Expansion circuit made with NE571

It can also be seen from the compression-expansion characteristic curve that the gain is I at OdB. Since
the actual reference level of the line input 0-level standard is 0.775V, the line input signal is lower than OdB most of the time. , the average is about
-8 to -10dB. The actual gain at this time is only 0.3dBo, so the expanded sound sounds reduced.
If Hukouguo switches the expansion to pass-through at this time, it will cause a sudden change in the volume. For this reason, resistor R3 is added in series to the feedback loop of the op amp to increase the gain of the op amp so that the gain of the expander is 1 when the input is -9dB. In this way , the listening experience of the volume is basically the same whether in the expanded state or in the straight-through state. consistent.     The internal bias circuit of the IC is designed to operate at the lowest power supply voltage of 6V, that is, the static DC voltage output by the op amp is 3V. When the power supply voltage is higher than 6V, an external resistor must be used to adjust o. There are two methods: one is to use the feedback circuit in series with resistor R3, but as mentioned earlier, R3 also has an AC negative feedback function, so the op amp must first be satisfied . Select appropriately under the premise of gain . The second is to connect a resistor in parallel between pins 5 and 12 and ground , such as R2 in the figure. When the power supply voltage and R3 are determined , R2 can be calculated according to the following formula.