Amps and Preamps
There are 3 primary things we can do with a signal: Split, Join, and 'Cut'
or 'Boost'. The most basic function is to boost it and send it to
a speaker so we can hear it. A signal from a microphone is not powerful enough
to drive a speaker, and when you think about it, it makes sense.
If you watch a microphone when it is doing it's job, it hardly moves
at all. You would not be able to see the diaphragm moving in it because it
moves very slightly. In contrast, the distance a speaker has to move can be
as much as 3cm. Not only that, but a speaker cone is at least 10 times bigger
than a microphone.
We use amplifiers to boost a signal so that it is powerful enough
to drive a speaker.
Line Level
A microphone signal is so weak in fact, that it is useless until it is amplified.
Effects units and recorders require a much stronger signal than a microphone
is capable of producing. We use preamps to boost the strength of a microphone
level signal so it can be used by recording devices and effects units.
There is a standard signal strength called line level that is used
by most recording devices and effects units. A signal that comes from a microphone
or electric guitar needs to go through a preamp to boost it to line level.
This is why most mixing desks have preamps ( called 'gain' or 'trim'
knobs ). Some guitars have inbuilt preamps, and some recording devices have
'mic in' sockets.
Generally if something says 'Mic', it means it has an inbuilt preamp.
A line-level socket will presume that the signal has already been through
a preamp and is a line-level signal.
Power Amps
A line level signal is a good standard that ensures that the signal will
be compatible with any device that you want to use, but it still is not powerful
enough to drive a speaker. It takes a lot of energy to make a sound, so a
signal needs to go through a power amp before it can drive a speaker. It is
common to see 60watt or 100watt power amps on home stereo's, and live
sound rigs for concerts often have over 10,000watts!
Watts are a universal measurement of power that can be used to describe
the power of sound and electricity.
Eg: 100w ( a common light bulb or home stereo ) / 240v ( NZ household power
supply ) = 0.417 Amps
Also note that the power rating of a power amp is the maximum signal strength
that it can produce, not the power consumption of the device. Devices use
extra power according to the law of energy transfer loss. When any form of energy
is converted to another, some of the energy is transferred to a new type of
energy. Thus, a 100w amp at max power could be drawing more like 110w from
the power supply. The other 10w would be lost to heat and sound energy ( hums
etc ).
Summary of Amps
Line-level is a universal standard of signal strength that most devices use.
There are a couple of exceptions to this:
- Microphones & Electric Guitars produce less than line-level
- Speakers require more than line-level
- Preamps convert a mic-level signal to line level
- Power Amps convert a line-level signal to speaker-level
