The NE570 can be used to make a high performance compressor FTA, except that the rectifier is connected to the input. This makes gain inversely proportional to the input level so that a drop of 20 dB input level will produce an increase of 20 dB gain. The output remains at a constant level. As shown, the circuit will maintain an output level of ± 1 dB for an input range of + 14 to -43 dB at 1 kHz.
Additional external components allow the output level to resolve.