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Preamp circuits perform gain and they are used in almost all electronics, in its simplest form based around a transistor. The most searched for preamps are audio preamp types; phono preamp, tube preamp, stereo preamp, microphone preamp, bass/guitar preamp. Of course there are many other applications as mentioned below.
The short form used for preamplifier, preamp, has become more used in spoken and written language simply because it is shorter. Other spellings are pre amp /pre-amp and pre amplifier /pre-amplifier.
A preamplifier is required to amplify a signal, when the source level is too low and has to be pre-amplified in order to be able for further processing, control or any other use.
A preamplifier measures signals from sensors or other devices in a variety of situations such as sound, temperature, light, movement, pressure etc. In equipment for industrial, scientific, telecommunications, space, fiber optics or data links, the frequency range may cover from dc up to many hundred GHz.
Preamplifiers are also in a lot of consumer and commercial products. Ranging from low to high frequency used in phones, radios, audio equipment or car engine systems.
USB preamp / preamplifier
A recent preamp type that has become more popular, especially
for laptops without a soundcard, is the USB preamp.
It combines the qualities that may be searched for both as a preamp
as well as an AD-converter.
It is compact and easy to install. With power supplied by the USB
bus, there is no need for an external power supply.
Tube preamp / preamplifier
Also called Valve preamp (British english).
When a different and/or better hifi audio quality is considered, there are those who prefer tube preamps over solid state/transistor preamplifiers. Most tube preamplifiers refer to use in home stereo systems. Then there are tube preamplifiers to be used for microphones or instruments.
The sound by tube equipment is often defined by presence, warmth and clarity. There is a hold of certain value to older vintage tube preamplifiers as well.
Relating to the typical amplifier circuits by tubes and semiconductors respectively, at over-loading, the overdrive/distortion by class A tube amplifier circuits increases smoothly and will also generate even order harmonic distortion.
So called FET transistors are known to have tube like characteristics. An interesting page in this subject is here.
Phono preamp / preamplifier
In all amplifiers/receivers or other devices with a phono input, more in older equipment than new, there is a phono preamplifier inside. The phono cartridge of the turntable is a device that gives a very low signal output. Therefore, the level of the signal has to be amplified.
Most older amplifiers/receivers have an MM phono input only. Many modern amplifiers though (example), don't have provision for a phono cartridge at all, so it is necessary to have an external phono preamplifier. So with that in mind, unless replacing the amplifier/receiver, an external phono preamp is needed. External phono preamps are either MM/MC switchable or only for MC or MM.
Stereo preamp / preamplifier
Stereo means two channels. When the term is stereo preamplifier, it usually refers to (home) stereo systems. It is like an ordinary stereo integrated amplifier (stereo preamplifier + stereo amplifier in one unit) with source selectors, but without an amplifier inside, which has to be added separately either as a stereo amplifier, or as two mono amplifiers. Therefore, a stereo preamplifier has the same design and size as usual amplifiers.
Why different equipment may sound different.
There are both solid state and tube stereo preamplifiers and with that, also the everlasting discussions in forums, blogs etc about tubes compared to semiconductors, transistor/mosfet amplifiers.
Mic preamp / preamplifier
As a stand alone unit, a mic preamp is an essential part of the recording equipment in a recording studio. Some old tube microphone preamplifiers, some with a compressor, have gained a certain status. Whether old or new, these can be very expensive.
Many preamps have an inverting phase switch that can be useful in situations where several microphones are used, to minimize poor phase interaction, or to minimize feedback in live use.
There are one, two, four or even eight channel microphone preamps. For condenser microphone compability, they have 48V phantom supply. If best audio quality is considered, only one channel could be chosen.
Guitar preamp / Bass preamp
A guitar preamp commonly has settings for input gain, tone/eq and master gain/volume control. Except for acoustic preamps, most guitar preamps have an input gain to be set as high as possible for a clean sund, or it can be higher for a distorted sound.
Bass/Guitar eq/tone control
An electric guitar as well as an electric bass have several strings. Even though its sound doesn't cover the full audio spectrum, each string has its own sonic character. A preamp in a guitar or bass amplifier, or an external unit, is usually equipped with 3-4 tone controls.
In general, those who want distortion may choose a transistor preamp, and those who favor an overdrive sound may choose a tube preamp or which is possible, a preamp with semiconductor circuits that are designed to clip smoothly as with a tube preamp and/or asymmetrically (to generate even order distortion harmonics) like an class A tube preamp. The sound can span from hard distortion to very subtle overdrive. Overdrive can be more easily controlled depending on how the electric guitar or bass is played, whereas distortion in diode/transistor equipment usually occurs more suddenly.
From tube overdrive to distortion pedals
Originally, guitar amplifiers did not have a preamplifier gain control. Therefore, they only produced overdrive/distortion when played loudly, as the final amplifier tube(s) were overloaded. Overdrive/distortion could not be achieved when playing at moderate or low levels. Later, some tried in different ways to overload the amplifier input. A few years into the 60s, so called fuzz boxes/pedals appeared. A heavier guitar (tube) sound got more used in the late 60s. In the 70s, transistor guitar amplifiers were made. At the same time, distortion pedals became very popular.
Although the sound of a tube preamp isn't considered same as of a loudly played tube amplifier with an undistorted input, it often competes over semiconductor preamps. Some of the most used vaccum tubes in guitar/bass preamps are 12AX7/ECC83 and 7025.
Antenna preamp / preamplifier
Other terms that can be seen are antenna amplifier, antenna/signal booster or signal amplifier. Besides, there are units called RF preamplifier meaning about the same thing. However those called antenna (pre)amplifiers/boosters are usually made to be mounted also on an antenna mast.
RF preamp / preamplifier
RF means radio frequency. As above, an RF preamp can be a separate unit but is also a circuit in a radio/receiver, tv etc, at its front end, to amplify the weak signals.
A preamplifier can be made of discrete or integrated semiconductor circuits where some circuits are specialized and called low noise amplifiers or preamplifier ICs. Some preamplifiers are also made with tubes.
Some people build own preamplifiers and there is a lot of DIY (do-it-your-self) pages with info on the web about various electronic circuits, both advanced as well as simple preamp schematics and diagrams. However it is very recommended to get some books in the subject, it is funnier and easier to concentrate over a subject with a book instead of only using internet. The most popular home made preamplifiers are maybe for home stereo, microphones and electric guitars.
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