Audio Preamplifier – I

The audio preamplifier as deduced from the word itself is a component used to amplify an audio signal before the final amplification.

Pre-amplification and final amplification are two separate things not to be confused.

The final amplification is the amplification of the signal with line level (0.770 v – 1.223 v) up to power values (tens of volt), realized by the use of power amplifiers called ultimate power terminals (fig 1) in the definition of the last amplification stage before the sound diffusion of which we will discuss in subsequent articles. This power is generally used to send the audio signal to the passive audio speakers (fig. 2) (fig. 3).

Fig. 1 Powersoft-K20-from-Old-Barn-Audio.png

Fig. 2 line-array-Axiom-AX2010A-elemento-attivo-small-9754-431

Fig. 3 2016-03-21_12-41-02.jpg

In figure 3, a conceptual diagram of the final amplification stage, in which the signal line level, for example the output from the master out of the audio mixer, it is sent to the input of a power amplifier with the task of raising the signal audio to a power level so as to be able to properly feed the audio speaker for sound diffusion.

The pre-amplification instead is used to elevate the microphone level signal (millivolts) to line values (0,770 v – 1.223 v), this is necessary and essential in order to qualitatively process the signal (equalize, compress, manage output lines, mix and all those operations that can be done with outboard processors, audio mixers, analog and digital devices with the audio signal to line-level management) before sending it to a final power.

The device that performs this type of amplification is called microphone pre-amplifier (fig. 4).

Fig. 4 1073dpafronthigh

In figure 5 a conceptual diagram of the pre-amplification stage.

Fig. 5 2016-03-21_12-24-38

In figure 5 it is understood that the microphone signal, such as the one output from the microphones, must enter a microphone preamplifier so that it can be elevated to line values and can be properly processed and sent to the amplification stage for audio diffusion.

They do not exist instead pre- amplifiers of line level because as we will see for work on a signal of a line level is necessary before lowering its value at microphone level to then be used microphone pre-amplifiers to raise it to line values useful for the processing and mixing.

n.b. A line-level signal within standard audio equipment such as those used in the professional audio world, as well as being able to create distortions and damage to components, would be unmanageable as it would always have values close to 0 dB and above without being able to handle quality adequate signal level to be used for processing, recording and mixing. So, it’s always lowered to microphone levels or close to then being pre-amplified and processed properly in the right level of amplification.

The only pre- amplifiers at line level used are those for balance-imbalance active line present in the output circuits at line level in the modern analog and digital audio mixer (Main – Aux – Group – Omni – etc. … Analog out), stage boxes – fixed gain active splitters and digital (analog out) and some multitrack recorders (analog in and out at line level), and the input circuits (analog input) at the line level in the power amplifier. But these are very different from those that we will be used to process the audio signal, as explained in Analog Audio Cables IV, are internal circuits at the hardware of the processor realized by the manufacture for the functions of balance the signal and are mostly at fixed gain or little more than 1.

 

Features

You cannot choose whether or not to use the pre-amplifier microphone, it is always and in any case necessary before any work on the audio signal at the microphone level otherwise it would risk the introduction of background noises and strong distortions on the signal, even more when it rises to power levels.

As regard to the shape, size, and components used in a microphone preamplifier these are varied, each manufacturer has its own depending on the time it was built, based on its quality and price.

The microphone pre-amplifiers can be divided into different categories such as:

  • Analog microphone Pre-amplifiers
  • Digital microphone Pre-amplifiers
  • Software microphone Pre-amplifiers

 

Analog microphone Pre-amplifiers

Analog pre-amplifiers are made using passive and active analog circuitry (so any microphone pre-amplifier needs external power supply for its operation) such as Valves (the first systems still used today to create vintage sound patterns) and Bipolar Junction Transistors (BJCs), which are still the most used today, whose diagram and component are in figure 6.

Fig. 6 bjt-terminals.jpg

The pre-amplifiers BJT are mostly manufactured using an amplification structure in class A or AB (example in fig. 7).

