How Do Amps Work?

What qualities do we look for in guitar amps? Come to think of it, why do guitars need special amps in the first place? We’ll try to find out the answers to both these questions.

The Sound Barrier
What is a 'clean' amp sound? You might be surprised at the answer. When a string is struck, the transient signal voltage peaks at around five times its average value (Fig. 1) – so an amp set at half volume can actually be overdriven for the first few milliseconds. It sounds clean because the ear takes time to perceive distortion.A guitar amp is considered to be distorting if the sum of all harmonics (that is, overtones added to the output by the amp itself, aka Total Harmonic Distortion or THD) is greater than 5 per cent, but this amount can’t be heard at less than 100 milliseconds (0.1 S), by which time a guitar's peak signal has fallen to its average sustain level.
Of course, persistent peaking beyond the output limit lowers this threshold, and it's important that the amp shows no other signs of distress such as instability during otherwise inaudible peak excursions beyond its clean limit.