Audio Preamplifiers – III

Pre-amplification controls

Almost all the controls to manage the signal to be pre-amplified are on the front of the hardware, whether it is Inboard or Outboard.
The main controls on the microphone pre-amplifiers Inboard are:

  •  Gain, Mic Gain, Mic Level, Gain, Input Gain, Input Level, Preamp Gain, Sens ….

Various names can have the control of the amplification level of the microphone signal.

The amplification of the signal level determines an increase in the value of the voltage as the signal is in the form of electrical impulses, this level change is called gain (Gain).

The gain, as well as how much amplification we can also identify the amplification ratio of 1: x, a ratio that represents the value 1 as the level of the input signal and X the level of amplification date.

The control of the gain at the analog level is generally a rotary button of red or white color (white almost always for the amplification control line if present) for Inboard, and various colors for Outboard forms with type (fig. 1), more modern and (fig. 2), more vintage.

Fig. 1 preview Fig. 2 1073dpafronthigh

The size and magnitude of the rotary button depends on the philosophy of construction, there is no standard.

The amplification is generally date by moving the rotary switch to the right, all in extreme left we have the minimum value of amplification, all in extreme right we have the maximum amplification value, for which an analog rotary button does not rotates 360 ° (only in some cases it can rotate through 360 ° but fulfill only one turn), has a radius of rotation established by the manufacturer also according to the resolution of date amplification. More precise is offering step amplification (for example a pre-amplifier which offers 3 dB of amplification every 10 mm of displacement of the rotary button is more decisive one which provides 5 dB for the same displacement), and much more could be the radius of rotation imposed by the manufacturer to allow the user to act on each step.

They are always linear buttons and not selectors, with a fluid movement of the pulsating (more professional) or at step (not professional).

In other (rarer) cases it is also possible to find them as in figure 3, in which the gain control is a generally sheared screw but sometimes also in a star. Useful especially for modules in fixed installations, so as to avoid an accidental modification especially by non-assigned personnel.

Fig. 3 pre-amplificazione con guadagno variabile a vite.jpg

At digital level instead are presented as a logical control which rotates endlessly (fig. 4) (fig. 5), the amplifier will follow the rotation (always a minimum and maximum to the left to the right) until its minimum and maximum values independently accomplished by the radius from the rotary button. A digital pre-amplifier generally provides a higher resolution than an analog one and allows to achieve higher amplifications with minimal distortion values.

Fig. 4 2016-03-21_19-07-29  Fig. 5 2016-03-21_19-05-46

The analog microphone pre-amplifier will wear out more quickly in time with respect to digital due to continual brought stress by the rotation of the rotary button, as it is connected to a mechanical movement, but has the advantage of suffering less of the moisture component and powder, and to have a very quick response.

The digital pre-amplifier will wear less because the amplification control is logic whereby turning the rotary button arrive electrical pulses to the amplifier module which amplifies (for which it is not directly connected to operate a mechanical movement). It wears more rapidly over time than analog when subjected to unfavorable conditions such as moisture and dust.

An analog pre-amplifier is it is why more faster in the amplification process, even if at the current state also controls the gain of the digital pre-amplifiers have very fast response times, next to the analogue ones.

The analog pre-amplifier also has the advantage of being faster in amplification processes, example, if port quickly the rotary button towards the extreme right, the analog pre-amplifier will arrive first at the maximum amplification value than digital, precisely because the control is mechanical and direct amplifier.

The digital pre-amplifier as well as all the digital components used in audio environment, offers better performance on the audio signal quality than analog, better linearity of the frequency response, reduced harmonic distortion and background noise, has a dynamic and a value maximum of amplification lower than the analog one, but the most modern come to be offering performance as analog ones also on these parameters.

Almost all of the digital pre-amplifiers that use digital amplification can also attenuate, of approximately – 2.5 dB.

Unity Gain, used mostly in analog rotary buttons, was used to define an amplification standard that determines the amplification with the attainment of the line level considering a line signal at the input and defines the fact that the amplifier has amplified the amount of compensation of the input resistance. Sometimes it can also be in reference to the incoming microphone signal, and goes to consider the attenuation given by the resistance present in the microphone input.
Some analog pre-amplifiers in the gain control indicate this value as 0 dB or as U (Unity Gain) (fig. 6).

Fig. 6 sddefault

Not all but many microphone pre-amplifiers have a minimum gain already basic and not attenuated (the minimum possible value), of about + 3 dB / + 5 dB sometimes even +12 dB / + 20 dB (every 6 dB of gain is a doubling of the voltage level), also visible from the numbering which indicates the gain values present in the rotary button as shown in figure 23 in which the minimum value is + 12 dB.

Even the pre-amplifiers that indicate how the minimum value of 0 gain have most often a minimum of basic amplification (fig. 7).

Fig. 7 2016-03-22_14-05-46

The numbering as can be seen in figure 7 indicates the level of gain given by the pre-amplifier, the higher will be and the more quality will also be the pre-amplifier capacity itself, but as mentioned is always more qualitative if not necessary, not amplify.

The control of the gain for the microphone input signal is always positive as can be seen from figures 6 and 7, while the range for the line signal control part from negative values to positive values of amplification, because there is a resistance input (fig. 8).

