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What's the difference between audio and pre amplifier ?

Discussion in 'Electronic Basics' started by bench, May 3, 2004.

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  1. bench

    bench Guest

    Hi all,

    I'm rather ignorant about audio circuits, I was just wondering
    is there some sort of convention regarding what would be
    called an "audio pre amplifier" as opposed to simply
    "audio amplifier"
  2. grahamk

    grahamk Guest

    An audio preamplifier takes the low level audio signal from microphone, turntable etc and amplifies it, to make it of a suitable amplitude for the input of an audio power amplifier. It often incorporates volume and tone controls.
    The audio power amplifier converts this input voltage to a power signal of sufficient watts to drive loudspeakers.
  3. Hi,
    'pre' simply means before, so a 'pre-amplifier' is an amplifier
    in front of another amplifier. It is usually of higher sensitivity
    and lower noise than the main amplifier and often combines inputs
    from several sources, levels and impedances together with such
    additions as RIAA equalisation.

    Cheers - Joe
  4. bench

    bench Guest

    Is there some convention regarding power of voltages, for example
    up to so and so voltage it is a signal which requires a pre-amplifier
    and beyond so and so voltage we use an audio amplifier proper ?
  5. John Fields

    John Fields Guest

  6. tempus fugit

    tempus fugit Guest

    kind of. You are usually looking at an input signal of 100mV (or less) to
    about 2V (for a CD player) going into an audio preamp. This could be
    considerably less for a microphone.
  7. Bob Masta

    Bob Masta Guest

    A preamp accepts millivolts and puts out a volt or so,
    whereas a power amp accepts a volt and puts out tens
    of volts. But a more important difference is that the
    power amp can drive a lot of current through a low
    impedance load like a speaker (4 or 8 ohms), whereas
    a preamp is only supposed to drive a high impedance
    like the input to the power amp, typically 10000 ohms
    or so. There are no numerical rules; the distinction
    is usually pretty obvious in practice because there aren't
    a lot of applications for things in between these extremes.
    An exception might be driving headphones; they are
    relatively low impedance (say, 8 ohms to 600 ohms or so)
    but they don't take much voltage to get high levels. So
    a preamp may inlcude a headphone driving stage (separate
    from the stage that drives the big power amp), and this
    headphone stage is really just a little power amp.

    So to make a long story short, the main distinction is
    probably the impedance they drive. (But somebody
    is sure to bring up tube amps driving high-Z electrostatic
    speakers or something...)

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis

  8. Hi Bob,

    Are those pre-amp useless, if your audio is already pre-amplified?

    I would like to hook up my stereo amplifier to the PC audio output. I
    think audio form PC is already pre-amplified and thus may not need any
    further preamplifier outside.

    By chance this stereo amp is already having a pre-amp, which is
    followed by a power amp.

    My problem is where to connect the input, to the pre-amp or directly
    to the power amp?

    Will the pre-amp burn out in the first case?

    Thanks in advance

    Best regards,
    Animesh Maurya
  9. Bob Masta

    Bob Masta Guest


    Most preamps have an input for things that
    don't really need any further amplification.
    Aux, CD, Line In and probably most Tape In
    fall into this category. The only input to avoid
    is Phono, which is definitely for low-level
    signals, or Mic, which few peamps have anyway.
    Even then, the most you probably have to
    worry about is grossly distorted sound; I
    doubt you will blow anything.

    The advantage of going through the preamp
    section is the volume, tone, and balance
    controls, plus the simple switching when you
    want to listen to another source.

    Hope this helps!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
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