Hi-Gain (descended from the more generic electric guitar amplification term high-gain) is the sound most used in heavy metal. High gain in normal electric guitar playing simply references a thick sound produced by heavily overdriven amplifier tubes, a distortion pedal, or some combination of both–the essential component is the typically loud, thick, harmonically rich, and sustaining quality of the tone. However, the Hi-Gain sound of modern pedals is somewhat distinct from, although descended from, this sound. The distortion often produces sounds not possible any other way. Many extreme distortions are either hi-gain or the descendents of such. The Mesa Boogie Triple Rectifier Series of amps are a great example.
Some Hi-Gain Pedals Include:
Boss MT2 Metal Zone
BBE Crusher High Gain Distortion Pedal
DigiTech Death Metal Distortion Pedal