Distortion

Passing any audio signal through a non-linear amplifier distorts it (changes its waveform).  This type of distorted sound is particularly desirable by many guitar players.  Originally this type of sound was produced using valve amplifiers.  The only problem is that a valve power amplifier is only capable of producing this sound at a very high volume.  This is alright for stage shows but not otherwise.

Dummy speaker loads (the good ones are not just resistive, they need to simulate the reactive load of a speaker) allow a player to use one amplifier in a variety of playing situations and styles by running the amplifier at the desired level, and using the dummy load to regulate the volume level.

However, the silicon age replaced these valve amplifiers by the new solid-state designs and now these options are available only to wealthy musicians!  On the other hand some solid-state non-linear amplifier designs can give similar distortion sounds as well as other types of distorted sounds.  A big advantage of this is that non-linear amplification can be done at pre-amp level and thus distortion sound is available at lower output volumes also.

Guitar Distortion Circuits

Virtually any non-linear amplifier will produce distortion.  Two commonly used types of non-linear amplifiers are Soft-Clipping (commonly known as Overdrive) and Hard-Clipping (commonly known as Distortion).

Soft-Clipping

A soft-clipping amplifier is made by placing two back to back diodes in the negative feedback path of an amplifier as shown in the circuit.

The output voltage as a function of input voltage is shown in above figure (+ve values only).

The output (red) for a sine-wave input (blue) is shown in this image.

Hard-Clipping

In a hard-clipping amplifier, signal is restricted within a range.  This is usually done by placing two back-to-back in shunt to ground as shown in the circuit.

The output voltage as a function of input voltage is shown in above figure (+ve values only).

The output (red) for a sine-wave input (blue) is shown in this image.

As you may realize, a hard-clipped sound is very rich in harmonic content.  This produces a kind of metallic sound used in “heavy/death metal” music.

Sound depending on diodes?

The transfer function (Figure A and B) at the clipping voltage (red circle) is not abrupt (assuming ideal diode behavior) but somewhat curved due to the smooth turn-on characteristics of the diodes.  This, to some extent affects the frequency content of output and thus the tone of sound.  We can get different sounds using different types of diodes (silicon, germanium, schottky or even LEDs!).  We can even one type for positive clipping and other type for negative clipping giving an asymmetrical waveform with entirely different harmonic content!

Effects of Distortion

Sustain

One of the key desirable feature of distortion is the sustain it produces.  Higher gain produces more sustain but generally reduces the tonal quality of the sound.  Players usually use the overdrive sound for single note solo work.

Inter-Modulation distortion

Distortion amplifier is a non-linear system.  It results in mixing of frequency components of the input.  This is known as inter-modulation distortion.

For just two notes played, inter-modulation distortion produces an additional note with a frequency of the difference between the original two notes.  For chords, where up to 6 notes are played, the combinations of note pairs can produce an unrecognizable mess of distortion.

On the other hand, this is actually desirable in musical styles which use mainly Power Chords (2 notes / 5th chords), because in that case, inter-modulation distortion adds a note which is tune with the chord.

My Distortion Unit (Old)

Here is the simplified circuit of the distortion unit that I built. It is designed to work on single supply.  First stage is a voltage follower (buffer) with input biased at Vcc/2 for single supply operation.  Second stage is a high gain soft-clipping AC amplifier.  DC gain is unity so the output of the amplifier varies around Vcc/2.  A low-pass filter at its output filters out high frequency components (too much fuzz otherwise).  Both input and output are AC coupled.

Along with this the unit also includes a 9V battery for supply, gain control at second stage, volume control at output and a foot-switch to bypass distortion (all not shown).

I use this unit with a guitar processor set to provide some delay/reverb.  It gives a nice sound useful for both solo leads as well as power chording.