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:
20logVO = (1/2)(20log VIN) = 20log VIN½
This reduces to: VO = VIN½
The graph in Figure 2 illustrates the gain plot using linear coordinates, along with a unity gain VIN versus VO 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 VO/VIN = 1/VIN½, and for VIN = 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 RN in Figure 3), the current added into the node of the rectifier “fools” the compressor into thinking it has an input signal. Normally, without RN, a 1-V signal into the rectifier produces an input current of:
IIN = 1 V / R1 = 100 µA
When resistor RN is added to the compressor ciruit, the dc current injected into the rectifier node is:
IIN = 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/VIN½ = 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.