Preamplifier info - preamp preamps pre amplifier amp amps

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Audio Mic Phono Stereo Tube Bass Guitar Signal
Preamp Pre Amp Preamps Preamplifier

Testing 2 mic preamps is the title of this diy video. Showing an example how to put an electret condenser mic and a preamp in a dynamic mic case. As we can see, some problems arise and it may not look overwhelmingly good but in the end, it works.

Guitar preamp processor front Phono preamp Differential preamplifier

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.

Preamp definition

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.
When preamps are of high quality or "high tech", the longer word is rather used. Pre is a latin prefix meaning before, in advance, in front of, precede.
Amplifier relates a latin verb, in its simple infinitive form, amplificare, to enlarge, to increase, to magnify, to extend.

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.

JFET preamp

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.

For different or several purposes, usb preamps may have some or several features:
Balanced microphone XLR connectors with +48V phantom power possibility, high impedance input for guitar or bass, phono/turntable use with MM as well as MC cartridges, line in and line out connectors.Some even have headphone output.

Unless the sample rate is 44.1/48kHz it may also be set to several other sample rates, 8, 9.6, 11.025, 12, 16, 22.05, 24 and 32kHz.

Since most soundcards don't have any phono inputs, a USB preamp with phono provision may be a popular unit in order to easily record vinyl sources even with old turntables.

USB phono preamp

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.
It can be seen, that when the shorter term tube preamp is used, on the net or elsewhere, the referred equipment sometimes looks smaller and cheaper. When the longer word is used, the equipment more often looks more qualified and expensive.

Tube sound

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.
In all the glory and sometimes, hype of tube preamps, an exception where tube preamps should not be used, unless it uses class B circuitry, is when recordings for an authentic reproduction is required. For that purpose, transistor/semiconductor preamps have to be used.

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.

Semiconductors only generate odd order distortion at overload unless they are designed to clip asymmetrically. The gradual class A tube amplifier distortion contains the 2nd, 3rd, 4th etc harmonic over-tone multiples of the fundamental frequencies. Common semiconductor distortion however, occurs suddenly and very unpleasantly if overloaded, with odd 3rd, 5th etc order over-tones. Distortion in a semiconductor preamp should not happen because semiconductor/transistor circuits are designed to have sufficient dynamic headroom to the clipping threshold, unless there is a soft clipping circuit. If such a soft clipping is designed to perform a gradual clipping asymmetrically, it will make a smoother even order distortion. So, some semiconductors preamps have circuits that may be of a simple design with softly clipping diodes or transistors to resemble vacuum tube clipping. Sometimes both semiconductors and tubes are combined.

So called FET transistors are known to have tube like characteristics. An interesting page in this subject is here.

Stereo tube preamp
Vintage tube stereo preamplifier Tube mic preamp Asymmetric even order harmonic tube distortion

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.

There are two types of phono cartridges, the MM - moving magnet which is the standard, common type, and MC - moving coil which provides better audio.
Usually, the signal of an MC cartridge is somewhat smaller than of an MM cartridge. Ortofon, a phono cartridge manufacturer to be mentioned as one exception, has made high-output MC cartridges to fit standard MM phono inputs. Otherwise, the amplifier/receiver may have the option to switch between MM/MC.

Turntable enthusiasts may want MC cartridges since they have better audio quality. Because the signal is low, many believe that its preamplification is a critical process that requires a phono preamplifier of very high quality. Unless the existing system is already equipped with such a preamplifier, an external phono preamplifier is needed.

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.
There are both solid state and tube phono preamps.

Phono preamp MM/MC cartridge input on a phono preamp Pioneer turntable

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.
And in the view of that, something to be considered, and worth knowing, is the fact that some manufacturers have used filters in the circuits to add just a little coloring, not more than still keeping the sound within the "hifi"-norm, but enough to make the sound "softer" or whatever words are to be used. Even very very small tonal differenses can be heard immediately in an a/b comparison by audio devoted listeners. And this, maybe just because of a pair of capacitor/resistor or two somewhere in the circuits, may ultimately lead some people to buy certain equipment that cost tons of more money. Some equipment really is very expensive.

So even though a tube preamp may have a certain sound mainly because of its (asymmetrical) smooth overdrive distortion, parts of the sound may also come from the transformer and/or internal filters.
When the shorter stereo preamp term is used, it also refers to studio related 2-channel mic/instrument preamplifiers.

Stereo preamplifier

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.
Tube mic preamps seem to be more valued because of its smoother sound, but for a totally unaffected sound, solid state mic preamps are better.
A mic preamp with semiconductor circuits can achieve a sound that resembles tube sound if it is designed to clip smoothly as with a tube preamp and/or asymmetrically (to generate even order distortion harmonics) like an class A tube preamp.

Some mic preamps, either solid state or with tubes, have input transformers that may color the tone or add smooth distortion. So, the "warmth", the subtle overdrive that is commonly associated with vintage or expensive preamplifiers, may (also) come from impedance matching input transformers. Even some solid state preamps use transformers.
Just as mentioned in the chapter about stereo preamplifiers above, "hidden" filtering (very little filtering though making a difference) may be used to make the sound of a mic tube preamp even "better".
The sound of tube mic preamps is often described as the low end and lower mids sound rounder, and the top end gets a silky touch and benefits from improved presence without sounding boosted.

In situations such as recording of birds where authentic reproduction is necessary, a clean sound with a semiconductor mic preamp is the only choice.

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.

Other common mic preamp applications are in mixer boards and computer soundcards.

Mic preamp module

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.
Like some guitar amplifiers, some guitar preamps have a switchable 2-channel design to perform switching between "clean" and higher gain overdrive distortion sound, where each "channel" has its own gain and tone controls.

Guitar preamps are built upon either semiconductors, transistors or ic (integrated circuits), vacuum tubes or any hybrid of these. There are guitar preamps also for acoustic guitars or basses.

Related links:

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.

Guitar overdrive/distortion

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.

Eventually, tube preamps and tube rack effects became common, reestablishing the smooth "tube sound". Among guitar pedals or preamps with semiconductors, some have been designed to perform asymmetrical clipping and later even a clipping that as much as possible are meant to resemble the smoother tube clipping.

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.
Many guitar preamps are analog but in recent years, there have come digital guitar "preamps" with further availabilities.

Bass preamp rack Overdrive preamp pedal
Telefunken ECC83 tube

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.
An external preamplifier is useful if the antenna is smaller, or if the signal is too weak. Antennas with preamplifiers built in, whether they are relatively smaller or not, are called active antennas.

Antenna preamp

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.

RF preamp circuit board

Preamp schematics

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.
This is an interesting article about a cheap preamplifier with remarkable results. Forewords. The IC in the $5 preamp can be found here.

Analog Devices low noise microphone preamplifier integrated circuit SSM2166 One of many simple preamp schematics and diagrams.

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