There's been speculation now and in the past that tube based products sound "better" than their solid state counterparts because of the differences in distortion they add. What's the truth behind these ideas? Let's first look at the distortion in question.
Harmonic distortion means the amplification device adds something that wasn't in the music to begin with. In the case of solid state, 3d order (odd) harmonics prevail, while tubes have greater second order (even) harmonics. So, what's that mean?
Each frequency has what engineers call a fundamental. Simply put, this is the frequency under discussion. For example, if we have a 25Hz tone, the fundamental is its namesake, 25Hz. Harmonics are added to this fundamental in increments of odd or even numbers. For example, if the fundamental frequency is 25 Hz, the frequencies of the next harmonics are: 50 Hz (2nd harmonic), 75 Hz (3rd harmonic), 100 Hz (4th harmonic). The extra tones are added at some loudness less than the original signal.
When you see figures of THD (Total Harmonic Distortion) it means all the harmonics, both odd and even, are added together and produce a number which is a percentage of the total loudness. If your preamp's specs suggest THD levels of 0.1%, that means when added up, the extra tones are 1/10th of one percent of the total volume level of the original tone.
Studies have shown that even harmonics sound sweeter and more musical than odd. Tubes produce more even than odd and this fact is thought to be a primary reason tube circuits sound more musical than solid state. But is that true?
We'll continue tomorrow.