Before analyzing the distortion and overdrive effects, lets try to understand what distorsion is.
Volume and gain are often interchangeable terms. There can be some confusion, especially for some instruments (such as mixers) that are equiped with both.
You can find true gain controls on many effect units, but only rarely on amplifiers.
The amplifiers that control the speakers are different small amplifiers connected together to form a chain, each of these stages of amplification has its own fixed gain.
The total gain of the amplifier is the sum of the gains from the single stages.
The volume controls act either on the input signal or on what passes through two of these stages. The signal level can be altered and be reduced to zero, but don't alter, however, the real gain of some stages.
The gain controls, instead alter the gain of a stage. The minimum position is 0, or a level of output signal equal to the input.
The maximum position can be set to get a special distorsion, but generally the purpose of this control is to ensure that the stage being checked supplies the ideal output with respect to the operational state of the successive stages
at maximum efficiency, regardless of the input signal level.
Basically, the gain controls are raised in order to have as high a signal as possible for the following stages without causing distorsion.
The amplifiers for guitars with more than one input channel (generally designated high and low) often provide different measures of gain for each channel, and therefore have different sensitivities. A high channel gain will have more sensitivity and probably less overload capacity. Since the difference between overall gain almost always depends on an alteration of gain obtained from the first valve, there is an increased possibility to produce distorsion in the first valve. This is desirable because with distorsion in the first valve, you can obtain a distorted sound at any volume. It is not as advantageous to have distorsion in the second valve since the effect depends on the tone level. Amplifiers equiped with adjustable gain controls can develop this concept, giving more versatility to the distorsion levels.
An electric guitar can supply a prolonged output signal of 50 or more millivolts.
A signal of this level can overload (or put into "overdrive") the amplifier.
Amplifiers for guitars are therefore designed to support conditions of broad and prolonged input overload. This overload can cause distorsion in the pilot and/or power stages when the guitar is played at high volume.
The sound quality of a valve amplifier in overdrive is, as you can see, the main reason why many guitarists prefer this form of amplification.
If the first valve cannot be overloaded by a single guitar, you can use a preamplifer to increase the voltage of the output signal of the guitar.
A preamplifier could be incorporated into a guitar ("on-board" preamplifier), as pedals, or even as a rack effect.
This is the basis of many fuzz boxes and of other distortion devices.
The majority of distortions are activated by an electronic trigger, generally called a "Schmitt trigger". This device produces an output signal of square waves at the same frequency as that of the input of the guitar. The result is a mixture between the signal of the amplified guitar and the square waves.
The proportion between non distorted signal and square waves is defined by a mixer control generally named distortion or gain.
Although many manufacturers have tried to create evolved fuzz boxes to simulate the distorted sound of the valve, only a few models actually sound like valves. Attention: a lot of transistor equipment have a patented sound, very different from that of valves.
I will never get tired to repeat how extremely important personal taste is when choosing equipment. You must always keep this in mind.
Overloading a solid-state circuit, the waveform suffers a "clip" at a determined signal voltage according to the manufacturers specs. When the voltage of the input signal has reached a level that provokes the "clipping", any further increase in output signal
won't change the output signal. Sustain units take advantage of this phenomenon. This unit produces the maximum effect as soon as the signal of the guitar surpasses the clipping value, entirely eliminating the normal characteristics of attack and decline of a guitar note.
Electronic trigger circuits that produce square waves perform the same job, since the output level of the square waves coming from the trigger is virtually independent of the input signal level. The piece of equipment in question is called the compressor, and in many cases is combined with the distorter.
There are many pedal distorters models on the market today. All the examples that we will talk about are very standardized.
Lets look at a common distorter and its controls.
There are also rack distorters on the market. With respect to pedals, they usually offer a more complete equalization and also the possibility to have two (or more) channels, and therefore two (or more) sounds. In some cases the pedal model is also produced in a rack with the previously mentioned variations.
There are many musical types that prefer the use of these types of effects; they are clearly found in rock, but most all musical genders have use for the distorter. In cases that require a precise combination of the effect for a specific musical gender, this is usually indicated by the manufacturer
and many times is written directly on the instrument. The fantasy and creativity of the musician shouldn't be influenced by a name. Today there are some particular combinations that come from sound research (i.e. metallic sounds in fusion).
This type of effect has a different constructive nature than distortion. While the latter distorts the note, modifying the original waveform, overdrive doesn't change the note at all, acting on the volume by bringing the output gain to saturation.
In the 1960's a guitarist who wanted to distort the amplifier had to do so by raising the volume of the amplifier practically to the maximum. The sound that resulted was certainly saturated and full, but in small rooms the ears (especially those of the neighbors) suffered the imaginable consequences.
If the preamplifer is saturated (giving signal distorsion), and placing a control after the preamp to regulate the volume of the saturated sound, you can get the distorsion at a lower volume. Today many amplifiers have adopted this type of circuitry.
The overdrive acts in this manner, simulating the saturation of a preamp. This all serves to have better control both of the tone and the sound volume.
Overdrive and valve distorters are now built in both pedal and rack processor forms, in order to produce sounds similar to valves.
The controls of a common overdrive are basically the same as those of the distorter.
An interesting use of the overdrive is to saturate the amplifier with the following settings:
This type of adjustment produces a major push of sound (useful when going from rhythmics to a solo, which requires a bigger sound) and is a very commonly used system with guitarists. Concluding, I believe that this effect is fundamental in the set-up for any guitarist.
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