While guitar and bass amplifiers are generally designed to amplify a signal and play it back through speakers, there may be times when you need to send a signal from the amplifier to another device instead. Many manufacturers now include a line-out jack on their amplifiers for this reason. Line-out jacks are usually found on the rear panel of the amplifier, near the speaker outputs. Line-out jacks typically accept standard quarter inch phono plugs, and in some cases, XLR connectors. There are three primary uses for line-out jacks.
Connect to Another Amplifier
Guitarists who play at large venues may occasionally need to play through two different amplifiers to create the necessary stage volume. If you don't have an easy way to split your signal between the guitar and amplifier, using one amplifier's line-out jack to connect to the second amplifier's input is an easy solution. Some amplifiers even include a "power amp in" jack on the rear panel that allows you to bypass the preamp. This means the tone from the second amplifier should be nearly the same as the first amplifier. This technique is also known as "slaving," and the line-out jack may be labeled "slave out" on some older amplifiers.
Connect to a Recording Device
While using a microphone to record the sound coming out of your amplifier generally produces the best sound, there may be times that you want to record something, but can't use this method because of volume constraints. This typically applies to people living in apartments, or who like to record late at night. In these situations, you can usually use the amplifier's line-out jack to connect to a recording console, or even your computer's sound card, while keeping the speaker volume low or completely off. Just add a pair of headphones to record your masterpiece without waking up the neighbors or the baby.
Connect to a PA System
Bass players often find that sound engineers at clubs prefer to run the bass directly through the PA system, rather than place a microphone in front of the bass amplifier. This can be done using a direct box between the bass and the amplifier, but many players prefer to use their amplifier's line out instead, This allows you to send a signal to the PA system that's closer to the sound actually coming out of your amplifier on stage, as the signal has gone through the amplifier's preamp first.
Not all line out jacks are created the same; some manufacturers provide options you can enable or disable. Common options on bass amplifiers include the ability to remove any equalization from the line-out signal, or adjust the volume level of the line-out signal separate from the overall amplifier volume. Some guitar amplifiers allow you to add speaker emulation to the line out signal. This is a special kind of processing intended to emulate the characteristic sound of an amplifier running through a speaker cabinet. This feature is often found in digital modeling amplifiers. Check your owner's manual to see what options your amplifier includes.