In contrast to an amplifier which has a constant gain, a compressor is a device that varies its gain in response to the level of the input signal. When in compression, large input signals result in less gain, thus reducing or "compressing" the dynamic range of the signal. In order to accomplish this goal, a compressor must have the ability to both sense the level of the input signal as well as control the gain of the signal path. There are many ways to accomplish each of these tasks, with each approach exhibiting its own characteristics and sonic qualities. The 1176LN and LA-2A provide two different examples of how to accomplish these tasks. The gain reduction stage of the 1176LN uses a field effect transistor (FET) to control the gain. In this circuit, the FET acts like a variable resistor whose resistance is controlled by the voltage applied to its gate. The larger the voltage, the less its resistance will be. This gate voltage is derived from the side chain circuit and changes in response to the level of the input signal.
The 1176LN and the LA-2A are two examples of classic compressors which use very different techniques for gain control. The 1176LN uses a field effect transistor as its gain reduction element, while the LA-2A uses an electro-optical element (el- op) in its circuit. What follows is a basic overview of the principles behind the gain reduction circuits in each of these circuits.
