A preamplifier is a type of electronic amplifier used in the audio chain, during sound production.
As in any amplifier, the purpose of a preamplifier is to increase the level of the signal and, for this, acts on the voltage of the input signal.
When the signals come out of the preamplifier, they will have reached the line level, standardized at 0dB.
The preamplifier is in charge of leveling the electrical voltage that comes from the different audio sources (each equipment has a different output voltage), and then, once equalized, send them, as an input signal, to another equipment (usually a power stage).
The relationship between the output level and the input level is the gain. Thus, the gain, expressed in decibels, indicates the degree of amplification of a signal.
Some preamplifiers have controls that allow them, in addition to regulating the output voltage, to regulate the tone, the balance, etc. In addition to reinforcing the loudness with low volume (loudness).
In spite of what has been said, normally, equipment for audiophiles does not include any of these controls, since they can distort the original signal. These actions can be carried out, without introducing losses in the signal during the process, later: in the power stage, in the mixer or, even, in the loudspeaker.
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What is a tube Preamp?
A tube preamp is an amplifier with one or more tubes (lamps). A tube contains a filament with therefore a cathode (emitter), a control grid and an anode. If the filament (usually 6v) is at a temperature and the cathode is heated up, the last electrons begin to emit.
A DC voltage is applied between the cathode and the anode (usually between 250V and 400V). The free electrons now run from cathode to anode. This current is influenced by the voltage on the control grid.
If the alternating voltage of a signal source (microphone, tuner, etc.) is added to the control grid, the signal is amplified at the anode.
In contrast to a transistor, a tube cannot amplify current, only voltage. Previously, 800-ohm speakers were used to display the signal from the tube, but in general, a transformer is used to make the output signal suitable for loudspeakers from 4 to 8 ohms.
What is the purpose of a preamp?
- A preamp is used to bridge the gap between an amp and a microphone. This is a primary system and not just in audio because its function is to format a signal for proper processing by the following systems. So its way of thinking depends on what you want to format, for example, a temperature sensor will not have a preamp with the same design as a microphone preamp.
- It amplifies the amplitude of the signal at its input
- It cleans the signal of the parasitic noises
- It ensures an impedance adaptation
Simple tube preamp schematic Diagram
Why a tube preamp?
A studio microphone transforms an acoustic wave into an electrical signal. Except for some electret pickups, the output level of a microphone is of the order of a few millivolts. As this level is very low, it is indicated in decibels by analogy to the gain it would take to bring the signal back to 0 dB volt (0.775 volts). In general, the microphones have an output level of -40 to -60 dB volts, that the preamp is responsible for enhancing.
This essential equipment is often present on professional high-end mixing boards or audio interfaces, but the integration constraints and their secondary aspect in the product specifications work against them: indeed, these preamps often have a sound. Too pale, and rarely offer advanced features that can be found on the “real” preamps.
Because this is the advantage of a preamplifier: it does not only amplify the signal from the microphone but also transforms! It can make it brighter, warmer, and invert the signal for example.
How does tube preamp work?
The preamps are divided into two major parts:
- Preamps with transistors (JFET or MOSFET), a recent and robust technology.
- Pramplis lamp, sound warm and vintage (very much sought sometimes).
We will dissect these two “families”.
The transistor was discovered in the 1950s. It is a silicon-based component that allows, under certain conditions, to amplify a signal. It is usually very small, and very simple to implement: it is, therefore, the ideal component of the preamps integrated consoles and audio interfaces (with exceptions, such as TL Audio consoles for example).
There are two modes of amplification: class A and A / B. The first is harder to design, but offers a sound of great purity in every sense of the word, and mainly at the level of distortion. Class A / B, on the other hand, is easier to produce, so it has the best quality/price ratio.
But it is often criticized transistor preamps a certain coldness. The sound is clear but gives a “surgical” impression: it is too clean. This effect, accentuated with the advent of digital (converters use transistors), is one of the reasons for the return of the lamps.
How to set a preamp
These devices teem small buttons in the mysterious sense that we will demystify:
The connector “IN”: It is the plug where we connect the microphone, usually in XLR for symmetrical connections (point of hi if it is not), and in 1/4 “jack for asymmetrical inputs intended for the instruments
A” + 48v “ button: This button will activate the external power supply necessary for some electrostatic microphones This is a direct current transmitted to the polarizing microphone Note that even if the dynamic microphones are logically protected so as not to burn when they are connected to a preamp with the phantom power turned on, this is not something to try … you are warned.
A button, or PAD attenuation, usually -20 dB: What is more surprising to find a button that attenuates the sound on a device that amplifies me tell me? Its presence has several reasons: taking sounds on high volume instruments (guitar/bass amps, drums) may require attenuation. The use of the amp in the direct box (Instrument Level -> Micro level) also requires the activation of such a button.
A gain potentiometer: it is he who will precisely control the preamp stage. It is, therefore, this button that must be adjusted to have the desired level: we often deduce the right setting from the vumeter placed on the preamp.
On tube preamps: there is often a function that controls the very contribution of the circuitry of the tube. Called “Drive” or through presets (as on ART preamps using V3 technology). If such a potentiometer is absent, it is directly with the gain potentiometer that manages to control the “heat” of the sound.
A VU-meter: sometimes dial (needle), but often LED, it indicates the signal level at the output of the preamp stage. It avoids any saturation that could degrade the signal (the effect that may be interesting).
A signal inversion button (“Invert”): it allows to invert the signal (to make a modification of the phase of -180 °). This kind of button is vital in the case of stereo or live sound, to ensure the consistency of all signals arriving at the mix. When the instruments are recorded one by one, or by single microphones, it has little interest.
