4.2. What does a preamp do?
A preamp usually is designed and used to do one or more of the following actions:
• Increase the gain from an audio source (for example a microphone)
• Changing the tone (by using filters, for example)
• Lowering the output impedance
• Converting from unbalanced to balanced
One of the most important controllers in a preamp is Gain. Gain just means the amount your
signal level is increased by the circuit. The amount a specific preamp increases your gain is
of utmost importance, because every device your signal will be sent to (in our case probably
the mixer) has an ideal range of signal level it expects in order to operate best. Not every
preamp can effectively drive a power amp, for example. The preamps we are looking into
were usually built to feed the mixing circuit, therefore they usually had a fixed gain amount.
Impedance can be described as the efficiency of the signal transferring from one piece of
gear to another. It is the opposition of a circuit to the flow of alternating current, a result of
the complex sum of resistance and reactance. A good illustration of this is cycling uphill. If
you are in the right gear, pedaling requires some energy; but if you are in the wrong gear, it
requires a lot more energy. An ideal impedance relationship is a very low output impedance
number connecting to a very high input impedance. That is being "in the right gear". If your
instrument or device has an output impedance that is too close to the input impedance of
the device you're connecting it to, you will be in the wrong gear, and your signal will be too
weak. That weakness may result in a lower signal level and/or a dull tone.
Preamps are usually "active", meaning they require a power supply. This is because it
requires energy to boost a signal. Currently, the preamps get the energy from the main
power circuit of the mixer. Older preamps like the V76 had their own power circuit built-in.
A balanced circuit is a signal-carrying circuit with two active electrical conductors of equal
impedance with respect to a common reference point, which is usually ground. Each
conductor carries the signal with a polarity that's the inverse of the other one. Usually, both
conductors are enclosed within an overall metallic shield, which does not carry the signal.
Balanced circuits can be found on professional-level microphones, XLR inputs on a mixer,
and balanced connections between an amplifier and speakers. The advantage of these
circuits is that the two signals are going to be decoded in the destination (remember they
were polarity-inverted), and the audio signal will be the difference between the two identical
signals. Any interferences picked up in transit will presumably not be polarity-inverted, and
therefore will have little to no difference between the channels. These will be eliminated,
resulting in an interference-free signal.
An unbalanced circuit is a signal-carrying circuit with only one electrical conductor and an
overall metallic shield. It is usually used in domestic connections, like Hi-Fi, for example.
Although there will be little to no problem in closer connections (input and output are
near to each other, therefore minimizing the risk of possible interferences), they would be
problematic in longer connections (a long wire basically works like an antenna).
That's why when we use microphones or other sound sources that are distant from the
mixer, we have to use balanced connections, and when they don't have them, we have to
connect them to a closer device that will convert the signal from unbalanced to balanced.
Arturia - User Manual V76-Pre - V76-Pre OVERVIEW
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Summary of Contents for V76-Pre
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