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Guitar Amplifiers, in their most basic form, are electronic amplifiers that are designed to make the signal from a guitar, whether it be acoustic or electric, louder so that it will produce sound through a speaker. Another thing that a guitar amplifier will do is modify an instrument’s tone, either by emphasizing or de-emphasizing particular frequencies as well as adding electronic effects to that sound.
Guitar Amplifiers come in two varieties. Those are, combination (combo) amplifiers, which include an amplifier along with up to four speakers set in a wooden cabinet, and the other type is a standalone amplifier (called a “head” or “amp head”) that does not contain a speaker. What it does do, however, is pass the signal to a speaker cabinet. Guitar amplifiers can range in quality and price from small practice amps without much power, designed for the beginner or student musician, that sell for around $$, all the way up to grand custom-made professional musician amplifiers that can cost in the thousands of dollars.
Amplifiers contain one or more circuit stages, each having a unique responsibility in the modification of the input signal.
The power amplifier or output stage will produce a high current signal, making the speaker emit that sound. At least one preamplifier stage precedes the power amplifier stage.
The preamplifier is just a voltage amplifier that amplifies the signal from the guitar to a suitable level that will drive the power stage. There may be one or more tone stages. If tone stages exist, what they will do is affect the character of the guitar signal: Before the preamp stage, (say you have guitar effects pedals) between the preamp and power stages, (multiple dedicated amplifier tone circuits, or effects loops) in between stacked preamp stages, as well as feedback loops coming from a post-preamp signal to an earlier pre-prep amp signal (in case of presence modifier circuits).
Tone stages can also have electronic effects, for instance: Equalization, distortion, chorus, reverb, or compression. Amplifiers make use of vacuum tubes, (they are called valves in Britain) or solid-state (transistor) devices, but sometimes use both as well.
An amplifier stack containing a head on top of a cabinet is commonly known as a “Half Stack”, while a head sitting on top of two cabinets is usually referred to as a “Full Stack”. The cabinet that the head sits on top of will usually have an angled top in the front, while the cabinet underneath of the full stack typically has a straight front.
The very first version of the Marshal stack consisted of an amp head on an 8×12 cabinet, which means it was a single speaker cabinet that contained eight 12″ speakers. After only six of these cabinets were produced, the arrangement was reconfigured to an amp head atop two 4×12 cabinets, meaning four 12″ speakers, to help ease the transportation of the unit.
A distinction usually exists between “practice amplifiers” or “recording studio guitar amplifiers” which typically have an output rating of anywhere between 20 watts all the way down to even a small fraction of a watt, and “performance” amplifiers, which are usually at least 50 watts. For the most part, these have been fixed-power amplifiers, and a few models containing a half-power switch that will lower the listening volume slightly, while still preserving power-tube distortion.
Oddly enough, the relationship between the perceived volume and the power output is not immediately obvious. A 5-watt amplifier is perceived to be around half as loud as a 50-watt amplifier (although the power is increased tenfold), but a half-watt amplifier is a quarter as loud as a 50-watt amplifier.
Doubling the power of an amplifier will result in a volume increase that is “just noticeable”. Therefore, a 100-watt amplifier is said to be just noticeably louder than a 50-watt amplifier, although that can also be subject to the human ear’s tendency to act as a natural compressor at higher volumes.