A last potentiometer is sometimes present, it allows to adjust the output level: it is very useful in the case of tube amplifiers. One can choose to push amplification of the tube circuit as much as possible to force the distortion effect and choose to attenuate the signal later.
The connector “out”: its use is similar to that of the connector In, except that it is used to connect the preamp to the console or the sound card 🙂
In addition to these generic buttons, each preamp can have its own functions. In addition, the mode is now “voice channel”: in a rack are combined preamp, compressor, and equalizer. So there is more than plug the output of the rack on a converter to try to make the optimum sound …. but prepare your scholarship! such devices are not trading at less than 400 euros for the lowest-end …
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Which preamp to choose?
After this little top more or less technical, you are aware of the usefulness of a preamp, as well as their various functions. But what preamp to choose? do you have to win the lottery to buy the vintage Avalon preamp at 5000$ to make a sound recording? Or DJ’s preamp mixer in the 70s be enough to record the sweet voice of my singer friend? Patience … we get there 🙂
In fact, everything depends on the sound:
For styles such as blues, jazz, and soft voices, we prefer a tube preamp, more or less good quality depending on the budget, associated with a static microphone.
For the stage, the small size and low cost of transistor preamps make them ideal in situations where the ears of viewers will not be as demanding as home listening to a CD.
For electric guitar and bass guitar: a tube preamp will add depth to the sound, but a transistor preamp may be enough since the goal is to reproduce as faithfully as possible the output of the amp of the instrument.
Finally, for acoustic pickups, the general properties of a preamp will be more important than its technology: it seeks the least noise and distortion possible. A class A transistor preamp may give a better sound, but will also be more expensive than a midrange preamp.
In all cases:
Do not choose a preamp for its technical features! Indeed, no need to have a reported signal-to-noise ratio of 90 dB if that of your microphone is 77 or 80 dB (case of some microphones Rode to 400 euros room for example). It is also useless to compare the distortion rates of lamp amps in front of transistor amplifiers: the distortion of transistor models is much more brutal and more unpleasant. Whereas we will admit a greater distortion on the preamps to lamps, even a big brand!
One thing to remember: the preamp and the microphone form an inseparable couple! It’s not because a preamp will sound good with a specific microphone that it will be objectively good, and vice versa. But as always a rule is in order: the more expensive it is, the more likely it is to be good.
Best audiophile tube preamp
The audiophiles are people with a special interest in reproductive systems and audio recording, in order to seek the highest quality and fidelity. The pleasure of listening to music in the most reliable way possible leads the audiophiles to take an interest in the underlying technology. Some extreme positions reject current techniques although these provide better frequency response and much less noise in the audio record.
Many times the results of a high-fidelity sound reproduction do not satisfy the preferences of the audiophile, so it uses the means available, such as electrical or electronic techniques to obtain audio effects that are in tune with their preference. This can initiate trends such as rejecting both digital media (CD, MP3, DVD included) and digital equipment when listening to music; opting for audio stored and treated in physical or magnetic media, such as cassettes, vinyl records, open reel tapes, tape recorders and equipment treated with valve amplifiers.
It is evident that a modern low-cost commercial equipment could well be of lower sound quality than one built with previous analog technology. An appropriate assessment of the fidelity and the acoustic phenomenon, in general, can only be carried out by means of instruments. A theory based on experiments explains the flexibility and loss of quick reference that the human ear possesses, and therefore the necessary regular exercise of good audio.
DIY tube preamp recommendations
You like to tinker, and you have at hand used equipment such as speakers or PC power. So here is a tutorial that can give ideas on tube preamp DIY. We provide details on the manufacture of a hand-made amplified box, on which one can connect a stereo preamp/tube amplifier or a simulator of the type Pod.
Here are the supplies:
Six boards of chipboard cut at Castorama 20 mm d Each thickness is in centimeters: 70×40, 74×30 and 30×40.
Eight squares with four screws: about 10 euros
Four solid wheels: 8$.
A PC power supply in AT format with an On / Off button. I recovered it on a wreck: 0 euro.
A car amp radio brand Fenton 2x120W, 4 ohms promo to 59 euros.
From painting black bomb, the fabric silver, the clothesline plastic … all amounts to 13 euros. You can get supplies from auto accessories stores for the fabric, which is supposed to cover the backs.
A small mixer used for the inputs (which can be replaced by simple 6.5 jacks). We find used to 15 euros.
Two 12 “Jensen speakers retrieved from a Beranger subwoofer In good condition, this type of speaker can be found for less than 50 euros each.
And now, let’s see the assembly.
It’s pretty simple. Just mount the box with the brackets. The planks are fixed with 5-centimeter screws.
The loudspeakers are then installed on the front panel. You can then paint and dress the amp in its own way.
At the back, I made cuts to gain access to the interior and install the console.
The wiring is just as simple. The console goes to the two inputs of the amp (Left / Right) that comes out on the two speakers. The amp and the console are powered by 12V PC power.
If you have questions about cabling, do not hesitate to ask them on the forum.
And here is the complete set.
What is the overall balance, and the result at the sound level?
Conclusions of DIY tube preamp recommendations
The amp itself is very powerful enough to not be pushed anywhere. Studio tests will give the verdict.
The sounds of the Vamp are well retransmitted, without the effect of compression and crushing hi-fi system speakers. In short, the operation is not very expensive and allows to have his handmade amp.
- The price
- The power
- Easy to assemble
- The versatility
- The weight (less heavy than a 2×12 combo)
- No lamp
- Requires a preamp
- The size (normal for a 2×12)
- The quality of the finish that depends on your handyman skills.