Fig. 7 (pre-amplifier scheme in class A, from ilprogettista.it)

product_195.gifFig. 8 (pre-amplifier scheme in class AB, from ilprogettista.it)

Fig. 8 product_198.gifIn a real context and more modern, to achieve levels of pre-amplification more qualitative, precise and higher amplification levels without introducing distortion and significant background noise, using up to “millions of these active components BJT” ranging has forming the so-called electronic amplifier Solid State or Integrated Circuit fig. 9)

Fig. 9 index.jpg

Valve preamps instead use a circuitry with Triodes valves and a Class A amplification structure (figure 10 – 11 – 12) is always implemented.

fig. 10 (Class A pre-amplifier valvular scheme, from audiovalvole.it)

bronzetti_finale_potenza_audio.gif

fig. 11 (Triode scheme, from audiovalvole.it)

 trido_composizione.gif

fig. 12 (Triode)

RS242_triode_2.png

The use of these components with this type of class in configurations A makes the input impedance of the pre-amplifier input is adequate to properly receive and transfer the input audio signal (microphone signal) towards the pre-amplification process, to amplify properly and get the best performance on the microphone signal level throughout the audible audio band (20 Hz – 20 kHz) and up to the highest dynamics.

Each manufacturer has different philosophies about what is the best input impedance and circuit to have to get a 100% efficiency (ideally), in real cases, the transferred signal to the amplification process even in the most modern and qualitative pre-amplifiers does not exceed 90% yield.

Some circuits are both active unbalanced that pre-amplifiers (the latest, allow to obtain higher yields), (because as we have seen on the argument Cables Audio Analogue III and IV, the balanced audio signal is unbalanced at the entrance before being pre-amplified to eliminate all interferences loaded along the line, but in general within the electronic valve and audio equipment the audio signal travels and is processed unbalanced).

Other unbalance the signal with a passive component (the transformer center tap) and then send the signal to the pre-amplification (older and less qualitative).

In general there is a better than the other system, if we look the quality and linearity as well as transparency of the circuit, without doubt, the pre-electronic amplifiers solid state class A or AB, which serve as both unbalancing that by pre-amplifier are the most qualitative, others have each its timbre,  especially the pre-tube amplifiers tend to have a hot stamp due to the introduction of even harmonics of higher order generated by distortions of the valves, the same distortion values ​​are more pleasing to listening in pre-tube amplifiers and electronic. So, it depends on what we want to achieve, if a warm sound and vintage undoubtedly opt for the pre-tube amp, if a pure sound and transparent opt ​​for pre-electronic amplifiers, otherwise all to experience, each pre-amplifier its timbre and its characteristics.

There are also pre-tube amplifiers and solid-state together for hybrid configurations.

Some manufacturers then use systems with multiple stages of pre-amplification (both solid state valve), which are considered qualitatively better than those single-stage, in that each individual amplifier used can be calibrated to work in a certain amplification range so as to introduce less distortion and noise can bottom (e.g. from 0 to +5 dB with the first amplifier, from +5 dB to + 10 dB with the second and so on), this is because each individual amplifier can be calibrated in its circuit for to work at its maximum efficiency and maximum performance only for a small range of amplification.

n.b. In general, at the level of use of microphone pre-amplifier in order to obtain a higher quality audio signal is always recommended amplify as little as possible, the more it amplifies and the more the pre-amplifier will introduce noise and distortion.

As we will see later the pre-tube amplifiers are best suited for use on external hardware, while those electronic also as pre-amplifiers internal to the audio mixer themselves, this is because the valves are very fragile, suffer much more of concussions, dust and humidity, and have a charging time to get up to speed (time required for the valve loading “that goes in temperature” so as to work at its best and provide the highest possible performance), for which they need more devices including:

  • Turn on time for the pre-valve amplifiers (the same is true for any device that uses amplification valves, such as pre-amplifiers for electric guitar and bass guitar). The charging time also depends on the number of valves present and their type, triodes used for the microphone pre-amplifiers 15 – 20 minutes are up to speed, for pentode used in pre-power amplifiers instead also needed 30 minutes.
  • Please do not send signals to the valves before they are fully charged to avoid altering the loading processes in the long run could ruin them. Perform sound check and recording only when the valves are fully operational.
  • From off to move the valve pre-amplifier with foresight and place it gently on the various surfaces, if the pre-amplifier is turned on, not to move it precisely.
  • As a charging time when lit also has a discharge time when turned off, so before moving to wait that the valves are completely cooled.

n.b. If the pre-valve amplifier is turned on, it is possible in certain cases to see the valves illuminated as loaded (access color as in figure 13, if still off means that there is probably a charge), bumping the hardware problem is in certain cases possible to perceive a sudden distorted sound, this indicates the extreme fragility of the components which if shaken tend to destabilize the circulating signal, up to rupture risk. It is of fundamental importance for which does not move the valve pre-amplifier when turned on.

