11.11 Do I need a preamp? Why?
Description
This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
11.11 Do I need a preamp? Why?
The tasks of a preamp are to:
Switch between various input signals,
Amplify any phono inputs to line level,
Adjust the volume,
Adjust the treble and bass if necessary,
Present the right load impedance for the inputs, and
Present a low source impedance for the outputs.
If you have a turntable, you NEED a preamp with a phono input.
This is because the turntable has an output which is too
small for driving amplifiers and because the output of the
turntable requires frequency response equalization. You
can't connect any other source to a phono input other than a
turntable (phono cartridge). Also, you can't connect a phono
cartridge or turntable to any input other than a phono input.
Microphones also require special preamplifiers. Some microphones
also require "phantom power". Phantom power is operating power
for the microphone which comes from the preamp. Microphone
preamps are often built into tape decks and microphone mixers.
If you only have high level inputs, such as the output of a CD
player and the output of a tape deck, the main value of a preamp
is selecting between inputs and providing a master volume
control. If you only listen to CDs, it is plausible to skip
the preamp entirely by getting a CD player with variable level
outputs and connecting them directly to a power amplifier.
Some caveats apply. One, the variable outputs on a CD player are
often lower sound quality than fixed outputs. Two, some sources
have high or nonlinear output impedances which are not ideal for
driving an amplifier directly. Likewise, some amplifiers have
an unusually low or nonlinear input impedance such that common
sources can't drive the input cleanly. A good preamplifier
allows use of such devices without sacrificing sound quality.
Unfortunately, the only way to be sure that a preamplifier is
of value with your sources and your amplifier is to try one.
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