Pre-amplifier vs power amplifier

Discussion in 'General Electronics Chat' started by Mazaag, Jan 10, 2010.

  1. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
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    Hi guys,

    I came across this definition online for a pre-amp.

    Preamplifier » The first amplifier in the chain, the preamplifier takes a low level signal from a guitar pickup, mic, or turntable, etc., and amplifies it. Technically speaking, the preamp provides significant voltage gain and small current gain, which makes a preamp good for recording applications. A power amplifier must follow in order for current to be amplified enough to power loudspeakers.

    Firstly, I would like to know why a voltage gain alone makes a preamp good for recording applications? Is it because we don't need large currents to drive speakers? but rather a strong voltage signal for line-in purposes?

    Secondly, is there a difference between a power amplifier and a current amplifier? I understand that POWER is voltage x current, so does that mean we are amplifying both the voltage and the current simultaneously? In terms of sound, does that mean increasing the volume and providing enough "force" to drive the speaks? or does electrical power translate to a different characteristic in sound?

    Lastly, why use a pre-amp at all? why not run it directly to a powerful amplifier (power) right away? Since (based on my assumption) a power amp will amplify both voltage and current as mentioned above.

    Thanks guys. I appreciate your help.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    The signal coming from a guitar pickup or a mic is very small, and sometimes very dirty. A good preamp will condition that signal and amplify it to a good base level for you power amp.

    Think of it this way. if you have a 100x gain on an amp and you start with 1 you will end up with 100. great. BUT if you preamp your 1 to 5, then after poweramping 100x you will end up with 500.

    Yes. You can put 500v to a speaker and it will just sit there. You have to fluctuate the current to change the vibrations of the speaker.

    See what I said above. You want to condidtion the signal before really boosting it. Also some amps come with built-in preamps. Look at most tube-amps. The smaller tubes are usually preamps and the larger are for main amping
     
  3. Mazaag

    Thread Starter Senior Member

    Oct 23, 2004
    255
    0
    Great. Thanks for your help retched.

    So now, i'm trying to understand a pre-amp circuit off one of the do-it-yourself kits. I think I understand most of it, but I do have a few questions on some of the components.

    http://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k2572_rev2.pdf

    The circuit schematic is on page 2.


    There are two inputs (left and right) and two corresponding outputs (hence stereo)

    C4 is a grounding capacitor (is that the proper term?) to get rid of any ripple or unwanted AC from the supply voltage.

    R2 and R1 seem to make up a voltage divider that feeds into the positive terminals of the op-amps (not sure why we need a vdivider here)

    The op amps make up an inverting amplifier with coupling caps C7 and C6 at the inputs. R3/R6 and R4/R5 provide the gain for the amplifiers.

    I'm not sure what the purpose of the rest of C1 across the R1 resistor.

    Rv1 and Rv2 are pots I believe that are current limiters? but doesn't a preamp amplify voltage with not much current gain? why do we need to limit the current.

    And I have no clue what the output caps are for, filtering of some sort?

    Thanks Guys
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    It is a filtering CAP. It filters out the ripple and such. ie. the 60Hz hum of household current.

    This is important, so read up on this.
    http://en.wikipedia.org/wiki/RC_circuit

    OPAMPs use a reference as an input. the resistors are basically turning the knob on the amp. Your not limiting the current to the output, just providing reference. Thats how the amp know how much gain you want. Also known as FEEDBACK. http://en.wikipedia.org/wiki/Negative_feedback

    Filtering. Exactly.
     
    Last edited: Jan 10, 2010
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