I am making an amplifier and came across the fact that, if I want to operate a microphone then I will need a pre-amp.
What is the difference between power amplifiers and pre-amplifiers? Do both of them have the same gain, and if so then why do we need pre-amps?
Detailed explanation would be more more helpful to get the concept.
Generalizing, power amps for audio devices all tend to have the same input characteristics to make them compatible and generally interchangeable. Pre amps tend to have an output level and impedance compatible with power amp inputs BUT, a pre amp is usually specific to it's signal source and you'll find that a pre amp suitable for a guitar is not suited for a microphone.
Within the microphone family there are many different types and a pre amp suited for an electric or condenser Mic will be pretty poor with a ribbon Mic and some moving coil microphones.
Pre amps also tend to incorporate tone controls and these tone controls are clearly just affecting the instrument that is plugged in whereas the tone control of a power amp (when it receives signals from a mixing desk) will make global tonal changes to all the instruments.
As mentioned in another answer, pre amps tend to have very low noise inputs, making them suitable for signals that are particularly weak. Pre amps can also feed phantom power to condenser microphones.
In short, pre amps are tailored to the specific instrument that they are intended for and another example is the pre amp for a moving coil pick up cartridge on a record deck. It has a specific frequency characteristic that de emphasizes the cartridge's incorrect characteristic thus making it "sound" correct.
Perhaps an analogy with a different industry will help. Think industrial sensors and data collection. This sort of system might use an ADC to digitize signals from various sensors but each sensor will require a different pre amp to feed to the common ADC. In this rather weak and off-the-cuff analogy, the input to the ADC can be thought of as the "standard" input to a power amp. Inputs can vary more so than in the audio world.
For instance, a thermocouple has a very low level output and may require large amplification but, it's bandwidth is usually small so low noise amplifiers may not be needed. A strain gauge can also have a low level signal but can be used up to many tens of kHz and can need very specialist pre amps and activation with a polarization current. RTDs tend to need low gain and most applications are limited bandwidth. Horses for courses.
A power amplifier generally takes line-level signals and amplifies them for loudspeakers - it produces the larger power needed to drive speakers.
A pre-amplifier takes the very weak signals from a microphone (for example) and boosts it to line-level.
Example signal levels for comparison purposes
Microphone -60 dBV
Consumer Line-Level -10 dBV
Pro Line-Level +4 dBV
Loudspeaker +24 dBV
From Understanding Signal Levels
Note that microphone sensitivity varies widely, that ouput levels obviously depend on the loudness of sound being recorded. Pro line-level is usually measured in dBU not dBV.
As Red says there's a difference in the input and output levels and impedance, but there also is another great difference: noise. As we know from the Friis formulas the noise in a cascade of stages is primarily established by the noise coming from its first stage. Moreover the noise contributions of the subsequent stages are all divided by the gain of the first one, so your pre will need to:
Usually a microphone pre is based on a couple of transistors, the first one being a (J)FET. FETs have a very very low input noise and can achieve very high gains in common source configuration, so the output signal is practically already line level.
The subsequent power stage will rise the voltage even more but will also have a very high current gain to properly drive a speaker: a power stage is often made with two stages, a common source/emitter to give voltage a boost (voltage gain), and a source/emitter follower that dramaticaly lowers the output impedance thus giving the wanted current gain.
Note that sometimes you can make a quite good pre out of an op amp: usually the op amp choosen, that must have a low input referred noise, will use FET for the differential input pair.
Yet another difference is frequency response. A magnetic phono cartridge (remember them?) has different frequency response requirements from a microphone or a tape head. See, for instance, RIAA preemphasis. These differences are best handled at a low level. Once the frequency response of the source has been equalized, power amplification is (pretty much) "just" a matter of providing a flat response. (My apologies to those of you with golden ears.)
