Below (Figure 1), a diagram of a hypothetical microphone pre-amplification circuit with the presence of additional controls as equalizer for high frequencies and equalizer for low frequency.
At the hardware level, basic, there are two types of analog microphone pre-amplifiers:
In figure 2 illustrates a generic scheme of the audio signal path using microphone pre-amplifiers Inboard.
As can be seen in the microphone signal output from the microphone enters incoming Mic In in which the microphone pre-amplifier will be present, once the signal is raised to values of line and possibly processed and mixed with other line signals is sent to input of a power amplifier with the task of raising the level of power line signal for sound diffusion.
The pre-Inboard mic amplifiers are those housed directly inside of audio equipment such as audio mixers, active and digital Stage Box – Splitter, various audio equipment with the presence of a microphone preamplifier inside them.
The microphone inputs of audio mixer analog and some digital (especially of old conception), always have pre-analog microphone amplifiers with the task of being able to amplify the signal line level before sending it to the various successive stages of the audio mixer (processing and mixing) , the same for the Active Stage Box – Splitter (variable gain) to amplify the signal directly from the Stage Box still before the signal transfer along the multipolar conductor ensuring a dynamic signal is transformed to those of the upper passive. For some digital mixer (especially for those who, like reference meter for the input signal level, have a meter with analog reference standard) the microphone pre-amplifier is present before the scheduled A / D converter.
The Inboard microphone preamps are those housed directly in audio equipment such as Audio Mixer, Stage Box – Active and Digital Splitter, various audio equipment with the presence of a pre-amplifier within the microphone.
The microphone inputs of analog and some digital audio mixers (especially old-fashioned) always feature analog microphone preamps with the task of amplifying the line level signal before sending it to the next stages of the audio mixer (processing and mixing), the same for the Stage Box – Active Splitter (variable gain) to amplify the signal directly from the Stage Box even before signal transfer along the multipole conductor ensuring a signal dynamic higher than the passive ones. For some digital mixers (especially for those who, as a reference meter for the input signal level, have a meter with an analog reference standard), the microphone pre-amplifier is present before the A / D line converter.
n.b. It is not mandatory pre-amplify the microphone input signal if it is already at a good level, the microphone pre-amplifier can be left to its minimum value of amplification, ensuring as seen in the previous article a better quality and less background noise (also though as we shall see when we discuss the mixing can lead to problems in some other way).
The input to the microphone pre-amplifier is always XLR (fig. 3) or XLR Combo in modern mixer and stagebox – splitter, while Jack TRS in audio mixer old-fashioned.
Generally, it is called Input or In (fig. 4) or Mic In (fig. 5) and then follows the reference channel numbering.
In the case of pre-amplifiers Inboard the input is almost always in the back of the equipment on which it is mounted, if we consider for example an audio mixer, microphone inputs and line are located in the rear.
At this input will in fact get a microphone level signal (millivolts, – 60 dBV / – 50 dBV) so as to be able to elevate levels of optimal voltage for the correct processing and mixing (we will see in future arguments which are the optimal levels that a signal audio must have according to the techniques used) up to a maximum value at line level or a little more before the distortion (0,770 v / 0 dBu or 1,223 v / +4 dBu).
The microphone pre- amplifiers Inboard have the advantage over those Outboard to have a performance and superior dynamic as that the circuit is wired directly on the machine and does not pass through external connections and connectors that can induce especially if poorly shielded introduction of noise and inductive phenomena. By contrast they have had to suffer the crosstalk (Cross Talk problem) between adjacent channels not present instead in those Outboard except in very limited extent multichannel. As a rule, it is useful that the crosstalk between two channels of microphone pre-amplification is separate of > 100 dB.
n.b. As also seen in topic Analog Audio Cables II, the signal it is unbalanced at the input to eliminate any noises and interferences loaded along the line and then only subsequently amplified.
At analog level the audio mixer always uses the same pre-amplifier for both the microphone signal and for that line (simply interposing the 20 k) resistance. When there is this situation if the mixer is level consumer or semi-professional has two signals can be sent simultaneously to the same input (but of little use not being able to manage separately the two signals), while at the professional level the mixer always has an input selector for deciding which line to send to the pre-amplifier (if that microphone input or line input), it is generally only a called Line button (fig 6) or Mic / line (fig. 7) that when clicked enables the line input and disables the microphone.
