Features and Specs
Amplifiers are electronic devices that amplify a sound. That is, they make it louder. When we talk about an instrument amplifier, we are usually describing three different components that work together:
- Preamplifier
- Power amplifier
- Speaker(s)
You'll hear these terms again and again when talking or reading about amps. These may all be in one box (often called a combination, or combo, amp), or they may be in separate boxes. Most musical instrument amplifiers are either "all in one" boxes, or the preamplifier and power amplifier are in one box and the speaker is in another box. Place one preamp/amp box on top of a couple of speaker cabinets, and it's called, not surprisingly, a "stack."
Each part of the amp is designed to do one specific job, even though they're supposed to work together as one. The preamp takes the tiny signal from the instrument and boosts it a tremendous amount (but it's still really small) and then routes the signal through its tone-shaping circuitry. You usually find a gain control that sets how much the signal is initially boosted by the preamp, followed by tone controls (bass and treble, or bass, midrange, and treble), and finally there's a master volume (or simply "volume") control that sets the overall loudness of the amplifier. The signal from the preamp is then passed to the power amplifier, which takes the tiny preamp signal and boosts it many, many times so that it can deliver enough voltage to a speaker to move a lot of air. The speakers are usually specially selected models that can handle the high signal levels that amplifiers typically deliver, plus they can take the sharp "spikes" that come from slapping a bass or playing machine-gun-like lines on a guitar or keyboard.
Amplifiers come in all sizes, are mostly rectangular, and are most commonly covered in gray, creme, or a brown color of vinyl. Some have wheels and multiple handles, which makes it easier to move them.
Caution! Most amplifiers produce enough volume to damage your hearing. This includes small amplifiers no bigger than a backpack. Therefore, always keep your head away from the speaker, and if you're playing loud, wear earplugs.
A second caution! Amplifiers use a lot of wizardry to boost the signal from an instrument, and this usually involves high voltages. So never reach inside an amp or monkey around inside the circuitry. Even if an amplifier is turned off and unplugged, it can sometimes hold a charge in some of its components for days, and this charge can be high enough to injure or even kill you.
Important Features & Specifications
Wattage
What's a watt? It's a measure of energy, but never mind that. When you
shop for an amp, you'll probably want to know how many watts it puts
out. Why? Because it's a good relative indicator of how loud an amp
will be. The manufacturer usually includes a "wattage rating" on the
amp itself or on the accompanying literature. This tells how much power
the amplifier delivers to the speaker, but there are many factors,
including speaker type and efficiency, cabinet type, etc., that keep
this from being an absolute way to tell how loud an amp is once the
speaker does its part of the job. However, within reason, you can guess
approximately how much louder one amp will be than another by reading
the wattage rating.
Distortion
How true to the instrument's original sound is an amp? How true do you
want it to be? These things have to be balanced. Sometimes a circuit in
an amp will make the sound a bit edgy. That's distortion. Most bassists
and keyboardists don't want it, but guitarists tend to love it. Some
amp makers will list their amp's distortion, but it's usually the
result of a lab measurement under controlled conditions. Let your ear
be the judge, and not the specs here.