6. Circuit designs

The electronic circuits of various compressors differ in the design of the operational amplifier, which is responsible for the automatic level adjustments. This would not be worth mentioning, if the type of operational amplifier didn't have an effect on the sound quality and sonic characteristics of the compressor. Below, we would like to give you an overview of the most common designs.

VCA

VCA stands for „Voltage Controlled Amplifier“. The control signal is derived from the input voltage and fed to the VCA in the form of a rectified control voltage. VCAs are readily available as integrated circuits (ICs) from manufacturers like THAT Corp. Their fundamental advantage is their linear frequency response. They also allow for very short response times. Of all designs, these models have the smallest effect on the sonic characteristics of the signal. VCA compressors offer great flexibility and can generally be used for all applications. Some well known VCA compressors include the SSL Bus Compressor, the API 2500, and the DBX 160.
DBX 160A Mono-Compressor

Opto

Opto-compressors are another popular variety. In this design, a photosensitive resistor (Light Dependent Resistor, LDR) or photo transistor is responsible for the signal-dependent amplification. The LDR is illuminated by a suitable light source (bulb or LED), whose brightness is modulated by the input signal. A change of brightness causes a change of resistance of the LDR. Optical compressors typically exhibit a certain sluggishness in their time response. In addition, their response is nonlinear. For enthusiasts, these „imperfections“ account for the unique charm of these devices, and the sonic quality is often perceived as very musical. However, certain applications require faster response times than an opto-compressor can deliver. Famous examples of opto-compressors include the Universal Audio LA-2A and LA-3A.
Universal Audio LA2A Teletronix

Tube / Valve / Variable MU

Today, tubes are added to the signal paths of many compressors, in order to add the popular „tube sound“ to the signal. However, most of these devices still rely on VCAs or optocouplers as their control elements. In a true tube or valve compressor, the control element itself is a tube (pentode). The manner in which the signal is fed to the tube causes a variable, signal-dependent amplification, which is known as Variable MU. These compressors do not have adjustable ratios. The ratio automatically increases with the level of the input signal. The higher the input level, the higher the ratio, and the greater the gain reduction. The tube causes a typical, smooth distortion, which is responsible for the characteristic sound of Variable MU compressors. The time response is typically between that of opto and VCA compressors. The Fairchild models 660 and 670 were designed this way, as is the Manley Variable MU.

Manley Variable MU

FET

Here, the operational amplifier is based on a field-effect transistor (FET). In the working range of the response curve, it functions like a voltage-controlled resistor and mimics the behavior of a tube. Like tube compressors, FET units have unique sonic characteristics. The best known example of this category is the Universal Audio 1176 LN.

Universal Audio 1176 LN

PlugIns

In the age of DAW-based recording studios, compressors are of course readily available as software plug-ins. Virtually all of the analog classics mentioned above have been emulated as software. Well-known producers of high quality emulations include companies like Universal Audio, Waves, SSL, Softube, Slate Digital, and others. In comparison, the compressor plug-ins that come bundled with DAW software tend to be of lesser quality. If the respective hardware units don't fit within your budget, consider high-quality plug-ins by these manufacturers. The audio quality is very close to the originals, with the added bonus of being able to use multiple instances of the plug-ins simultaneously.

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