What Harmonic Distortion "looks like".
This short tutorial is intended to explain what different kinds of distortion look like (as seen on an oscilloscope). Although by no means is this an "exhaustive" display of every type of distortion vs amplitude and phase of harmonics, this is representative of the type of waveforms you are likely to encounter. By studying the displays shown in this article, you can get a reasonable idea of what the distortion in your amplifier is, by comparison.
Just to calibrate things, so to speak, the first picture is a pure sinusoidal wave:
Notice the nice smooth symmetry of this waveform.
Next is a waveform with exactly 5% second harmonic added:
If you look carefully, you will see that the top of the waveform is narrower, and the bottom of the waveform is wider than the true sine wave. The degree of asymmetry is correlated to the distortion. Note that the waveform is still smooth, as there is ONLY second harmonic distortion present in this waveform.
Odd order distortion, on the other hand, is symmetrical around the "middle" as shown in this waveform that has exactly 5% third harmonic distortion:
What you should notice here is both the top and bottom of the waveform are fatter.
When higher order distortion components are present, their effect becomes slightly more visible on the scope. Here's a waveform consisting of 5% second, 2% fourth and 1% sixth harmonics. Notice this represents ONLY even order distortion.
Comparing this with the second harmonic ONLY case, you can notice the top is definitely sharper, and the bottom definitely flatter. The "sharpness" is a give-away that there's higher order distortion components.
The same kind of thing happens with odd order distortion. Here is a waveform with 5% third and 2% 5th order distortion products:
Again, the symmetry is a giveaway that the distortion is ODD order, and the flatness of the top and bottom is a giveaway that there's higher order components.
Many real world amplifiers have both ODD and EVEN order distortion products. Here is a waveform with 5% second, 2% third, 1% fourth, 0.5% fifth and 0.2% sixth harmonics. This might be representative of an amplifier reaching its maximum output:
Interestingly, this waveform doesn't LOOK too bad. Lets reverse the amplitudes, just to compare things. In this image, the harmonic distortion is 0.2% second, 0.5% third, 1% fourth, 2% fifth and 5% sixth harmonc:
Notice that the THD reading of these last two figures would be the same, yet the bottom one LOOKS (and would sound) much worse. This is one of the reasons why a decreasing spectrum content amplifier sounds better than one in which the higher harmonics predominate, even though the latter might have "lower THD".
I hope this gives you some idea what different distortion components LOOK like, so you can judge what your amplifier is producing. This also allows you to correlate what you hear with what you see.
Technical Note: The waveforms here were created from the form:
y = cos(x) + %2nd*cos(2x) - %3rd*cos(3x) + %4th*cos(4x) - %5th*cos(5x) + %6th*cos(6x).