Fig. 6 Fig. 7
At other times it may be without a selector but present nonetheless separate two pre-amplifiers.
These pre-amplifiers (inboard) are generally calibrated during the construction phase in order to give the best of their performance on a microphone level signal, whereby for the management of a signal line level even though interposed a resistance of 20 KΩ as that present at the input line may generate some distortion and background noise more.
A highly professional level is constructed instead a pre-amplifier for the microphone input and one for the line input, wherein the amplification is independently adjustable (fig. 8).
These differences determine the quality but also the price of the pre-amplifier.
The pre-amplifiers are always Inboard mono (a pre-amplifier or double if even for mono line for an audio channel).
Fixed or Modular
Whether analog or digital mixer in the mixer Inboard pre-amplifiers can be fixed or modular.
Those fixed (fig. 9) are those present in the mixer and the consumer of average level but sometimes at a high level even if they can find, are pre-amplifiers installed directly on the electronic board that composes the internal architecture of the mixer itself, and if it is necessary to replace a pre-amplifier or damaged due to aging malfunctioning, or simply to clean it from dust and any oxidation is necessary to fully open the hardware of the mixer, remove the internal electronic board and then it is possible to work on it.
In most professional mixer but also expensive, the microphone pre-amplifiers are presented as modules (fig. 10) (fig. 11) installed in the hardware of the audio mixer (fig. 12) (fig. 13), (in which each generally component of each channel is modular, thanks to which it is possible to extract each individual module, clean and / or replace it with others of the same category modules or even with pre-modular amplifiers of different types to have different timbre in the input channels of the audio mixer.
Fig. 10 Fig. 11 Fig. 12
Generally, the modular component is more present and widespread in analog field, rarer are the cases of modularity in the digital environment as more complex and costly is the architecture of the components used.
In Figure 14 is illustrated a generic scheme of the audio signal path using microphone pre-amplifiers Ouboard.
n.b. The output of the signal at line level from the microphone pre-amplifier will have to enter the audio signal path without passing through additional pre-amplification in order to avoid the oscillatory problems and distortions seen previously, whereby in case of wanting to enter incoming microphone an audio mixer, whether it is analog or digital you will need to enter in the Insert line (Insert in) so as to bypass the pre-amplifier which may be present to the microphone input of the mixer itself. The same goes for an active and digital Stage Box – and Splitter that has a pre-amplifier circuit.
In the event that the pre-amplifier are further checks on the signal such as an equalization circuit, and that’s what you want to use to work on the audio signal, it’s good to set the insert of the audio mixer post eq, so as to bypass the equalizer of the audio mixer and introduce less noise. Not all mixer, however, especially those not of high level have the possibility to choose the position of the insert, in this case then it is good to exclude the equalizer (inactivating it through the appropriate button). If instead you want to use the equalizer present in the audio mixer, then turn off in the pre-amplifier.
The Outboard pre-amplifiers have the same characteristics as those Inboard but only built on external rack for housing precisely in the rack and easy transportation, being able to ensure their replacement in case of breakage. In addition to this by having more space and constructional dimensions can be created more sophisticated circuits and often the best performance.
In the case of Outboard preamplifiers, the input is almost always in the back of the equipment on which it is mounted, but it can also be located in the front (fig. 15).
N.B. Do not put in bridge different microphone pre-amplifiers thinking to realize the configuration of the multi-stage amplification seen in the first part of this article, which is used by manufacturers to ensure better quality and performance, as that is necessary to realize a real circuit, while putting simply bridge two or more microphone pre-amplifiers on the rack (the audio signal that comes from the first you send it to the input line of the second, the audio signal coming out from the second pre-amplifier you send it to the input of the third and so forth, remembering not to return ever incoming microphone pre-amplifiers in bridge to avoid the risk of bringing in distortion the pre-amplifier immediately in the next stage), it’s gradually generate an increase of the background noise and poor quality of amplification due to the sum of each single amplification stage that generates natural phenomena self-induction and countercurrents, even more so if the pre-amplifiers used have different characteristics from each other. It takes for which purpose-built circuits for the control of these phenomena by the inclusion of more stages of amplification.
