concert halls, guitarists have favoured environments to get their best "room tone". Even in an ideal room, the sound is often captured with a microphone for recording, or live reproduction. It should be no surprise that microphones have their own signature sounds that can enhance (or destroy) good guitar tone. Popular choices include the Shure SM57, Royer R121, Sennheiser 421 and Neumann U87.
As you can see, every item in the guitar signal chain plays a role in modifying the guitar signal to produce something desirable at the end. If you plug a guitar directly into a recording desk or hi-fi for comparison (keep the volume down!), you can hear how radically different your favourite tones are! As a guitarist, you have a wide range of products to choose from with each of these items. With some trial and error, you're likely to gravitate towards the gear that you like best. Skilful use of your gear combined with the playing style you develop is what makes you a unique player.