In addition to Duke's comments, fatigue can be caused by the presence of odd-ordered harmonics in the amplified signal. Our ears use the odd orders (5th, 7th and 9th) as a means to determine the loudness (sound pressure) of a sound. If these harmonics are distorted (enhanced) by amounts of only 1/100th of a percent, we will perceive the sound as louder. In addition, these harmonics also contribute to brightness, in fact we audiophiles have terms like bright, hard, harsh, brittle, clinical, etc. to describe this type of distortion.
One of the primary ways that these harmonics get emphasized is through the use of loop negative feedback in electronics. Loop feedback is used to *reduce* distortion, but due to timing effects that occur in any gain stage, the loop feedback is always a little late in doing its job. Its this 'lateness' that enhances odd ordered harmonics. Not by much, mind you, but it happens that this is a fundamental violation of what you might call human hearing rules; how we figure out the volume of a sound is pretty important.
This is why you see some audio manufacturers pushing zero feedback circuitry, as it is less fatiguing to listen to if they have their ducks in a row.
One of the primary ways that these harmonics get emphasized is through the use of loop negative feedback in electronics. Loop feedback is used to *reduce* distortion, but due to timing effects that occur in any gain stage, the loop feedback is always a little late in doing its job. Its this 'lateness' that enhances odd ordered harmonics. Not by much, mind you, but it happens that this is a fundamental violation of what you might call human hearing rules; how we figure out the volume of a sound is pretty important.
This is why you see some audio manufacturers pushing zero feedback circuitry, as it is less fatiguing to listen to if they have their ducks in a row.