Better for which to purchase a microphone pre-amplifier which may already provide more amplification stages in its interior.
To date fashion (a bit for the costs is widespread, a bit to follow the fashion) to use different microphone pre-amplifiers outboard as an alternative to pre-amplifiers inboard of the audio mixer only on the signal of the voices, especially lead vocal.
It’s a fashion would say to overcome (for those who have the ability) as that use different pre-amplifiers on different instruments, whereby also for example on percussion, guitars, basses, etc., can be determined qualitatively different sounds sometimes more beautiful and useful to their mix than in the basic audio mixer.
The pre-amplifier Outboard need for external power and presents the power button on the front or rear of the rack, the size is on average from one standard rack at 1 – 2 – 3 units.
The Outboard pre-amplifiers may be single-channel (fig. 16), two channels (fig. 17) and multichannel (fig. 18) (fig. 19).
In figure 15 a pre-amplifier tube mic at 1 channel.
In figure 17 a microphone pre-amplifier at the solid state at 2 channels.
It is seen as in the Outboard pre-amplifiers is easy to find components of a different class as passive, active, digital, an example is the optical input and output of this pre-analog amplifier. Even in Outboard the input is generally balanced XLR connector or TRS jack, the same for the outputs at line level. Some also provide the Insert input for the possible connection of dynamic or frequency processors.
In figure 18 a pre- microphone solid state amplifier to 4 channels.
In Figure 19, a microphone pre-amplifier solid-state 8 channels with ADAT digital interface, microphone inputs with XLR combo for housing also TRS and logical checks on the buttons.
n.b. A logical button is used mostly in digital hardware components (audio mixer, stage boxes, various processors), but it can also be found in some analog hardware components, compared to normal buttons and position switches (which once pressed remain in the position, as if this push down a button remains down, while if they push this back again on), are of superior quality as failing to provide mechanical components for the movement that with time (from wear) induce noise within the signal that passes through. The logical button is a button that when pushed and released always returns to its natural position.
Some microphone pre-amplifiers especially those vintage present selectors instead of buttons activation and deactivation of any processors and controls on the audio signal (fig. 20).
Especially the analog pre-amplifiers microphone on Outboard can also present a selector to eliminate the ground (GND Lift) (fig. 20) (fig. 21) (almost always positioned in the rear hardware as in Figure 19), which is useful in case of strong disturbances of the power line, especially in this pre-tube amplifiers. They can also file a voltage adapter for power supply, generally 230/240 v – 100/110 v, useful as that in some countries, eg USA, the power supply is not at 230/240 v as in Europe but 100/110 v.
Some also have an input for the signal level directly from musical instruments with low voltage values and high values of impedance (Instrument Input) (as can be as low passive electric and electric guitars), so that they can amplify and record directly as an alternative to miking. In this case the input impedance is greater than 100 KΩ (fig. 22).
Especially in those Outboard you can be found sometimes also a monitor output for generally connecting headphones or active monitor for pre-listening of the output signal (fig. 23).
Circuit and Output Impedance Outboard
The outputs as shown in figure 24 are always in the back of the hardware and are proposals on XLR connection or TRS always balanced, there may be both solutions or only one of the two (fig. 25).
The analog input impedance of a microphone preamplifier output will be ideal for the connection with an insert input of an audio mixer or the input of a multi-track recorder or any audio component, but which does not provide as seen further preamplifier microphonic or that it passes through a line resistance for the reamplification as that the signal is already in post-amplification line values.
The output impedance values of the line output of pre-amplifiers are generally standard from 50 Ω to 600 Ω also according to the type of output whether balanced or unbalanced.
Main Features Inboard and Outboard
As can be seen from figure 26, the microphone pre-amplifier may contain not only a possible control of the amplification level but also further parameters for the complete management of the audio signal before sending it to the mixing process, in Outboard pre-amplifiers it is easier to find several passive components, active, digital or a mixture of these respect to Inboard pre- amplifiers, and many more components there will be and the quality and the more the pre-amplifier cost.
In the next article we will see in detail the components that most are in microphone pre-amplifiers for the management of audio signal.
More on Audio Preamplifiers:
Audio Preamplifiers – I ( Principles of Operation, Analog Microphone Preamplifiers )
Audio Preamplifiers – III ( Preamplifier Controls II )