Fig. 13 2016-03-22_15-12-55

 

Input Impedance

The output of a microphone signal as said has values ​​of a few millivolts of voltage (generally not exceeding 20 mV – 50 mV) and the output impedance of the microphones considering the most commonly used (dynamic and condenser) is on average from about 50 Ω – 100 Ω to 500 Ω – 600 Ω

To optimize the load transfer of the microphone signal to the amplification stage will be necessary that the impedance of the microphone pre-amplifier input is much higher than that output from the microphone or from any device whose signal has voltage values ​​similar to one microphone to be sent to a pre-amplification can be input as the output signal from a D.I.Box. The same principle also seen in topic D.I.Box for interfacing of instrumental lines and D.I.Box inputs.

The impedance ratios used are generally in the order of a minimum of 1: 6 – 1:10 even up to 1:20 and more, the greater will be the transformation ratio and much more quality and therefore expensive must also be the components used.

n.b. The impedance ratio is not to be underestimated, since the lower and the more the microphone will tend to generate distortion with less incident sound pressure, hence a lower output voltage value from the microphone. For example, in figure 14, the highest sound pressure values are shown before generating a THD distortion value of 1% considering two different input impedances (preamps).

Fig. 14 2017-07-06_17-05-29

As can be seen in fig. 14, a higher input impedance favors correct signal passage and thus minor distortion up to a higher voltage value (hence sound pressure).

A different transformation ratio is often also determines a different timbre and a unique characteristic of frequency and dynamic response, such that it is useful to try different types of pre-amplifiers with different types of transformation ratios in order to obtain the sound helpful for their own mix.

The pre-valve mic amplifiers generally tend to have higher impedances for natural characteristics of the valve itself, while those on the integrated circuit are those with lower input impedance, and as seen along with the quality of the components used and the flow rates harmonics from the same components used in the amplification circuit is able to obtain the most colorful and warm sounds (valves), or more transparent sounds and clean (integrated circuits).

In general, it is useful to know the output impedance of the microphone used to associate to it the microphone pre-amplifier with the most suitable input impedance (which complies with a minimum transformation ratio of at least 1: 6 – 1:10).

n.b. In case of background noise on the line to try to use a larger transformation ratio.

But even this may be just the opposite of all because there are quality pre- amplifiers mic with low transformation ratios that sound better than pre-amplifiers with high conversion ratios, is helpful then try following always and in any case these guidelines that are the fundamental basis for knowledge of the structure and the microphone pre-amplifiers operation.

n.b. They use not instead pentode or higher order valves as they present an impedance too high and input characteristics are not suitable to amplification of low-level audio signal such as the microphone, the same goes for the use of electronic amplifiers such as mosfet, both pentodes than mosfet as we shall see that views of their characteristics are used for the final power amplification.

To give some examples in the case of using a microphone with an output impedance of 250 Ω is useful to interface to it a microphone pre-amplifier with an impedance of at least 1500 Ω input.

n.b. On average the entry of pre-amplification microphone used in analog and digital audio mixer with analogue input has an impedance which is never less than 1500 Ω. At the level of pre-amplifiers outboard impedances can also be found in the order of 4000 Ω – 5000 Ω, 300 Ω as well.

 

Line level signals

If we have a balanced signal or unbalanced line level what we do?

An example may be the output signal from electro-acoustic guitars, keyboards, electronic drums and samplers.

As defined it will be necessary to lower the voltage level at mic values, and to do this interposes a resistance of 20 k before the arrival of the signal at the microphone pre-amplifier (fig. 14).

