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Difference between Power Amp and Preamp?

  1. Mar 18, 2010 #1
    Hello all,

    I was wondering what the difference between a power amplifier and a preamplifier was. Someone told me that a power-amp provides current and voltage gain whereas a pre-amplifier only provides voltage gain.

    If this is the case, why are circuits like the following considered Class A Power Amps?


    Does this circuit provide current gain? I think it only provides voltage gain, but I could be wrong.

    Attached Files:

  2. jcsd
  3. Mar 18, 2010 #2
    I can answer a couple of your questions. The preamp amplifies what is called a low level signal like a guitar pickup, turntable cartridge or microphone signal. These are typically very low like 3mV. The preamp amplifies the signal to line level and provides filtering, ie tone control. The power amplifier amplifies the signal from the preamp so it is "powerful" enough to operate the speaker and produce sound.

    I don't know if you attached a schematic or not but I can't see it. There are 4 main types of power amplifier classes for audio.

    Class A amplifier operates in push pull mode which means one transistor amplifies the positive cycle and the other transistor amplifies the negative cycle. The key here is a class A has both transistors active at all times so it's considered inefficient.

    Class B amplifier has each transistor on only half the time. This helps with power efficiency but introduces something called crossover distortion when the signal switches from pos cycle to neg cycle.

    Class AB tries to remedy this by keeping both transistor very close to being active to minimize crossover distortion but keep Class B efficiency.

    Class D is bit more complicated and there are different types. Basically it uses pulse width modulation and relies on filtering to extract the audio signal.
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