[Photo] Low-voltage dynamic noise reduction IC-LM832

Both LM832 and LM1894 are dynamic noise reduction ICs, but the operating voltage of LM1894 is 4.5 to 18V, and its input signal level is high, which is not suitable for ultra-small audio devices with low voltage and low input signal level, and the work of LM832 The voltage is 1.5 ~ 9V, the input signal level is about 30mVrms, very suitable for small stereo audio equipment such as stereo walkman.
LM832 is the same as LM1894, and it is also a dual in-line 14-pin package. Except for the different functions of the {14} pin, the other pin functions are exactly the same, but the component parameters are slightly different. The {14} pin of LM832 is a stereo / mono selection switch. When a stereo signal is input, this pin is floating; when a mono signal is input, this pin is grounded. The picture shows the typical application circuit of LM832.
SW is a DNR switch. When the DNR action is not needed, just close the switch. C1 is the Power Supply decoupling capacitor, and C2 and C11 are the input coupling capacitors. C3 and C10 can set the minimum and maximum bandwidth of the control channel. When it is used in stereo, it takes 0.022μF, and the channel bandwidth is 1 ~ 30kHz; when it is used in mono, it is only 0.015μF, and the channel bandwidth is 1.5 ~ 44kHz. C4 and C8 are output coupling capacitors. C5 works with R1 and R2 on the control channel. The value of C5 is small, and the high-frequency components will be attenuated. C5 has a large capacity and can increase the bandwidth. C6 and C7 are similar to C5. C9 and IC internal resistance together set the rise and fade time. C9 time is short, C9 time is long. The partial pressure of R1 and R2 determines the sensitivity of the control channel, requiring R1 + R2 = 1kΩ. The values ​​of R1 and R2 should be adjusted carefully to achieve the best noise reduction effect. R3 is used as the load of the gain amplifier stage when the DNR is off. The value of R3 affects the maximum bandwidth, load gain amplifier stage output and distortion. It should be noted that the control channel filter components (C5, C6, C7, C9, R1, R2) and integrating capacitors (C3, C10) generally follow the recommended values ​​of this circuit, and should not be changed easily unless from FM or tape sound The noise characteristics of the source are fundamentally different. Improper use of component values ​​will cause system performance degradation.
To further reduce the system noise, two LM832s can be used in cascade. C3 and C10 use 0.015μF. At this time, the system noise can be reduced by 17dB. The method of cascading is to connect the two output terminals of the first LM832 directly to the input terminals of the second LM832. If the signal source is mono, a LM832 can be used to achieve cascaded noise reduction. The method is to input the pin {2} as the input, the pin {13} is directly connected to the pin {4} with a wire, and finally the pin {11} is output. C3 and C10 also take 0.015μF. If dedicated to the FM stereo receiver, an LC resonant circuit needs to be connected at the {8} pin, where L1 is 4.7mH adjustable and C is 0.015μF, the resonance frequency is adjusted to 19kHz, and the 19kHz pilot signal is removed Noise, so the noise reduction performance of the system will be better.
The working principle of LM832 is the same as that of LM1894. It uses sound quality masking and adaptive bandwidth circuits. The system can adjust the signal channel bandwidth according to the strength of the input signal and the characteristics of the signal-to-noise ratio, so as to achieve the purpose of noise reduction. There are two other points to note: First, the DNR must be placed in front of the treble volume control level, otherwise changes in tone and volume will affect the adjustment of the dynamic noise reduction circuit; second, the use of LM1894 for signal sources with high input levels .





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