Analog processors, such as preamps or consoles, also add harmonic distortion, because their internal components (transistors, tubes, transformers, etc.) are not entirely linear.
When this linearity breaks, the audio dynamics change and consequently, new harmonic content that before didn’t exist, is now created.

When we say that the analog gear is not linear, we refer to the fact that the input level is different from the output level, forming a curve shape if it is graphed (x and y axis). This curve is known as transfer function or transfer curve.

In the above graph, the X-axis is the input level, and Y-axis is the output level, when the graph is linear there is not distortion because the input level correspond to the output level, but when the graph is curve shaped, the harmonic content is generated, and the audio dynamics changes.