Preamps, Amps, Compressors And Power Amps

[TheMacDonaldMan]

So on my ever ending journey to learn about guitar setups and rig setups I started looking into Racks. I grasped the concept of all that after a few solid days of internet-ing and what not. I started looking into my favourite like Iron Maiden, Metallica, Shinedown etc. I understand why each member may want many different preamps in order to shape their sound, but why would they need so many amps? Or am I getting confused by the definition of an amplifier? For example, Here's Adrian Smith's (Iron Maiden) Rig Rundown:

***Amplifiers***

-Two 300-Watt Marshall 1960A Angled-Front 4x12 Cabs loaded with 75-Watt Celestion G12-T75 Speakers
-Two Marshall 30th Anniversary 6100LM 100-Watt All-Tube Heads
- Two Marshall DSL100 JCM2000 Amplifiers
-Marshall 9200 Rack Power Amp
-Marshall JVM410H
-Blackstar Series One 104EL34
-Blackstar Series One 1046J6
-Blackstar HT-5

***Effects, controllers and processors***

-Dunlop Uni-vibe Wah Controller
-Dunlop Cry Baby Wah
-Yamaha MFC10 Midi Foot Controller
-DigiTech Whammy Pedal
-DigiTech Eric Clapton Crossroads
-Two Ibanez TS-808 Tube Screamers
-Duesenberg Channel 2 overdrive/distortion
-Mike Hill B.I.S. Isolation hole
-Lexicon MX200 multi-effects unit
-Boss DD-3 Digital Delay
-Boss CH-1 Super Chorus
-Boss CS-3 Compression Sustainer

Now as far as I understand, the amp id the part that the overall sound comes out of; correct me if I'm wrong. Also, why might there be a few bits of the same equipment in the setup?

[Togo]

Right then young Mac, first we need to understand what an "Amp" is doing. When a note or a  chord is played on an electric guitar it is picked up by the.... pickup. You have no doubt heard the result of a guitar that is not equipped with an acoustic body cavity (acoustic guitars are) and agree that it is zilch. Now when the guitar is plugged in, ie powered, the pick up converts the strings almost inaudible sound waves into electrical impulses. These used to be analogue but are increasingly digital. Now the feed from the guitar is fed to what is commonly called an amp. The "amp" has 3 basic components. First the pre amp. This takes the very weak signal from the guitar pick up and enlarges it or shall we say pre amplifies it. This is important. It must not distort or allow spurious electronic "white" noise to enter the signal stream but must enlarge the signal enough to enable it to drive the main amplifier circuit which will in turn drive the speakers. Because the main amp drivers are basically a muscle output they rely on very clean input from the pre amp circuit. Now as you can by now imagine the wealthy or top end users of such equipment have needs well beyond the everyday requirements of most people and can afford to split these stages down into a dedicated pre amp, output amp and separate speakers for the massive audio levels that they require. I myself was astonished at a recent Stone Roses gig at thee size of what looked like tower mounted ceramic speakers, and wondered what the hell was going on between guitar pickup, microphone and re transmitted sound. Keep on enjoying your music.