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Harmonic Distortion and Sound
Many audiophiles believe that 2nd harmonic is to be preferred over 3rd harmonic. Certainly it is simpler in character and it is well agreed that orders higher than third are more audible and less musical. However, when given a choice between the sound of an amplifier whose characteristic is dominantly 2nd harmonic versus 3rd harmonic, a good percentage of listeners choose the 3rd.

I have built many examples of simple 2nd and 3rd harmonic 'types' of amplifiers over the last 35 years. When I say types, I mean that they used simple Class A circuits described as 'single-ended' versus 'push-pull' and so tended to have a 2nd harmonic versus 3rd harmonic in the character of their distortion but were not made to deliberately distort.

Anecdotally, it appears that preferences break out roughly into a third of customers liking 2nd harmonic types, a third liking 3rd harmonic and the remainder liking neither or both. Customers have also been known to change their mind over a period of time.

However, the issue is partially obscured by the fact that the 3rd harmonic type amplifiers usually have lower total distortion. Third harmonic usually appears with a negative coefficient, resulting in what we think of as 'compressive' - the example in figure 3. It's worth noting that odd orders on nonlinearity also can be seen altering the amplitude of the fundamental tone - something a distortion analyzer doesn't ordinarily display.

Audiophiles have been accused of using 2nd or 3rd harmonic distortion as tone controls to deliberately alter the sound. I suppose that there are people who like it that way but I don't think this is generally the case. For reasons which will become clearer when we talk about intermodulation distortion, high levels of any harmonic become problematic with musical material having multiple instruments, and the argument that 2nd or 3rd adds 'musicality' doesn't quite hold up.

The sound of 2nd order type circuits is often praised as 'warm' and by comparison 3rd order type circuits are often noted for 'dynamic contrast'. 2nd order type amplifiers seem to do particularly well with simple musical material and 3rd order types generally seem to be better at more complex music. Figure 4 shows a distortion curve of two power stages operated without feedback - the blue is single-ended Class A, the red is a push-pull Class A.

In Figure 4 we see that the 2nd order type declines inversely to the output voltage (the square root of power), and the 3rd order type declines inversely to the square of the voltage (inversely proportional to power). There may be also a relation between this and the perception of 'warmth' versus 'dynamics', but it is not clear to me at this time.

Nevertheless, whether you prefer 2nd or 3rd order type amplifiers, let's agree that we wish to minimize the total amount of distortion. And assuming that we have to put up with some distortion, let's also agree that we prefer 2nd and 3rd harmonic components over 4th, 5th, 6th, 7th and so on.

To get lower order harmonic character, we want smoother 'bends' on the device's transfer curve. This usually means Class A operation. Figure 5 shows a harmonic comparison between the same push-pull circuit operated in Class A versus Class B with a 500Hz signal. In the case of Class A operation, we trade off energy efficiency for a smoother transfer curve, with both halves of the push-pull gain stage smoothly sharing the load and mutually conducting current at all times. In Class B, there is no time when both halves share the load. A popular compromise design, Class AB, smoothes out the transition between the two halves by having them both share the load for a portion of the transfer curve.

The high harmonic content of Class B amplifiers brings us to the word monotonicity. Monotonicity describes the relationship between the distortion level and the output level.

The smooth transfer curves of Class A amplifiers have a characteristic which is monotonic, that is to say the distortion goes down as the output declines. It implies a low-order harmonic characteristic, which we have previously agreed is sonically preferred.

If you see a curve with distortion levels climbing as the output goes down, it implies crossover distortion caused by the gap between the two push-pull gain elements. This implies high-order harmonics.

The examples of Figure 4 showed distortion declining smoothly with power in Class A amplifiers, but Figure 6 compares a push-pull Class A (red) with a Class B amplifier (blue).

In real life of course, the distortion of the blue amplifier would likely be reduced through the use of negative feedback, and the marketing department would be able to say that the distortion is "less than .05%". (To avoid any confusion, please note that graphics you see in this paper show performance without feedback unless otherwise noted).