Using the SA571/NE571 as a compressor can create a problem in some applications. Unlike most conventional compressors, which only compress above a threshold level, the SA571 compresses a 100-dB dynamic range to 50 dB. When viewed on a log-log plot, the compression curve is observed to be a straight line (Fig. 1).

The equation of the line shown in Figure 1 is:

20logV_{O} = (1/2)(20log V_{IN}) = 20log V_{IN}^{½}

This reduces to: V_{O} = V_{IN}^{½}

The graph in Figure 2 illustrates the gain plot using linear coordinates, along with a unity gain V_{IN} versus V_{O} for comparison.

For an input of 100 µV (−80 dB), the gain of the compressor can be seen either from Figure 1 as −40 dB/−80 dB = 40 dB or 100. Or from Figure 2, the gain is equal to V_{O}/V_{IN} = 1/V_{IN}^{½}, and for V_{IN} = 100 µV, the gain is 100. This high gain, with little or no signal, presents a big problem in some applications because it raises the noise floor by a factor of 40 dB. Therefore, if the noise floor were −105 dB referred to the input of the compressor, the output would have a noise floor of −65 dB (−105 dB + 40 dB) which is totally unacceptable in many audio applications.

By adding one resistor (denoted as R_{N} in Figure 3), the current added into the node of the rectifier “fools” the compressor into thinking it has an input signal. Normally, without R_{N}, a 1-V signal into the rectifier produces an input current of:

I_{IN} = 1 V / R1 = 100 µA

When resistor R_{N} is added to the compressor ciruit, the dc current injected into the rectifier node is:

I_{IN} = 5 V/500k = 10 µA

This is equivalent to the rectifier having an ac signal of 100 mV. The new gain is now given by: Gain= 1/V_{IN}^{½} = 1/(0.1)^{½}= 3.16

Therefore, the starting gain has been reduced from more than 100 to approximately 3, lowering the noise floor in this example from −65 dB to −95 dB. The complete compressor circuit is shown in Figure 3.