Hello,
I don't know much about audio circuits. I was making an audio project, although I used a LM386 for amplifying my signal directly, I have heard many people talk about preamplifiers. I tried to search google, but the terminology and purpose is not directly clear to me.
Can somebody explain, why using a preamplifier is better than using an amplifier directly ? And what is the difference in both of them?
In audio, increasing the gain of a small signal up to the and being able to drive enough current to move a speaker voice coil (for instance) all in one step is difficult and can add noise or signal distortion - you may have an input signal that is 5mV and you need say 5V across an 8 ohm speaker.
So you use a pre-amplifier to bring the signal from 5mV to say 500mV, a gain of 100, and then use an amplifier to get the current/voltage up to have 5V (a gain of 10) across the speaker so you can hear it well.
Thank you for the clarification. 
So, are there any special requirements for a preamplifier or is the circuit is just a number of amplifiers with different gains in cascade ?
I ask this because I have never seen an LM386 preampliying a signal for another LM386 although I have seen circuits which use LM358 operational amplifier used as a preamp for an LM386 ardio amplifier.
A good pre-amp will have low noise, low signal to noise ratio, etc. Many op-amps can do that.
The LM386 is the power amp that gives you some current/voltage to drive speakers.
LM386 as preamp into another LM386 would probably be an expensive way to get two gain stages.
Vn Equivalent input noise voltage
RS = 100 ?, VI = 0 V, f = 1 kHz
40 nV/?Hz
http://www.national.com/ds/LM/LM386.pdf
Low distortion: 0.2% (AV = 20, VS = 6V, RL = 8ohm, PO =125mW, f = 1kHz)
So the two are not even described the same; 40nV/Hz of noise introduced vs 0.2% of distortion - but the loads that can be driven are also quite different.
Thank you for your help. I got the idea.
The function of the preamplifier is to bring all audio sources upto the same signal level (be it a 50uV MM phono cartridge, 400mV line input or 1mV microphone input). It has a supplementary function of incorporating tone controls or graphic equalization. These days everything is digital anyway so preamps aren't really needed.
Power amplifiers are concerned with providing enough power for the speakers in question. Any preamp should be able to drive any power amp because the output of a preamp is always line level.
If you want an OpAmp for audio purposes you want a low noise opamp (many opamps are too noisy for amplifying a microphone signal, for instance) The industry standard used to be the NE5532 but now there will be low-voltage single rail opamps that are reasonably low noise too (that means 5nV/root Hz)
National Semi did have a dual pre-amp chip to go along with there many analog chips offered in the 80s and 90s. Their pre-amp offering was the LM-381:
http://www.national.com/ds/LM/LM381.pdf
Lefty
Thank you MarkT and retrolefty.