Improved dynamic expansion circuit

It is a dynamic expander circuit with better performance composed of NE571 and LM 1894N . A variable bandpass filter composed of LM1894N is added to the dynamic expander circuit. The reason is that ideal dynamic expansion requires the use of an "RMS rectifier" to prevent the so-called "noise modulation breathing effect", and the "RMS The production cost of "rectifier" is very high and cannot be generally accepted. By adding a variable band-pass filter composed of LM 18ic)4N before the expander , the impact of "wheezing noise" on the sense of hearing can be eliminated.     The signal after band processing by the noise reduction stage is input into the dynamic expansion amplifier composed of N]ester 71 and NE5532N through c9. Since the conversion rate of the operational amplifier inside NE571 is only 0.SV/pLS, and its passband is only effective in kilohertz, NE5532N is selected as its operational amplifier unit, leaving the operational amplifier unit in the original IC unused (see figure The 5-11(a》o circuit uses only the two unit circuits necessary for dynamic expansion, the variable gain unit and the average rectifier of the NE571, and the additional NE5532N forms a complete, high-performance dynamic expansion circuit.     This circuit The circuit 's adjustment of the expansion ratio is achieved by connecting resistor R4 in parallel between pins 5 and 3 of NE571. Through experiments, using the value 100k as shown in the figure, a real and natural sound reproduction effect can be achieved. C12 is a built-in function of NE571 The filter capacitor of the rectifier . Its value and quality have a great influence on the overshoot, rise speed and delay amount of the expander . It is advisable to choose tantalum electrolytic capacitors with stable charge and discharge. At the same time, the l and 16 of the left and right channels The feet should be used in parallel.