Fig. 15 (illustrative example of the presence of a resistance on the signal line before the microphone pre-amplifier).

sch_att_01

The input for the line-level signals is often called Line In in analog audio mixer (fig. 15) or pre-amplifiers on external hardware or analog input of old conception digital audio mixers (fig. 16), and is always on TRS own connection because the tools whose output signal is at the level of line passes on Jack TS or TRS connection, while for the stage box – analog active splitters this type of input is not expected because that their purpose is to transfer microphone signals over long distances, while the line signal output from different musical instruments is always unbalanced (if it is balanced to ensure that the signal level is the one microphone otherwise place a D.I. Box or get to the line input audio mixer).

Some audio mixer especially well-crafted high-level, may eventually provide balanced XLR line inputs.

Fig. 16 2016-03-17_16-38-12

Fig. 17 images.jpg

In figure 18 a graph example of the signal connection at line level in microphone pre-amplifiers in order to be processed, for example, and mixed with the microphone audio signals.

Fig. 18 2016-03-21_13-03-59.jpg

This example refers to a configuration with the audio mixer in the vicinity of the instrument that sends the unbalanced signal line (within 10 meters).

In figure 19 an example instead of considering to send the signal at distances longer than 10 meters, which as we know is the need to transform the unbalanced signal to balanced.

Fig. 19 2016-03-21_13-14-31.jpg

In the example it is clear how interposing a D.I. Box it is possible to bring the unbalanced line signal in a microphone input may be balanced by interposing attenuation pad directly from D.I. Box or from the pad present in the audio mixer if the incoming signal is too high.

n.b. In reality, the signal line level even after the insertion of the resistance from 20 k or a D.I. Box is close to that microphone, generally +5 dB / + 6 dB more than the average microphone level. There is an ideal resistive value to be placed on the line to get a real mid-level signal as the microphone of the line signal level that will be after the resistance from 20 k depend strongly from the output impedance of the instrument from which it picks up the signal line level, from the total impedance of the circuit including that of the cable in which the signal passes and many other factors.

If the line signal you want to insert into a microphone input as seen on topic D.I. Box and in the example in figure 16 is in fact necessary to introduce the D.I. Box that in addition to balance allows to adapt the signal level and impedance for the microphone standards. In this case it is possible, if the signal is still too high level, inserting additional resistors (pad) present in D.I. Box generally with attenuation of the signal level of 10 dB and / or 20 dB to be used in lieu of the resistance of 20 k at input line of the audio mixer or external microphone pre-amplifier, being able to connect the signal directly to the microphone input balanced.

In analogue or digital field of old-fashioned in any case with the use of D.I. Box is always more qualitative lessen upstream, then use the attenuators of D.I. Box and then enter the hall microphone pre-amplifier, unless you have some internal damping pad to the audio mixer used much more qualitative or generate less noise than those of D.I. Box used.

In modern digital audio mixer field a few have the attenuation pad digitally and / or software, but the principle is the same, use the attenuator of D.I. Box unless the mixer pad does not generate a noise lower down.

At level digital audio mixer of modern conception, the input line does not exist (fig. 20), but are present only balanced XLR microphone inputs.

Fig. 20 2016-03-21_13-45-44.jpg

This is because it uses the same A / D converters for both the imbalance of the balanced signal processing, analog / digital conversion that pre-amplification, and guarantee to obtain a very high input impedance level even with transformation ratios of 1:20 1:40 (example 5 KΩ to 10 KΩ) ensuring high performance without loss of quality. So, as you understand are microphone inputs that can also be useful to receive a signal line level, generally then introducing a pad with 10 dB of attenuation.

In some cases, it is still possible to find thought limited analog line inputs with A / D converter but called AUX IN (fig. 20), generally used to pick up signal from external outboard line level (as frequency processors and dynamic) in place of those within the digital audio mixer.

Fig. 21 2016-03-21_13-48-06.jpg

Given the resistance of the 20 KΩ input sometimes 40 KΩ, even if the overall input impedance is determined by the A / D converter, it is also possible to use them as line inputs to connect the output signal from the musical instruments, multitrack recorders, etc …

To send a pre-amplified signal by a pre-external microphone amplifier is possible in the internal routing level to the digital audio mixer set this AUX IN connection as Insert input, so as to bypass the pre-amplifier present in this input.

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