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Newbie Question - What does a pre-amp do?

Discussion in 'AV Receivers & Amplifiers' started by Neiliboy, May 27, 2003.

  1. Neiliboy

    Neiliboy
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    Hi All -

    I was wondering what is the advantage of having a pre-amp.
    Is it the same as a power amp?
    How would it advantage a home cinema set up?
    How are they all connected??

    Many thanks in advance.......

    Neil:smashin:
     
  2. hornydragon

    hornydragon
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    pre amp is basicly the volume control as power amps are fixed gain. It is more comlex but thats the basis the HC equivalent is a processor.
     
  3. sounddog

    sounddog
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    A pre-amp takes the output from your CD player (or other source) and adjusts the level to something compatible with the poweramp. It also (generally) controls source switching and have the volume control, etc. In an integrated amp ... you will have both a pre-amp and a power amp in the same box. High end equipment often has separate boxes for preamp and power amps to make for better electrical separation, and to allow a degree of mixing and matching to get the best sound.

    In an AV system, your integrated amplifier contains a pre-amp, various digital processing functions and power amps. Some people have preamp/processors which contain just the preamp and processing stages and have separate power amps.

    A pre-amp could be used in an AV system with a separated preamp/processor to provide a more direct and therefore higher quality path from a CD player to the poweramp, but running it in parallel to the preamp/processor. If this last bit confused you then don't worry ... in most AV setups - like yours for example - a pre-amp on its own would be of no use.

    The normal path is ...
    Source ---> Preamp ---> Poweramp ---> Speakers

    Hope this helps ... if you have anymore questions / need clarification just ask.

    Vikki
     
  4. NicolasB

    NicolasB
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    But does he know what a power amp does? :)

    The purpose of an amplifier, or rather, let us say, the purpose of amplification, is to take the relatively small electrical signal produced by something like a CD player and convert it into a much bigger electrical signal, which is powerful enough to drive loudspeakers. The signal fed to the speakers should be the same "shape" as the original one, but bigger. Imagine that you start with a scale drawing of a football pitch and, based on that drawing, you have to mark out a full size one. You're amplifying the original drawing. :)

    Now, as well as turning a small signal into a large one, the amplification needs to have a volume control. In other words, the amount by which you scale up the original signal has to be variable. (Working from the same original scale diagram, changing the volume means that you end up with different sizes of football pitch). With an "integrated amplifier" all of this stuff goes in one box. You feed the output from your CD player in one end, and plug your speakers in the other.

    But with a more expensive system it can be useful to split the amplifier into two parts. The power amplifier is the bit that actually turns a small signal into a large one - but it has no volume control. The amount that it scales the signal up by is constant. To control the volume you put another box in between the CD player and the power amplifier which actually makes the CD signal smaller, before feeding it into the power amp - and you can vary the amount smaller that it makes it. So the first box, the pre-amplifier, controls the level of signal that is fed into the power amplifier. Then the power amplifier scales it up.

    So a pre-amp is a volume control. As Vikki says, it often does other things too. It probably will allow you to plug several different sources in (e.g. a CD player, an MP3 player, a computer sound card output, a VCR) into it at the same time, and let you switch between them. Only one of the signals is actually fed through to the power amplifier. It might also have a headphone output, etc.



    With a home cinema setup it gets a bit more complicated. In principle the chain looks like this:

    DVD Player ---> Processor ---> Pre-amp ---> Power amp ---> speakers

    The DVD player produces a digital data stream. That is fed into a processor, which decodes it into 5 or 6 signals that resemble the two signals you would get out of a CD player. You then need a six-channel pre-amp (where what one would normally think of as being a pre-amp is two channels). The output from that is fed into a power amp.

    In practice you never actually separate the processor from the pre-amp, as there isn't too much point. So the chain actually looks like this:

    DVD player ---> Processor/Pre-amp ---> power amp ---> speakers

    Cheaper home cinema systems also put the power amplifier in the same box as the processor/pre-amp, in the same way that cheaper stereo amplifiers put the power amp in the same box as the pre-amp.

    Phew!

    Hope that's a little clearer.
     
  5. MarkE19

    MarkE19
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    NicolasB,
    I already knew all that, but trying to explain things simply without missing large chunks out is near impossible, but I think you managed very well.
    Now if you could just talk me though Quantum Physics I would be grateful :laugh:

    Mark.
     
  6. NicolasB

    NicolasB
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    OK.
    • Matter and energy are interchangeable.
    • Atoms absorb and emit energy in discrete amounts.
    • On the quantum scale things behave like waves when moving through free space and like particles when interacting with large-scale matter; this means they don't have a well-defined position or momentum until one of those two things measured or restricted in some way, and the uncertainty in one of those two measurements increases the more precise you are about the other one. (Among other things this means that a quantum particle can move from point A to point B without passing any point in between).
    • The way that any given particle behaves is random - but when averaging over large numbers of them, the overall behaviour becomes predictable.
    I think that pretty much sums it up.
     
  7. Neiliboy

    Neiliboy
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    NicolasB - That was an absolutely fantastic explanation - I can see clearly now the path to audiophile enlightenment.

    So do many people have a preamp/processor combo outputted to a power amp? Or is this fairly high end (expensive!) stuff??

    Neil
     
  8. menalaus

    menalaus
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    i wonder if you could help me now?
    i have bought, been given or stolen more Bic biros than i have had hot dinners, could you please tell me where they all are now?
    thanks in advance

    (P.S have look down the back of the sofa already)
     
  9. NicolasB

    NicolasB
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    It depends what you consider expensive.

    If we consider new prices, the cheapest you can get a stand-alone processor and multi-channel power amp for is a little under £2000. So that roughly coincides with high-end integrated systems like the Pioneer AX10 and Denon A1SR. Once you above that price/quality range, you're definitely dealing with separates. Below that you're definitely in integrated territory.

    (That's not including the price of player, speakers, or subwoofer of course).
     

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