To understand and to use the different kinds of compression pedals.

In this article I am going to give a guidance to the use of compression pedals based on the type of circuit and the musical needs of guitar and bass players . Another article on the Soundsation blog dials with historical hints and descriptions of common compressor types.

Why compressing?

In the first article about compression we have seen that compression is the automatic level control operation of an electric signal (guitar sound, recorded track, and so on), setting its dynamic range. In short, the sound levels that exceed a certain threshold are reduced, whereas sound levels under it are amplified.

Requirements for a signal dynamic reduction can be adaptation inside a circuit, levelling in audio production, automatic processing of the volume of a recording, peak control , including those peaks created by a guitar or bass player's playing , up to specific sounds of music styles as funk, fusion, disco and country.

Compression cuts threshold exceeding peaks and raises lower levels, levelling out the output signal and producing a sustained sound ( sustain).

Compression is very appreciated – though not always perceivable to everyone – when assigned to modulation and delay effects.

Compressor and booster

Especially among rock and fusion guitarists it is popular to use a compressor like a booster, placing it between the guitar and an overdrive pedal or the amp input in order to raise the signal level and take it more easily to distortion without raising too much the general volume of the amp. At the same time, the compressed and overdriven sound will be longer, warmer and more controllable by the fingers, although it has to be pointed out that a distorted sound is pretty much inherently compressed.

If you want a boost effect only, it is possible to get it just raising the Volume or Level knob, setting the Sustain or Compression one low or completely counter-clockwise, though these two controls are pretty interactive. In fact, the first one usually acts as the effect output level, whereas the second one as the compression gain.

Some compressors feature a clean/clip option, that is a choice between a clean full wave form and a squeezed one. Find an elemental example of clipping in the schematic below .

Other models offer an integrated booster with different effect routing options, as it happens in the Nux Masamune NBK-5.

Compression and noise

Especially in outdated and cheap compressors, the hidden enemy is the risk to produce noise or, rather, to amplify the existing noise in our sound chain. As a matter of fact the compressor “sees” the input signal inherent noise as a low level signal to be raised, causing a very unpleasant effect during performance pauses. For this reason compression circuits are often combined with a more or less declared noise gate section. Anyway, putting the compressor at the first place in the effect chain is recommended, thus avoiding it perceives possible noises from other pedals, especially overdrive and distortion boxes. Nevertheless, sometimes it is possible to see a compressor at the end of the effect chain to level out the sound before amplification.

Which compressor for which use

I remind you that the typology and the parameters of compression are dealt with in the first article of the Soundsation blog. Let's see now the most popular compressor types and some real models as references.

OTA

The most common compressor for guitar features an OTA circuit (current amplification), as it happens in the MXR Dyna Comp, in the Ross Compressor and in the following Ibanez CP-9, Boss CS-2 and DOD FX-80B, as well as in most of current ones with some twists: for example, MXR Dyna Comp Deluxe, Wampler Ego, Keeley Comp Plus, Electro-Harmonix Tone Corset, Fender The Bends, Maxon CP9 Pro, Nux Sculpture NCP-2 and Komp Core Deluxe.

OTA compressors can tend to “clip” and “squeeze” the sound adding a little of “dirt” with some settings. They can be somewhat noisy – though an uncommon flaw in today's quality products – or change the instrument original sound. Obviously much depends on the product range and the circuit advancement.

Music application fields are the most varied, from funk to disco, from country to progressive, from pop to fusion, including certain clean or light crunch sounds of Texas blues (think about Stevie Ray Vaughan's Riviera Paradise ). Bass players love OTAs too for traditional music genres and if a killer attack is not needed.

VCA

Compressors based on VCA circuits (voltage control) follow by popularity. A classic example is the Boss CS-3. VCAs are more precise and faster in reaction, with a sharp attack when requested. Often they are are not so true to the original input sound. They represent an effective and inexpensive solution in studio rack-mounted compressors, though guitar pedals can result “cold” or “clinical” for someone's taste. With their numerous parameters, they are very useful in metal music for tight rhythm playing, legato, palm-muting, prog wet arpeggios, in short when dynamic control, definite attack and a good “punch” are requested.

