(Transferred from the wiki by Peter)
Updated 13 October 2006
1.0) Background - The original version of this PET was posted by me on Linux Junior. It detailed how to use a few common components with a small guitar amplifier to make a subwoofer for your existing speaker system.
The advent of inexpensive multi-speaker systems for computers has made this information obsolete for the most part. You wouldn't want to run out and buy a guitar amp and these other parts when a really sweet Logitech 5.1 system with a 10" sub goes for less than $100 these days. If, however, you already have or can acquire a small guitar (or bass) amplifier cheaply enough, this might work for you.
2.0) Start With Your Sound Card - Read up on your system's sound card. If you have a laptop, it might only have one output, but most desktops (and some higher-end laptops) have a speaker output and a line output. Some even have the capability of switching the line output in software to become a secondary speaker or subwoofer output. Don't neglect to look at your sound card's capabilities - it might keep the hardware requirements down and save you a few bucks.
2.1) Don't Forget Your Computer Speakers -
Some computer speakers also provide an output to which a subwoofer or second set of speakers can be connected. Such an output will usually be labelled with "Sub Out", "Out 2" or some such. This output might work as well as or better than one from your sound card and be more accessible.
3.0) Hardware - What's required
3.1) Stereo Splitter or Second Sound Card Output Channel. If your system can provide a second speaker or dedicated subwoofer output, do what is needful in software to make that happen. If not, go to Radio Shack, Best Buy, Circuit City or some similar electronics store, or a decently stocked Wal-Mart and buy a 1/8" stereo splitter (also called a "headphone sharing" connector). Be careful that this is not a stereo-to-mono splitter - these will split out the stereo output into left and right channel outputs - not what you want.
The whole point of the above is to provide you with a second stereo output from the sound card to which your subwoofer will be connected.
3.2) Multimedia Adapter Cable - Even if you don't need the splitter, go pick up a multimedia adapter cable that has a 1/8" stereo male plug on one end and a pair of RCA plugs on the other. This will be used to connect the second output to the subwoofer/amplifier. Belkin makes a nice one.
(Note: you could, if you chose to, use this cable alone to hook a sound card speaker output to a home stereo's "aux" inputs, effectively making your computer a part of your home audio system, so this is definitely a handy item to keep in your kit.)
3.3) [Optional] RCA to 1/4" Phone Male Adapters - You may also need a pair of adapters which convert RCA to 1/4" phono male plugs to connect to your amplifier, but keep reading.
4.0) Connections and Setup - Whether you are using a splitter or two sound card outputs, plug your speakers into one, and your multimedia adapter into the other.
Now make sure the amplifier is off and connect the other ends of the multimedia adapter into two of the input channels on your amplifier. Some small practice amplifiers (e.g., the Fender Frontman and Frontman Bass) have RCA female inputs for playing a CD player through the amp to practice with. If you have one of these amps, just connect the RCA males to these inputs. Otherwise, you need the 1/4" male adapters to connect to the instrument inputs.
Once everything is connected, turn down the volume on the amp all the way, and set the tone controls mid-scale (or "flat") to start out. This will protect your amp's speaker from any unholy noises during setup, and your ears, too. Now turn on the amp's power switch.
5.0) Adjustments - Start a soundtrack (play a CD or something) and adjust your speakers to a normal volume level. Now begin to raise the volume on your amplifier slowly until you hear a balanced sound from both sources. Note that if you are using a second channel of your sound card, you might have to turn it up in the mixer before your amplifier gets a good signal.
Experiment a bit with volume settings on the amp and the mixer to get what works best for you. Don't forget to also try different settings on the tone controls to see what sounds best. If you have reverb, tremolo, or distortion features on the amp, you will want to turn them off at first to get the cleanest sound.
Have fun with your new cheap subwoofer! Even a small amplifier will get an amazingly loud, clear sound in comparison to your computer speakers, so you will almost certainly have to fool around with it a bit to get what you want.