A Hi-Fi compressor circuit with pre-emphasis

This is a Hi-Fi compressor circuit with pre-emphasis. The picture above is a high-fidelity compressor circuit that uses an external op amp with high gain and wide bandwidth. Stability requires input to the compensation network.

Another feature of the circuit in Figure 20 is that the rectifier capacitor (C9) is not connected to ground but is connected to the output of the op amp circuit. This circuit is built around the LM324, which speeds up the compressor's attack time at low signal levels.

When the rectifier input level drops from 0dBm to -30dBm, the time constant increases from 10.7k×CRECT to 32.6k×CRECT. In a system where there is unity gain between the compressor and expander, this will not result in an overall error. Gain or loss between compressor and expander will be false tracking of low signal dynamics. For systems where unity gain cannot be guaranteed, a circuit with an LM324 will greatly reduce this problem.

The compressor in Figure 20 includes a high-frequency pre-emphasis circuit (C2, R5 and C8, R14) to help solve this problem. The expander needs to be matched for de-emphasis. More complex designs can make pre-weighting variable and further reduce breathing.