Fig. 8 2016-03-22_14-18-17.jpg

As seen in Figure 8, the minimum value is – 20 dB to indicate that the input of the line there is a resistance which attenuates the signal level of – 20 dB to bring it to mic values. The rotary button amplification of the line signal has a smaller scale than that of the rotary button of the microphone signal because as mentioned in the previous article the line signal does not reach its values similar to those mics but has a few decibels more and for which necessity of minor amplification to achieve the line levels.

When using the same rotary button to operate on the gain level of a pre-amplifier that amplifies both the level of the microphone signal at that line, this will have two scales, one for the microphone gain and a gain for the line (fig. 9) (fig. 10).

Fig. 9 2016-03-22_15-18-37.jpg

The scale which also has negative values is always the one referring to the signal line, while the one with only positive values is referred to the input microphone.

Fig. 10 1073dpafronthigh
When there is this solution you can move up and down the rotary control, usually when it’s down will work on the amplifier in the input microphone and when it’s up work in the input of the amplifier line. The position to hold the can be seen generally from where are positioned the written mic (below) and line (above).

In other cases there is a selector to decide which send input to the amplification circuit.

In some cases, as for the pre-amplifier with the gain control in figure 10, when the cursor is positioned on Off it means that the pre-amplifier does not provide gain to the signal passing through it. This type of rotary buttons generally has stairs type that in figure 9, different from the stairs views to the pre-amplifiers used both for amplifying the microphone and line input signal seen in figure 9.

Some rotary controls, especially those of the past, may look like the one in figure 11.

Fig. 11 DSC_0412.jpg

The words sens is no longer used except in vintage equipment, the scale is expressed in dBm, as seen in topic Decibel Meter I one-time used circuits to maximize the power transfer and not voltage as now.

The scale as can be seen partly from a positive value + 4 dBm which indicates the voltage level required to reach the value of the operational level of line, gradually numbering goes on decreasing up to the value of – 50 dBm which indicates how in less than amplification is necessary to obtain a voltage level at the operating standard line level (measurements made by means of a test signal, so not to be taken as a reference for the common values of microphone signals used, and this is also the cause for which this type of scale has fallen into disuse).

For analog microphone pre-amplifiers with double amplifier (one for the microphone input and one for the line input) we will have precisely two separate rotary buttons to control the gain, with its own scale, a microphone and a line (fig. 12).

Fig. 12 2016-03-22_18-36-54 ( rosso is mic, white is line )

At digital level instead of the rotary buttons do not have almost never the gain numbers (fig. 13), this is because already there are many graphics and indications received dates from digital meter and software present in the digital audio mixer or digital outboard and especially because the button it can spin endlessly for which impossible to define its position according to its values.

Fig. 13 2016-03-22_16-33-01

Some analog microphone pre-amplifiers such as the one in figure 14 or 15, can have a control over the microphone input impedance variation, so as to put more or less resistance depending on the impedance of the microphone output also connected to it on the basis at the timbre of the sound generated.

Fig. 14 2016-03-22_16-26-39

Those in figure 10 and 14 are controls with fixed impedance variations, while in Figure 15 an example of control with impedance variations determined by the user.

Fig. 15 2016-03-22_18-37-14

Near or in the vicinity of the gain controls, there is always a led for signaling the values of distortion called OL (over load), clips, and other names, (sometimes place directly in the meter or in substitution to the same meter) (fig. 16) (fig. 17).

Fig. 16 2016-03-22_14-46-24 FIg. 17 2016-03-22_16-09-59.jpg

Some microphone pre-amplifiers especially analog Outboard may submit Insert connections (fig. 18) (fig. 19), useful to insert along the line frequency processors and / or external dynamic with analog connections.

In figure 18 a connection with Insert send and return in the same connection.

FIg. 18 2016-03-22_18-19-21

In figure 19 an Insert connection with separate send and return.

Fig. 19 2016-03-22_17-38-07.jpg

Both connections can be easily finding in the back of the hardware.

Other control parameters

The most common control parameters present in the microphone pre-amplifiers Inboard and Outboard are:

  • Phantom Power to + 48 V (fig. 20) (fig. 21) (fig. 22).

Fig. 20 2016-03-22_14-45-34 Fig. 21 2016-03-22_15-47-11.jpg Fig. 22 2016-03-22_16-25-01

In some pre-amplifiers it is also possible to find values of different phantom, useful to feed equipment with a different voltage.

  •  Pad attenuation (fig. 23) (fig. 24)

Fig. 23 2016-03-22_14-45-45 Fig. 24 2016-03-22_16-17-29

Useful if the input level, especially the line has too high values that lead to distortion levels or next.

Generally, the pad has an attenuation value of 20 dB but you can also find a 10 dB and 30 dB, and even with a variation of the attenuation level.

  • Reverse Phase (fig. 25) (fig. 26) (fig. 27)

Fig. 25 2016-03-22_14-45-23 Fig. 26 2016-03-22_16-29-34 Fig. 27 2016-03-22_16-38-01

Useful for reversing the polarity of the audio signal in the event that this will lead to an antiphase in the process of mixing with other audio signals.

