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The
role of the amplifier
The electric guitar needs amplification to strengthen the signal produced by its pick-ups when it's played. It cannot be directly connected to the loudspeaker, whose operation requires a much greater amount of energy than that supplied by the pick-ups. Therefore, the use of an amplifier is essential.
This ring in the electro-acoustic chain needs electric energy coming from an external source (electrical current or batteries) and it uses the tension of the signal coming from the guitar to check the dispatch of such energy to the loudspeaker. The amplifier allows you to modify the tone and the volume and also to add characteristics such as distorsion and reverberation. Many musicians strictly think about amplification in terms of power amplifiers used for operating the loudspeakers, but the true sense of the word amplifier can also mean many sound elaboration devices (equalizer, expanders, pressers, etc.).
The amplifiers can be built with two different technologies: with valves or transistors. The philosophies are different, and in a certain sense the characteristics of the amplified sound are also different. For the average amplifier, the input signal of the original sound can be reproduced at the output (loudspeaker) with great precision. And this, basically, is the meaning of high fidelity. Hi-fi amplification (high fidelity) is a fundamental requirement for acoustic guitars, bass guitars (which require clean sounds), keyboards, wind instruments and particularly the human voice.
An amplifier can also harmonically enrichment the signals that are sent to it. Many electric guitarists prefer this quality to the true sound, from the moment they become determinating factors in creating your own personal sound. The effect can also be useful for some keyboard instruments, but for the majority of the other instruments it is inadequate.
Amplifiers combined with the loudspeaker(s) are called combo amplifiers. They are easy to transport and be systematized, but in cases where you need more flexibility (i.e. to add extra power), its better to use separate amplifiers and speakers. The blocks can be stacked and the amplifier can be placed a safe distance from the speakers to avoid vibration damage.
How an amplifier works
The diagram below illustrates the different stages of operation of a typical valve amplifier with two input channels and reverberation. The quality of the power source depends on the effectiveness of all the rings in the chain that follow.
The performances of an amplifier is greatly diminished when under-powered. In this preparatory stage, the alternating current is elevated to high voltage and converted to direct current using diodes, semiconductors or valves. A system of resistors, condensors and an inductor stabilize the direct current and supplies the various stages of amplification with different voltages. Lets analyze the individual parts of the amplifier.
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The first stage, or preamplifer, consists of a valve and its relative components. They apply a fixed voltage gain to the input signal coming from the guitar
(in other words: preamplifying the signal).
The tone and volume controls often use a passive system of resistors and condensers, analogous to the controls on the guitar. However their operation can be slightly more complex since in this case the signal is of a higher tension, produced in the first stage. Some amplifiers are equiped with active tone controls that influence, within a range of suitable frequencies, the gain of the second stage.
The second stage consists of a valve that, like the first stage, is a voltage amplifier. Its function is to recover the voltage loss in the signal, due to the use of the passive tone controls, as well as to supply a gain in additional voltage.
The reverberation block contains a pair of metallic springs used for giving a delayed signal. One or two valves act as the power amplifier that pilots a connected transducer to an extremity of the springs. This vibrates the springs, collected at the other side of a second transducer that feeds a voltage amplifier to one or two valves. The reverberation control is essentially a volume control situated behind the first of these valves. A transistor amplifier obviously won't have the valves, but as we will see, has "solid state" circuits.
The general volume is a passive control that acts on the overall signal level sent by all the inputs to the power amplifier.
The power amplifier has three stages: the phase inverter, the pilot and the power stage.
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The phase inverter (or phase separator) produces two output signals, one of which is out of phase with respect to the other by 180°. The voltage of these two signals is amplified in the pilot stage. The two valves used in the phase separator are also often used for getting such amplification, therefore the two stages are often combined. Normally, all the valves of the circuit used up to this point are triodes. To save space, double triodes are used, which combine the functions of two valves into one. Therefore, the theoretical function of a valve can be developed by half a valve; the functional remainder can be used for other purposes.
In the power stage, one or more pairs of valves are used to convert the signal voltage into a flow of current. This current is passed through the output transformer and is finally sent to the loudspeakers.
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