Origin and description of the different kinds of compressor pedals.

In this article you will find historical notes and an overview of the most common types of compressors for electric guitar and bass. Another article on the Soundsation blog suggests a guidance to their use based on types and musical requirements.

Audio compression

In the pro audio sector compression is the automatic level control operation of an electric signal (guitar sound, recorded track, and so on), setting the dynamic range for specific purposes. In short, the sound levels that exceed a certain threshold are reduced, whereas sound levels under a certain threshold are amplified. Hence a compressor is used to adapt a signal dynamic within desired values.

The limiter is an “extreme” compression effect that radically limits – as the name says – a signal level, and it is used in noise gates and de-essers. At some extent, a limiting effect can be obtained also with a common compressor; very fast attack and high compression ratio (typically 10:1) are needed. I am going to talk about this parameters further on.

Opposite to the limiter is the expander, an effect that increases higher signal dynamics and further reduces lower ones.

Compression parameters

The compression procedure requires setting several parameters:

• Threshold is the signal level (expressed in dB) above which the compressor starts acting; in compressor pedals it is usually fixed by design.
• Ratio is the amount of compression (expressed as a ratio, for example 4:1) that is applied to the signal when it exceeds the Threshold value.

• Level is the effect output gain control that compensates for the compression determined volume loss.
• Attack is the time (expressed in milliseconds [ms]) it takes for the compression to start acting on the signal when it exceeds the Threshold value.
• Decay is the time (expressed in milliseconds [ms]) it takes for the compression acting since the signal initial level to that of the set compression.
• Release is the compressor action time (expressed in milliseconds [ms]) on the signal until there is no more compression.
• Knee or compression curve sets out the kind of compression reaction, from sharp to gradual.

Most common compression parameters

In the studio rack-mountable compressors and DAW (Digital Audio Workstation) plug-ins parameters can be even more, but in the most common compressor pedals they are usually three:

• Attack, sometimes not present, often fixed or adjustable via a little internal trimmer; sometimes it is actually a Release control (ex.: Boss CS-2).
• Ratio, more frequently called Sustain, Compression, Comp or Sensitivity.
• Level, also called Volume, Output or Gain.

Less commonly, and mostly in refined bass guitar pedals, you can also find:

• Tone, to reduce high frequencies.
• Input, to adjust the effect input signal level.
• Blend or Mix, to blend the original dry signal and the compressed wet (parallel compression).

As you can see, the nomenclature is rich and varied, sometimes also used in a confused way by manufacturers. On the Internet there are a lot of tutorials dedicated to compression and its detailed parameters in the recording studio as well as in the effect chains of guitar and bass players. In this article as well as in the one dedicated to compressor uses for guitar and bass players, we are limiting ourselves to a practical and schematic approach in order to help the unsure musician in identifying the most suitable compressor category for his/her needs.

Typology of compressors

Based on their circuits and components, compressors can be divided in these main types:

• VCA, Voltage Controlled Amplifier . Examples: Boss CS-3, Keeley Comp Pro.
• OTA, Operational Trans-conductance Amplifier , a kind of VCA in which output current varies instead of voltage. Examples: historic models as the MXR Dyna Comp and the Ross Compressor, also many of the current pedals, more or less boutique and with more or less variations: Wampler Ego, Keeley Comp Plus, Electro-Harmonix Tone Corset, some Nux models.
• FET, another type of VCA circuit, but based on FETs (Field Effect Transistor) and an operational IC. Examples: the historic Dan Armstrong Orange Squeezer, Electro-Harmonix Soul Preacher, Analogman Juicer.
• Optoelectronic or Optical, based on the use of bulbs or LEDs as light sources and photo-resistors that modify the signal according to light intensity variations . These compressors are slow in release, soft in attack and produce a “warm” sound. Examples: Diamond Compressor, Demeter Compulator, Electro-Harmonix White Finger, Mooer Yellow Comp, BBE Opto-Stomp.
• Tube, based on the use of tubes, usually one in pedals. Examples: Electro-Harmonix Black Fingers, DV Mark Compressore, T-Rex Squeezer, Effectrode PC-2A.
• Digital or DSP, based on AD DA conversion and processing, often providing different physical modelling simulations of vintage compressor pedals. Examples: Boss CP-1X, TC Electronic HyperGravity, Source Audio Atlas, Line 6 Constrictor.

A brief history of compressors

Early huge and heavy audio dynamic compression devices appeared in recording and broadcasting studios in the '30s and '40s of the past century. Today '50s and '60s tube compressors are much sought after. In the pictures below, the Universal Audio 175B and the Jimmy Page popularized RCA BA-6A are showed.

They performed according to the input signal level and did not feature the Ratio control but just the Threshold: the higher the input level, the more intense the compression, with a typical soft-knee. They are also called variable mu (µ) or vari-mu compressors.

As regards guitar and bass players, in the early '70s the first well known MXR pedal effects appeared. The little Rochester (NY) company's name was an acronym for MiXeR, because it was specialized in those devices before dedicating to historic effects as the Phase 90 and the Distortion+. In 1972 the Dyna Comp pedal was offered. It was based on the noted RCA CA3080 OTA metal IC, later a plastic one.

The Dyna Comp was an immediate great success in Nashville studios, thanks to its handiness and to the compression quality, though a little noisy.

Largely inspired by the MXR Dyna Comp, in 1974 the Ross Compressor appeared, the second great name in the history of compression stomp-boxes. It featured the same integrated used by MXR, but it produced a slightly different sound, warmer and more laid down, also less noisy.

Another '70s pioneer was guitarist and technician Dan Armstrong, with his coloured aluminium effects that were directly attachable to the guitar jack socket. The line included the Orange Squeezer, a compressor with no controls, except for an internal trimmer setting the FET BIAS. Transistors worked together with a JRC4558 IC. A hole was often drilled in the box front for a fast access to the trimmer.

It was at the base of the sound of guitar players as Jeff “Skunk” Baxter (Steely Dan, Doobie Brothers), Dire Straits' Mark Knopfler and bass players as Tony Levin with Peter Gabriel.

The big OTA compressor family

Starting from these early models, current and most popular compressors were developed. As simply as possible, the MXR Dyna Comp is the classic effect for the squeezed sound of a country solo or a funky riff. The Ross Compressor is squashed as well, but also milder and best usable for arpeggios and chord melody. The Orange Squeezer is a very simple effect (on/off) and also a little “dirty”; so is is great for boosting an amp to saturation. The choice is a matter of tastes and opinions...

Instead of the old, expensive and now little available CA3080, today is often used a modern and more performing double trans-conductance op-amp as the LM13700, though it requires new circuit designs.

This is the case, for example, of three OTA compressors in the Nux effect line: the mini-pedal Sculpture NCP-2, the advanced Komp Core Deluxe (in the photo) and the Masamune NBK-5.

We are going to talk about these and other compressor pedals in the second article dedicated to compression on the Soundsation blog.

Fabrizio Dadò

References
M.Maratea, Axe Tech column on Axe Guitar Magazine n.222 (Edizioni Palomino, September 2017)