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Thread: Pre-amplifier

  1. #1
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    Pre-amplifier

    Hello everyone!

    May be someone can help me with the following question:

    What does a pre-amplifier at the input to a mixer channel do and on average, by how much?

    Thank you!!

    Nik

  2. #2
    A pre-amplifier boosts the signal from a microphone so it's ready to be processed by the rest of the system.

    Most things in a sound system happen at "line level," which is on the order of a volt. If you could open a mixer while it's in use and poke around in it with a meter, you would find line-level signals at every point. It's a good, useful all-around level that is neither so weak that it's overly susceptible to noise and interference, nor so strong it would require expensive circuitry to generate. RCA connectors, 1/4" plugs and the XLR connectors that carry a mixer's signal to a power amp are all line level.

    But microphones put out nowhere near this voltage unless they're dealing with really loud signals. A person speaking into a dynamic microphone from a couple of feet away will generate a signal of a few millivolts. This has to be boosted before it's of any use. As a very rough guess, an amplification factor of 300 would be common.

    If you're lucky, this gain will be adjustable. With loud sounds, the microphone output will be so strong that, if amplified another 300 times, the preamp will overload. A dynamic mic inside a kick drum can put out peaks of 5 volts, which is high even for line level. The old way of handling this would have been to physically insert an attenuator in the mic line. The current method is to adjust the preamp gain.

    Why can't you avoid the problem by simply turning down the channel gain? Because the channel fader comes after the preamp. These are called "gain" controls but a fader is actually a loss control. Full up, the loss is zero. At half-mast, the loss is typically about 15dB. (The gain is made up by amplifier circuitry elsewhere.) If you took a typical weak mic signal and attenuated it by 15dB, it would have so much noise in it that it would be useless. Thus the practice of preamplifying the signal before it hits the fader.

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