QUOTE (sonickid23 @ Wed 17 May 2006, 19:36)
so what is this preamp gonna do exactly? Does it make the sound clearer? Or just boost it some?
Well, the typical preamp does several things, but there are three basic functions:
(1) Nearly all mics produce output levels that are very, very low compared to
a typical "line" level. If you were to plug a mic directly into, say, a power amp
input, you wouldn't hear much if anything. The first thing the preamp does is
provide additional gain, to raise the mic's output level up to "line" level.
Most mixers and some A/D interfaces have mic preamps built in. Among
the gear that is affordable to us non-celebrity types, built-in preamps
range from fair to awful. They can be all right for, say, miking a guitar amp
or a drum kit, but their deficiencies tend to get exposed when used to
record demanding tracks like lead vocals or accoustic guitars.
Really high-end mixing consoles have great preamps, but we can't
afford them. So manufacturers have taken to building separate preamps
for the purpose. Most of us don't need a whole console full of great
preamps; we only need one or two, so we can buy a box that contains
a mono or stereo preamp of superior quality and not have to mortgage
our houses for it.
(2) Some preamps are designed to emphasize or "color" certain
frequencies for specific purposes. For example, preamps that contain
tube front-end circuits are often said to make vocals and keyboards
"warmer". In theory, you can accomplish the same thing with EQ,
but it's sometimes more practical to just use a good preamp made
for the application than drive yourself crazy trying to tweak an EQ.
(The same consideration applies to mics.) Also, many preamps
have some basic filtering funcitons designed to solve certain
problems. For instance, mics that have a cardiod pickup pattern
are notorious for "proximity effect", which means that the bass
response goes up as the singer gets closer to the mic. To keep
the sound from becoming too boomy, many preamps have a high
pass filter that can be switched in to roll off the low bass some,
and compensate for the proximity effect.
(3) Nearly all modern mics, except for certain dynamic and ribbon
types, have "pre-preamp" circuitry built into the mic itself. There are
several reasons for this, but let's just say for simplicity that most
mics require some source of power. Some mics can be powered
with batteries. But there's a long-standing practice in the studio
equipment world that mics usually get their power supplied to
them from the preamp; this is called "phantom power". (It uses
a clever way of feeding the power to the mic back up the mic
cable, using the same conductors that the audio comes in on.)
A switch that is often marked "48V" or some such is the switch
on the preamp that turns on the phantom power to the mic.