Theory of Operation
Compressor Basics
Before we dig in to a description of the LA-2A circuit, it is useful to examine the general characteristics of compressors and review some terminology. Figure 1 depicts the input/output characteristics of a compressor, an expander and a perfect amplifier. When operated within its specified range, an amplifier provides a constant amount of gain regardless of the level of the input signal. In Figure 1, the middle line depicts a perfect amplifier with a gain of 10 dB. To see this, notice that a signal with an input level of –30 dB will result in an output level of –20 dB, which is an increase of 10 dB. Similarly, an input level of 0 dB will result in an output level of 10 dB, hence the gain stays fixed at 10 dB regardless of the input level.
Expansion
Perfect amplifier
+10
0
Compression
Output
Level (dB) -10
-20
-30
Input Level (dB)
Figure 1 - Input/output characteristics of a compressor, an expander and a perfect amplifier.
In contrast to an amplifier, whose job is to present a constant gain, a compressor varies its gain in response to the level of the input signal. Large input signals result in less gain, thus reducing or “compressing” the dynamic range of the signal. Referring again to the line marked “compression” in Figure 1, we see that an input level of –30 dB results in an output level of –20 dB, indicating a gain of 10 dB. Repeating this for input levels of –20 dB and –10 dB, we see that the compressor exhibits gains of 5 dB and 0 dB respectively. From this, it is clear that the gain decreases as the input signal increases.
Referring to the diagram, we see that the compressor will increase its output level by 5 dB for every 10 dB that we increase the input level. The compression ratio is defined as the ratio of these two numbers. In this case the compression ratio would be 10:5, which can be reduced to 2:1.
