A diode is an electronic component that allows electricity to flow in one direction, much like a valve does letting water go one way but not the other. Diodes can be seen in nearly all effect pedal schematics and are noted as this symbol (left). They are integral in creating that overdrive or distortion sounds in your favorite dirt pedals, and come in different packages, materials and sizes. Each diode has a positive side (anode) and negative side (cathode). This polarity is what resembles a valve, and dictates the flow of electricity.
But how does a diode contribute to that overdrive sound? If we look at an audio signal (represented as a sine wave) we can distort the signal by “clipping” the tops and bottoms of the wave. This clipping produces harmonics that we hear as overdrive.
If a signal is clipped evenly on both top and bottom of the wave, this is referred to as symmetrical clipping and is used in pedals such as the Ibanez Tubescreamer. If a signal is clipped more heavily at one peak than the other, this is called asymmetrical clipping as heard in the BOSS SD-1 Overdrive. The more heavily a peak is clipped, the more a sine wave moves closer to a square wave, and closer to distortion and fuzz.
The location of clipping diodes has an effect on the sound as well. Soft clipping involves having two diodes connected on the feedback-path of an opamp or a transistor (Tubescreamer). Hard clipping is when two diodes shunt to ground, creating distortion as in the ProCo RAT. The material of the diode also affects the clipping. Silicon diodes have been described as being “sharper” and “tighter” whereas germanium diodes are described as being more”compressed” or “spongey”. You can also use LEDs (light emitting diodes), Zeners, Schottkys, MOSFETs etc… the experimental possibilities are endless.
