preamp...?

[jimmy]

hi all

one thing ive never understood, what exactly does a 'preamp' involve??

ive heard people describe it as just a distortion circuit, and ive seen designs that are just gain stages set to 1.  so what is actually in one of those mass rack preamps you see?

thanks
Jim

[smoguzbenjamin]

I'd say tone shaping and a distortion circuit. Maybe a little gain, say about 20 or so.

[Peter Snowberg]

Put simply, a preamp is just an amp that is designed to amplify a tiny signal into a small one, but not an amp designed for high current output such as one that will drive a speaker.

Preamps often have the characteristic of high input impedance and low output impedance, which is good for sending a signal down a cable. They also boost the "program material" to a level well above the "noise floor" so that you can mess around with it and not get swamped with things like thermal noise in processing circuits.

It's a nebulous term when the marketing people get a hold of it.

Preamps often have tone shaping in them (well... almost always even if they don't have external control to modify the tone). Preamps are also frequently able to amplify a signal to more than the available headroom of the next amp stage, which is where overdrive style distortion is produced. Some preamps are designed with the primary purpose of overdrive in mind.

A good example of a simple preamp is the LPB-1. On the more complex side they'll consist of 2 to 4 stages, each of which work like an LPB-1.

If the gain is set to one, technically it's not a preamp but rather a buffer.

What's in a rack preamp box varies so widely that I can only say look for schematics to see what's in a given box. The simplicity of some may surprise you. The complexity of some of them is... well... high.

I hope that helps.

Take care,
-Peter