The problem has three elements. If you have ever wondered why a tube amp and a transistor amp can have the same bandwidth but the transistor amp sounds bright here's why:
Our ears are tuned to listen for the odd ordered harmonics in order to determine the loudness or volume of the sound. When electronics distort these harmonics it will sound bright to us. BTW, our ears are also tuned to be the most sensitive to **bird song** frequencies, not the human voice!
Now when you add negative feedback to an amplifier you will subtly increase the odd-ordered harmonics. Most transistor amps (not all) use this technique to improve linearity. In addition, nearly all semiconductors have a non-linear capacitance that exists in the junction of their devices. This capacitance is also responsible for adding to the odd-ordered harmonic distortion.
Now the odd orders that we are talking about are very slight, but it happens that the ear is very sensitive to this issue.
Finally, on a speaker like the Quad ESL 57 or ESL 63, the speaker really prefers an amplifier that does not double its power as impedance is halved; both of these designs existed before such was common. So a transistor amplifier, if it is behaving like a voltage source, will be bright on these particular speakers even if no odd-ordered harmonic distortion were present. for more info regarding this phenomena:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
Our ears are tuned to listen for the odd ordered harmonics in order to determine the loudness or volume of the sound. When electronics distort these harmonics it will sound bright to us. BTW, our ears are also tuned to be the most sensitive to **bird song** frequencies, not the human voice!
Now when you add negative feedback to an amplifier you will subtly increase the odd-ordered harmonics. Most transistor amps (not all) use this technique to improve linearity. In addition, nearly all semiconductors have a non-linear capacitance that exists in the junction of their devices. This capacitance is also responsible for adding to the odd-ordered harmonic distortion.
Now the odd orders that we are talking about are very slight, but it happens that the ear is very sensitive to this issue.
Finally, on a speaker like the Quad ESL 57 or ESL 63, the speaker really prefers an amplifier that does not double its power as impedance is halved; both of these designs existed before such was common. So a transistor amplifier, if it is behaving like a voltage source, will be bright on these particular speakers even if no odd-ordered harmonic distortion were present. for more info regarding this phenomena:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php