Thursday, January 5, 2012

Introduction to bass guitar amplification

Most bass players get started with an entry level bass and a practice-grade combo amp. But to play with a full band, more power is needed than even loud practice amps can provide. Curious about bass amplification? Here's the bare-bones of how it works and an introduction to the different types of equipment in use.

Bass Amplification 101

Here's the part you probably figured out for yourself. You pluck a string. The string vibrates. The vibrations of the metal string over the magnets in the pickups on your bass create an electric signal. That signal is sent from your bass to your amplifier through your instrument cable.

Before that signal becomes noise, however, a few things need to happen.

The pre-amp
The pre-amp is where the signal from your bass guitar is processed. If your amp has an EQ to adjust things like bass, mids, and treble, it does those things in the pre-amp stage. If your amp has a built-in overdrive channel, this takes place in the pre-amp as well. After travelling through the pre-amp, the signal from your bass guitar is now ready to be... AMPLIFIED!

The power amp
This section is what gives amplifiers their name. The signal from your bass has originated in the pickups and traveled through the instrument cable to the pre-amp, where it was shaped using EQ tone controls. That signal continues on to the power amp.

The power amp is where the power of that electric signal is turned up. It's that simple: the power of the signal is amplified. The power amp then sounds that signal on to the speaker, where it makes noise.

The speakers
After your signal has been processed in the pre-amp and juiced up in the power amp, it's sent to speakers. It might seem like the least complicated step, but there's some things to keep in mind. The size of the speaker affects the sound. In addition, bass guitar amplification requires speakers designed to handle the frequencies a bass guitar puts out. More on speakers below.

Putting it all together
But this doesn't look like the combo or head and speaker cab combination people actually play. What is the equipment that you're likely to see in a store and play on yourself?


Combo amp are an all in one combination of pre-amp, power amp, and speaker cabinet in one (usually) portable box. Power levels of combo amp vary, from the low wattage practice amps you might use when you're at home to beastly 500 watt combos with four 10 inch speakers. Usually, these tend to be on the smaller wattage scale.

As a rule of thumb, you need at least 100 watts to play with a drummer in a practice setting. Anything lower than that is probably what would be considered a practice amp. For loud drummers, or for loud bands in general (say hard rock or metal) you'll likely need more wattage than a combo amp can provide.

Combo amps:
  • Combine pre-amp, power amp, and speaker cabinet all in one package.
  • Have great portabilitiy, usually sacrificing power/volume.
  • Have no compatibility issues; all components are selected by the manufacturer to work together properly.
  • Can possibly use extension cabs, but don't have enough volume to get LOUD


A head is a pre-amp and power amp in one package. These are often seen sitting on top of a speaker cabinet (hence the nanme "head"). The head processes the signal, amplifies it, and sends it off to a speaker cabinet (or multiple speaker cabinets).

When matching a speaker cabinet to a  head, keep in mind that you MUST match the impedance (the minimum resistance, measured in ohms) of the amplifier head to the resistance of the speaker cabinet.
In addition, the wattage of the head (the amount of power in the signal it puts out) changes based on the resistance of the speaker cabinet. Figuring out how to the "ohms" situation can be a tricky topic, so be aware that it's something you'll need to learn about if you decide to get a head and cab.

Pre-amp / Power amp

It's also possible to just run a pre-amp and a power amp, mounting each on a rack, to power a speaker cabinet. This is usually more of a high-end, high-volume setup and not something most bass players use, especially when starting out.

Speaker cabinets

I'll only touch on speaker cabinets briefly, as I think they deserve a post all to themselves. At the end of the day, this is the other end of the line that began with you plucking a string.

A speaker cabinet has a few parts to know about as well. There are three main parts to most bass guitar speaker cabinets.
  • Drivers: These are what you think of when you think of speakers. Round, usually in 10, 12, or 15 inch varieties.
  • Tweeters: Small speakers (sometimes called horns) that are used to recreate high frequency sounds.
  • Ports: Some speaker cabinets are sealed. Some have ports, which allow air to move into the speaker cabinet. These definitely have an effect on the sound of a cabinet, and whether you like the sound of that effect is a personal preference.
All speakers aren't made equal; in fact, bass cabinets require speakers that are specially made to handle the frequencies that a bass guitar creates.

And that, ladies and gentlemen, is a whirlwind tour of bass guitar amplification.

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