FET

VCA models with a FET stage (Electro-Harmonix Soul Preacher, Analogman Juicer) enhance VCA described features adding a stronger attack, emphasized transients, percussive sound and great push. Metal, funk and fusion bass players love FET compressors for their sound and dynamic qualities and for an often greater number of controls, as Attack and Decay. Instead, the old, simple and successful Dan Armstrong Orange Squeezer in our picture had no controls.

Optical

Optical or opto-electronic compressors are based on a light source and photo-resistors that modify the compression effect according to variations of light intensity , for their part variable according to the input signal level. These compressors have a pretty slow release, a soft attack and a warm sound. Their kind of compression closely resembles a studio device one, so the effect sometimes is little perceptible, intended for musicians with discerning taste and fine playing. Anyway there are exceptions, preferably integrated with boost and EQ circuits. In this case they perform very well in styles like modern rock fusion. Examples: Eleven Electrix Opt-Compressor, Diamond Compressor, Demeter Compulator, Electro-Harmonix White Finger, BBE Opto-Stomp, Friedman Sir-Compre.

Tube

Tube compressors use one or more tubes as the old studio forerunners. Sometimes the tube is just used to “warm up” a solid state circuit sound. They feature a refined and soft sound, a slow attack, a smooth decay and generally offer few parameters. They need a dedicated power supply, thus making them not the preferred choice for guitar players' pedal-boards. Models with a good output, lots of controls and a DI out are recommendable for bass and electric-acoustic guitar players. Some examples: Electro-Harmonix Black Fingers, DV Mark Compressore, T-Rex Squeezer, Effectrode PC-2A.

Digital

Over the years, thanks to the technology experience made in physical modelling designs , some digital compressors have appeared, though it seems they were mostly unsuccessful. Exceptions are those models in whose marketing the word digital does not show too much, as well as those that can be used with download and editing apps . According to types, they can offer an original way of compression; more often, they emulate well-known analogue compressors, as in the case of the TC Electronic HyperGravity, the Source Audio Atlas and the Line 6 Constrictor. Other ones, as the Boss CP-1X, are multi-band compressors, meaning they process different portions of frequencies separately. In my opinion, placing digital compressors in pedal-boards has more sense in a totally digital domain or inserted in a loop.

Conclusions

A lot of guitar players snub the presence of a compressor in their pedalboard, but this is an essential, indispensable effect in the post-production of studio recordings for any great player, from Jimmy Page to Steve Vai, often used as an out-board or mixer effect. Session players and live pros always keep one in the reach.

It has to be said that the compressor effect requires a certain music and performance maturity to be appreciated by a musician. A good level of awareness of the several aspects of the sound and touch on the instrument is needed in order to include the right compressor in the right way among the effects and create fine natural sounds.

I suggest to novice guitarists to start with an OTA type compressor if a classic smooth sound is desired. If you are searching for less noise, cleaner sound and open highs, a model with the LM13700 op-amp is probably advisable. I also exhort to compare OTA and VCA compressors, just to understand their sounds before taking a direction.

Instead, I would suggest a bass player to make the inverse choice: a good VCA to start and then a comparison with an OTA or another kind.

I would suggest an optical compressor to the musician who is searching for balance and finesse in the harmonic texture of his arpeggios and phrasing; but a high class VCA can also work.

I would like to invite mature musicians who have not adopted a compressor yet, to take advantage of this little technical and stylistic guide.

If you have diversified needs and a good touch, try also compressors with more features, including equalisation and boosting. In this regard, I absolutely have to refer to the Masamune NBK-5 and similar ones to point out the different behaviour of the boost and the compression sections when they are connected in series or in parallel, an option this Nux effect offers: a whole world of dynamic and sound possibilities will spread under your fingers. Experience to believe!

Fabrizio Dadò