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There are many types of distortions produced in an audio system. Some will not believe, such as parts distortion, and as such each part is perfect in their eyes. Each type manifests itself in a different way, although a few may overlap. Some distortions are in plain view, like bass bloat, where the bass is obviously distorted. Other distortions are hid from view, like a dirty little secret, and when brought to the surface, is explained away in some mystical fashion. This paper will attempt to describe a few of these distortions and, in some cases, how they affect the sound.

1. The first type is Harmonic Distortion. Harmonic distortion is the production of harmonics (say from a tube) from a given signal, whether fundamental or a natural harmonic from an instrument/voice. For example, using a single test tone, the second harmonic will be double the fundamental tone, third harmonic distortion will triple the fundamental tone. If we switch to an instrument, and it produces a natural 2nd harmonic, 2nd harmonic distortion will produce twice the natural second harmonic, or 4th harmonic. If a third natural harmonic of an instrument is present, the 2nd harmonic distortion will cause a 6th harmonic to be produced. Imagine a band or orchestra playing, the sheer number of instruments and interactions. Definitely not good.

   Another problem besides the output stage is previous stages. The more drive signal it takes to drive the output stage, the more distortion is present in the previous stages, which passes through and also combines with the output stage. The drive signal required for an output tube can vary from a few volts peak to peak to several hundred volts peak to peak. It is best to keep the drive signal as low as possible to keep the distortion low from the driver and preceding stages. It is easy to see how the natural harmonic structure of an instrument can be changed. I have seen some comments suggesting 5 to 10% as ok. However, the next point will debunk this idea.

2. Next is Intermodulation distortion, which almost no one will discuss. IM distortion is generally 3.2 times the HD figure. Thus if the HD is 5%, the IM is around 15.2%. If the HD is 10%, the IM distortion is around 32.6%.

   IM distortion is an insidious type that produces sums and differences (instead of harmonics) of the tones/music, and is totally unrelated to the tones/music. Thus the sums and differences will not blend in with the natural harmonics. For example, if we apply a 4khz and a 400hz tone generator to a component, we obtain sum and difference tones, 3.6khz and 4.4khz. (IM distortion will also combine the fundamental and natural harmonics of a single instrument or voice.)

   To go one step further, when a singer's musical signal is passed "through" a component, the fundamental and all her natural harmonics combine to produce alot of "garbage" unrelated to the singer's fundamental or her harmonics. Imagine what happens when a band or orchestra plays, with all the fundamentals and harmonics.

3. Parts distortions are another category. These distortions vary as widely as there are different parts (see our reprint of resistor comparisons as an example). Parts distort in all sorts of ways, such as: masking inner detail, smooth sounding, harsh, grundge, bright, lack of brightness, bass bloating, dull, lack of depth, lack of width, lack of dynamics, thinness etc. In otherwards, we want to use those parts that have the fewest sonic weaknesses or compromises.

4. Frequency distortion is another. By that I mean the lack of flatness of response with varying frequencies. This kind of distortion causes a component to sound bright or lacking, midrange predominance or lacking, too much bass or lacking, or combinations of the above. This type of distortion can be caused by inferior "sounding" parts, frequency dependent feedback through the power supply (see our page on feedback through the power supply).

5. Another distortion is soundstage distortion. Examples of this are lack of depth, smeared imaging, lack of width, lack of inner detail, etc.



6. Speed distortion is the last we will discuss. Speed distortion can either be perceived as "attack" times or transients, being either too fast or too slow. Again, poor parts quality, distortions, and poor frequency response (the highs drop off or rise too much) can give the appearance of attack times being too slow or too fast.

In conclusion, I hope you come away with a greater appreciation of the number of distortions and the scope of the problem. Distortions, other than HD, are generally not discussed, which is a shame as it definitely influences the sound. One can see why high amounts of HD distortion are so detrimental, because IM distortion is almost always present in prodigious amounts.

These articles are written as a public service.

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