Pre-amplifier and Amplifier difference

Discussion in 'General Electronics Chat' started by myheart22, Dec 5, 2010.

  1. myheart22

    Thread Starter New Member

    Dec 5, 2010

    I am a 3rd-yr ECE student from the Philippines.

    Until now I am confused with the difference between Preamplifier and Amplifier.
    We had a design proposal on Audio Power Amplifier but then, our work is always been trashed by our professor because I can't answer him what is the purpose of the preamp if there is already a power amplifier. :/

    Moreover, can you please tell me what is the advantage of having a preamp. And what may be the output if I didn't include the preamp on our design.

    Any kind of help will be much appreciated. Thank you! :)
  2. tyblu

    Active Member

    Nov 29, 2010
    Preamp is a highly linear, high input impedance amp. It is meant to buffer the input and amplify the signal to levels that can be used by processing or further amplification circuitry. They often follow and precede crossovers. You have likely made a multistage amp -- the first few stages could be called a preamp. Some signals are low power and cannot directly drive components in a typical power amplifier, like power transistors.
    maqsudh and myheart22 like this.
  3. bertus


    Apr 5, 2008

    A Pre-Amplifier is the part that amplifies the signals from the Mic. MD-head or other small signal source to line level.
    In the pre-amp. you can also switch or mix the signals.
    Also you will find a tone-control in the pre-amp.

    The Power-Amplifier amplifies the line-signal to the high power level needed to drive the speakers.

    On this site you will find a lot of information on it:

  4. spinnaker

    AAC Fanatic!

    Oct 29, 2009
  5. myheart22

    Thread Starter New Member

    Dec 5, 2010

    Ughm? Speaking of Line level? What does it really mean. I remember when my professor asked me what is the use of preamp, I said that it amplifies low-level audio signal into a line level. But then he asked me again how can I determine if the signal is a low-level or a line level. I wasn't able to answer him because i do not fully understand what's written on the net. I just read it and that's it. In short, I can't analyze it just by reading. i just need further explanation. :/
  6. bertus


    Apr 5, 2008
  7. hobbyist

    AAC Fanatic!

    Aug 10, 2008
    Think of it this way then,
    In more simpler terms, the laymans, way of doing this,

    For the moment forget about audio amplifiers, and think in terms of basic electronics.

    If you have a resistor of 10K ohms in series with another 10K ohms and apply a voltage of 10v. to this network, you will have 5v. dropped across each resistor, and the supply current will be 500uA.

    Now if you put another 10K ohm resistor in parralel with the one connected to ground, and applied the same 10V. you will have a decrease in the voltage drop across the parallel pair.

    The voltage across this pair will be 3.33V.

    However the current from the battery will increase to 667uA.

    Now what current would need to be supplied to the parralell pair of 10K ohm resistors, to once again put 5V. across the pair.

    That would need 1mA. of current to flow through the entire network to make 5V. drop across the parrallel pair, which is the same voltage reading before the paqrralell pair was introduced.

    So now what voltage would the supply have to be in order to make 1mA. flow through the network.

    That would be 15V.

    NOW, that is exactly what the power amplifier is supposed to do, it is to supply the proper amount of current while the proper voltage is maintained across a given LOW impedance substance.

    For that to be so a power amplifier has very little voltage gain, it's main interest is to supply all the current needed to keep the proper voltage maintained across the given load.

    Now back to the original example.

    The 5V. output represents the voltage needed across the output impedance, and this voltage must be maintained at the level, but for that voltage to be maintained at that level, the current needs to be 1mA.

    However your signal source is 50mV. and can only deliver lets say 10uA.
    This is where the PREAMP comes into play.

    The preamp is designed to have maximum voltage gain, while the current gain can remain small because now the PREAMP is able to take this small 50mV. and amplify its voltage to 5V.

    A VOLTAGE gain of 100, while at the same time its current gain can remain small.

    The power amplifier takes this increased voltage output from the preamp, and sends this to the VERY LOW load impedance, remember the load needs 5V. so the preamp has 5V. at its output,
    BUT it cannot deliver the 1mA. needed to drive the load, so the poweramp, takes the same voltage from the preamp, and adds to it ALL the current, that the preamp could not supply, and supplies it to the output load.

    Poweramp has very little voltage gain.
    But has very high current gain.
    Preamp has very high voltage gain,
    But has very low current gain, with respect to the power amp stage.

    The transistor itself has high current gain, but with respect to the poweramp current needed, it is not enough to drive the load as the power amp can do.

    So by combining the two amp stages together, you have the preamp, supplying the final needed voltage for the load, and the power amp takes that voltage and supplies the current needed for the load, so both working together the load now has the proper voltage, due to preamp, and the proper current due to power amp.

    From which the entire system has taken a very small signal voltage and amplified both the voltage and current to drive the load, this is called power amplification.

    Hope this helps in some way.
    maqsudh likes this.
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    Preamps have the task of switching between several inputs, providing a volume control, and ensuring the output is the same level for the power amplifier. That is the primary purpose, anyway.

    The secondary purpose is addons such as equalizer or tone controls, DSP Delays for room size, and other signal enhancement before amplification.

    Think of the power amplifier as a device having one input and one output. Input is Line Level from preamp, output drives a speaker. There are no controls in the ideal power amp. Add as many power amplifiers as you have channels to drive. The preamp does "everything else".