The symbiology used is mostly the one in figure 25-26-27, but sometimes it is also possible to find Phase Rev, or Reverse Phase (fig. 28).

Fig. 28 2016-03-22_15-02-51


  •  High or Low Pass Filter Cut (fig. 29).

Useful to eliminate from the signal frequencies do not wish within the mix (then we will see in subsequent arguments the use of filters and their structure).

Fig. 29 2016-03-22_14-45-13

Very often, with the possibility to choose the cut-off frequency as shown in figure 29 (especially in Outboard, Inboard high level, and filters in digital mixer) or with fixed cutting (fig. 30) (more prevalent in analog Inboard and mixer generally at 75 Hz, 80 Hz or 110 Hz, is sometimes indicated the cutoff frequency and also the filter slope) (fig. 31).

Fig. 30 xfgdf Fig. 31 2016-03-22_15-23-59

Some Inboard – Outboard professional and digital mixers also have the low-pass filter or hi-cut (fig. 32) (fig. 33).

Fig. 32 2016-03-22_15-04-18

Fig. 33 2016-03-22_15-30-41.jpg

The filter as well as all other parameters is always to be activated if you want to use, pressing on its mechanical or logical button.

Some professional level may also decide the slope of the filter (fig. 34).

Fig. 34 2016-03-22_15-29-13

The scale which is can be in dB/Oct or Orders.

Those just seen are buttons at the analog level, while at the digital level is presented as an infinity rotary button (fig. 35), generally with a scale greater than analog ones and almost infinite step, since the cutting of the frequencies is performed at the software level (can have also step 1 frequency at a time), while at the analog level the filter is less selective and works with higher margins.

Fig. 35 2016-03-22_15-39-54

They can also be present Mute buttons (to disable the pre-amplifier), Bypass (to feel the difference in level between the original signal and the amplified and / or processed).


  •  Circuit Compression (fig. 36)

Fig. 36 neve5015.jpg

Especially in analog Outboard the pre-amplifier may also present a compression circuit as shown in Figure 36, sometimes also mixture of other dynamic processors such as gate and expander (we will see in other arguments the structure and operation of dynamic processors).

This useful for working on the dynamics of the signal, especially if it is better than that found in other Outboard and Inboard within the audio mixer used.

On the use of the dynamic processor in microphone pre-amplifier instead of the audio mixer it will be necessary if you can set the connection insert of the audio mixer post-dynamic, and if not possible, be sure to turn off the dynamics of the audio mixer. Disable instead the dynamics of the pre-amp if you prefer to use the one in the audio mixer.

In case of use of a dynamic Outboard processor it will be necessary to withdraw the line output from the pre-amplifier and send the signal to the line input of the processor, so that it can lower the signal again to values close to the microphone for the correct audio signal processing, and in turn by the dynamics processor will fetch the signal output at line level to be sent at the input of the audio mixer insert (fig. 37).

Fig. 37 2016-03-23_11-37-31.jpg

n.b. At the level of dynamic processors some technical tend to put in bridge different processors working step on the audio signal, an example from the first compressor compress 2 dB, from the second in bridge will compress of other 2 dB etc … . in order to have in output at the end of the processors in the bridge, the compressed signal properly, and being able to work every single compressor so as not excessive and with little stress by obtaining a higher quality. In this case it is possible to do so without the introduction of significant background noise as there is the presence of microphone pre-amplifiers, but always make sure that this does not generate more noise, alteration of the frequency response and distortion. If you choose this solution, then it is possible to use both the compressor present in the microphone pre-amplifier that any Outboard or Inboard in bridge.


  •  Equalization circuit (fig. 38)

Fig. 38 1073dpxfrontlights.jpg

Always in analog pre-amplifiers can also find the equalization section to affect the tonality of the audio signal before the output (control low frequencies, medium, high), the more will be the professional microphone pre-amplifier and could be much more articulated if present the equalization section.

Again, as seen by the dynamics processor you must disable the equalizer not used (that of the microphone pre-amplifier or that of the audio mixer), the insert must be placed if possible post-eq, are not put into bridge more equalizers because in this case there are amplification circuits (fig. 39).

Fig. 39 fddfgd.jpg

n.b. As we understand the audio signal undergoes amplification, attenuation, amplification, attenuation, increasing to each processor we are going to insert along the audio chain. This leads to alterations of the frequency response, dynamic, harmonic distortion and everything that can make change the starting sound. It is therefore necessary to use all of these components only when necessary and useful indeed to improve the sound quality, and it is crucial that they are of excellent quality.

n.b. Sometimes even the pre-amplifier Inboard to analog audio mixer interior can be composed of the pre-amplification more dynamic more equalizer section.

At digital level the audio mixers have the compression and equalization processes mostly in software, so when using an external processor, you just disable their operation.

Even Outboard digital pre-amplifiers follow the same principle, once the signal is converted, is all at the software level.
More on Audio Preamplifiers:

Audio Preamplifiers – I ( Principles of Operation, Analog Microphone Preamplifiers )

Audio Preamplifiers – II ( Preamplifier Controls )




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