The Difference Between Preamps and Power Amps
Amplifying Your Sound
Before diving into what preamps and power amps are all about, it helps to understand the basic idea of what happens when you're plugged in and you strum that epic power chord to hear bone crunching audio goodness. The signal is "picked up" by your guitar pickup, which translates your string vibrations into an electrical signal, which is fed to your amplifier's preamp, which is then sent to the power amp, and then finally through a speaker to your ears.
What Does the Preamp Do?
The signal created from your guitar strings is inherently weak to start off. The right off the bat, the signal has to be amplified to an acceptable level before it is processed by the rest of the guitar amp (hence the "pre"). That level is called "line level". Many valve amplifiers use tubes, commonly 12AX7s, in this stage to produce a pleasant sounding compression and breakup (distortion/overdrive). With a sufficiently meaty signal, it's ready to be processed by the power amp.
What Does the Power Amp Do?
The power amp is what amplifies your guitar signal and is typically the last stage before hearing it through the amp speaker. This is what can amplify your signal into the loud (or quiet) volumes you want to hear through an amp. This loud signal has characteristics such as headroom (how dynamic the sound is), and the tone of distortion or overdrive. The tubes associated with power amps vary a lot here, whether it's the British style EL34 or the American style 6L6. Power amps provide the variance in tone when it comes to a guitar amp.
Why the Two Stages?
You may be wondering why two stages are needed to amplify a guitar signal. The answer might be akin to why you stretch before a workout, or why you would preheat an oven before cooking a meal. Going from an incredibly weak metal string vibration to audio bliss takes "warming up". If a power amp were to amplify a signal solely, you may have noise, high temperatures, and electronics that are just working too hard to produce something quiet. The preamp stage is the preparation, but can also work in concert with the power amp to give you a desired tone. A "hot" preamp where the signal is on the verge of distortion before going into a power amp will more easily and more naturally distort in the power amp stage, which could give you a warmer overdrive. If a preamp excels at boosting a signal as cleanly as possible, the power amp could more easily amplify that signal without it breaking up.
This is a basic way to look at it, and with anything tone related, it can be passionately debated. But it's worth taking stock of what the preamp and power amp is the next time you plug in and fall in love with the sound of an amp.
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