Preamps and Gain Stages

[Legion6789]

I don't have an electronics background, I'm just a guitarist. But one thing I've noticed a lot in audio electronics is the use of preamps. What I don't understand is why there is a separate preamp and power amp. My simple understanding is that we have a signal that is basically a sinusoidal function F(x). When the signal comes from say an electric guitar, it's amplitude is very small. By amplifying it we're just multiplying F(x) by some constant. I know in reality all the components color the signal and introduce distortion, but putting that aside, why would you amplify a signal, by a factor of 10 and then amplify it again by a factor of 100 instead of just amplifying it by a factor of 1000 in one go?

 

[dick_freebird]

Power amps have a fairly high input line level (~1V?)
and an impedance that is mismatched to sources like
mics and guitar pickup coils.

Preamps may perform not only gain, but impedance
transformation as well; they may include shaping of
frequency response (like a RIAA phono preamp).
The noise qualities may receive more attention and
the performance at very low input levels as well.

 

[BradtheRad]

I take it you're talking about the basic concepts of amplification.

There are two different types of amplification needed for low-level signals such as guitars and mics:

* voltage amplification to obtain a usable signal
* current amplification to power a speaker

It's possible to do both in one stage if you use suitable components. However it is more convenient to do it in two stages.

One reason for amplifying in two stages is because many audiophiles like to use class A amplification when possible. It generates the lowest amount of distortion (as compared to class B).

However a lot of watts are wasted when you use class A in a power output stage.

So typically when you want voltage amplification alone it's feasible to use class A. It hardly wastes any power.

With class A you can run the transistor at a high supply voltage so as to have a wide range of linear operation. The greater linearity you have, the less distortion you have. This is optimal when you're starting with weak signals as from guitars and mics.

Then you can use class B in your output (current amplifying) stage. It draws current only in proportion to the sound level you set.

All this doesn't stop some audiophiles from insisting on class A in their output stage, and who don't mind building massive heatsinks to dissipate all the heat generated.

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My mistake saying class B above. Change to class AB.