The compressor

The term compressor involves two distinct signal control systems. These are the limiter and the compressor. They are two instruments used for reducing the dynamic range of a sound signal before its sent to the power amp (the last stage of amplification before the loudspeakers). Because the amplitude of the dynamic range in an amplifier is so important, it would seem strange to reduce it. But this is one of the most significant techniques of sound elaboration, which is often used in the recording studio.

Limiter

limiting is the simplest of the techniques used to reduce the dynamics. The limiter can be used in any situation where there is too much signal, which could cause serious damage to the musicians ears, the speaker cones and the equipment in general if not controlled.
Limiting is also the technique used to control the automatic volume and the automatic recording level. The gain is checked by a circuit that analyzes the level of incoming signal. The gain of the amplifier is linear up to a certain level of the input signal. When the circuit receives a signal that surpasses that level, it reduces the gain in order to maintain a constant output level.
Most limiters have a threshold that allows you to regulate that level. When the input signal goes below the threshold, the gain of the amplifier returns to its original level. A release control determines the velocity at which this happens; it can vary from a few milliseconds to one or two seconds (depending on the model). You need to adjust the control in order to cause the least amount of disturbance to the music (for instance, signal distortions can appear with the limiter at its minimum effect on the dynamics of the same signal). If the time of release surpasses 5 seconds, the limiter acts as an automatic volume control.

Compressors

The compressors are finer and more sensitive than limiters, even if they work practically the same way. They are normally predisposed as limiters, and some are able to compress or limit a signal. The compression helps to reduce the effect of the variations in the output signal level of the guitar as well as other instruments. These variations are caused by the differences between high and low notes between one chord and another or by irregularity in the action of the right hand. The compression can improve the sound and help create a compact and incisive sound, particularly nice in rhythmics. In a solo the compressor can be used to increase the sustain without causing distortion.
Like the limiter, the compressor also has a threshold and release that work in similar ways. The difference is when the circuit receives an input signal above the threshold; instead of altering the gain to keep a fixed output level, as the limiter does, the compressor changes the ratio of the amplifier. Before the compressor intervenes, the ratio is 1:1 (which means that if the input voltage reaches 1 volt, then the output voltage will increase to 1 volt). If the ratio is 2:1, the output voltage will increase only by 1/2 volt for every increased input volt.
In the graph below we analyze the relationship between the input and output signals of a compressor as well as the way in which the introduction of threshold alters the ratio. Different ratios can be chosen in order to create small or large variations in the dynamic range of the signal or over the entire range.

The characteristics of the various types of limiters and compressors are different. Some don't have varying thresholds, but perform the same task thanks to input and output volume controls. Others operate only at specific frequencies in order to eliminate drawbacks, such as the hiss of the voice.
This, seen above, is a technical graph of the compressor effect. We can add an easier graph to understand. In graph A we see the length of the note, expressed in time and played naturally without effects. In graph B the same note with the use of the compressor. In graph A, we see a naturally played note that slowly decays. In graph B the note remains at a constant volume (due to the effect of the compressor) until the compression finishes then dies almost immediately.

Recommendations for use of the compressor

Using the compressor properly can surely bring some advantageous results. Lets look at some possible applications.
The use of the compressor with acoustic guitars gives a more dense and constant sound. A problem that can occur when amplifying acoustics is the incorrect equalization of the volume between the various chords: the risk is that you might hear more bass than treble or vice versa. By therefore compressing the dynamics it's possible to linearize the global balancing of the chords. Regulating the entry speed between 10 and 40 milliseconds, you can maintain the natural entry of the pick, which is very important in phase accompaniment.
With the electric guitar, there are many situations in which a compressor can be used. When you need clean rhythmics (such as funky rhythm) we can regulate the compressor with a very present entry and a low sustain, which allows you to maintain a constant volume when the pick strums the chords, when stops are played, and "dead notes".
The note dynamics of distorted sounds are often uncontrollable: in this case the compressor helps to keep the sound constant, also increasing the feedback in an easily controllable way. Attention: many amplifiers already have controlled dynamics (naturally compressed sound). In this case little or no compression is required. If you want to increase the sustain without increasing the saturation, you can simply compress the input using a fast entry and a release of about 250 ms. Depending on the amount of compression you want, the control rate can be set to 4:1 or more and reducing the gain by at least 20 dbs.
As with any effect, abusing the compressor will negatively influence the performance. Too much compression limits the dynamics and therefore is used only when it is really needed. It's much better to use the compressor at a lower volume, because at a higher volume the natural compression of the amplifier produces enough sustain which will eliminate distorted sounds.
Remember that the compressor increases the dynamics of the sound, but also increases any background noises in the signal. For this reason some compressors (especially the rack type) are equiped with noise reducers. We will talk more about this in the section devoted to noise gate (noise reducers).

Connections

The compressor can be connected before the other effects (for pedals), and immediately after the guitar: remember that the pedals have an input resistance equal to or greater than 0.5 Mohms. The rack effects generally have a programmable resistance, therefore during the connections it is necessary to check that all the values correspond (the resistance selector is usually found on the back): the manufacturer always indicates these values in the instruction manual. A rack compressor can be connected to the send-return of the preamplifer, or after the preamplifer in line. Depending on the use and the type of equipment that you have (pedal or rack), you can try different combinations to get the best results. Also try different amplifier output volumes to better evaluate the dynamic response.