Self Powered Compressor

The most unique feature of this dynamic compressor circuit is that it needs no power supply, looks like  as passive connector or adapter cables. The attenuation is good enough, about 20dB, works for  input range of 100mV to 10V signal level. Here is the schematic diagram of the circuit:

The circuit consist of a power rectifier and attenuator. Some portion of signal is rectified to generate a DC current. This rectification is done by D1 and D2. The rectified signal is used to control the attenuation diodes D3 and D4.  The attack time of this dynamic compressor circuit is fixed, and depends on the value of C1, C2, R2, and the impedance of the signal source. We can slightly adjust the decay time by trimming the R7 variable resistor. The best performance is obtained using germanium diodes because they have lower forwad bias voltage and smoother conducting transition curve. This dynamic compressor circuit is suitable to limit the signal level before feeding it to a transmitter. Make sure if the input impedance of the device  that receive the output of this compressor is high enough, since the output impedance of this circuit is over 470k. [Source: Elektor]