When using a digital audio workstation program, such as Pro Tools or Logic, you can record the guitar “clean,” without any distortion. Do this by recording direct to the audio interface or computer sound card via a standard instrument cable. Once recorded, you can add post-production audio by assigning a specific virtual processor to the audio channel. The advantage is that you have greater control over the sound. The downside is that the resultant sound is less-authentic and less-natural than real amp distortion.
Rather than recording a guitar “clean” to the computer, if you connect it via your preferred distortion pedal, this adds a little more authenticity to the sound while enabling you to tweak the tonal parameters after. This approach is a compromise however, because you can never remove the distortion from the pedal once it is recorded and your sound will always lack a degree of warmth and authenticity because you recorded it direct to a computer, rather than via an amp. However, it is suitable for recording distorted guitars that aren’t going to be prominent in the mix.
When you crank an amp up really loud, the power amp section has to work hard to achieve the volume. This creates overdrive, which is a mild form of distortion. This is the authentic type of distortion that characterizes “classic rock” records, such as those by AC/DC. To record this type of distortion, you can either connect a cable to the “Line Out” jack on the rear of the amp or place a microphone in front of the speaker.
When using a microphone to record an instrument, you always capture a degree of room sound. This because the sound reflects off the walls and back into the mic. In some cases, this creates a pleasing, atmospheric sound. Because distortion has crunchy, powerful overtones, it works well in an ambient environment. By placing the microphones away the amplifier, you can capture even more room sound, adding an ethereal element to the guitar sound.
This is the antithesis of ambient microphone placement. By placing the microphone so close that it touches the grill cloth of the amplifier and angling it so the capsule is pointed at the center of the speaker, you eliminate a large degree of room sound. This creates a much more immediate, punchy and tight distortion sound. This technique creates the illusion of increased volume.