The lm1875 is a power amplifier integrated block that is a power amplifier integrated block developed by the US National Semiconductor Corporation. It has few peripheral circuits in use and has perfect overload protection. It has a five-pin shape, one pin is the signal positive input, two pins are the signal negative input, three pins are grounded, the four-pin power is positive input, and the five-pin is the signal output. LM1875 produces power amplifier circuit. The LM1875 adopts TO-220 package structure, which is like a medium power tube. It has small size, simple peripheral circuit and large output power. The integrated circuit is internally provided with overload overheating and inductive load reverse potential safe working protection.
LM1875 main parameters: voltage range: 16 ~ 60V; quiescent current: 50MmA; output power: 25W; harmonic distortion: <0.02%, when f = 1kHz, RL = 8Ω, P0 = 20W; rated gain: 26dB, when f =1 kHz; operating voltage: ± 25 V; conversion rate: 18 V / μS.
Circuit principle:
The LM1875 power amplifier board consists of an attenuated tone control circuit controlled by a high and low sound, and a LM1875 amplifier circuit and a power supply circuit. The tone part adopts an attenuated tone circuit controlled by high and low sounds, among which R02, R03, C02 C01, W02 form the bass control circuit; C03, C04, W03 form the treble control circuit; R04 is the isolation resistor, W01 is the volume controller, adjust the volume of the amplifier, C05 is the DC blocking capacitor, to prevent the LM1875 DC potential pair of the latter stage The influence of the pre-tone circuit. The amplifier circuit mainly adopts LM1875, which is composed of 1875, R08, R09, C066, etc. The amplification factor of the circuit is determined by the ratio of R08 and R09. C06 is used to stabilize the drift of the DC potential of the 4th pin of LM1875, but has a certain sound quality. The effect of C07, R10 is to prevent the amplifier from generating low frequency self-excitation. The load impedance of this amplifier is 4 → 16Ω.
In order to ensure the sound quality of the power amplifier board, the output power of the power transformer should not be lower than 80W, the output voltage is 2*25V, the filter capacitor is connected in parallel with two 2200UF/25V electrolytic capacitors, and the positive and negative power supplies share four capacitors of 2200UF/25V. The monolithic capacitor of 104 is a high frequency filter capacitor that is good for the sound quality of the amplifier.
Assembly and commissioning:
Tool preparation: 20W electric iron, one universal meter, one needle-nosed pliers, one screwdriver, solder wire and pine perfume.
Prepare for welding: first weld the jumper, then solder the resistor, then solder the capacitor, then solder the rectifier, then solder the potentiometer, and finally solder the LM1875. Before soldering the LM1875, fix the LM1875 on the heat sink, otherwise install it at the end. It is difficult to get in the screw when the heat sink is used. The part of the LM1875 that is in contact with the heat sink must be coated with a small amount of thermal grease for heat dissipation.
Debugging: After the board is soldered, connect the transformer. The output of the amplifier is not connected to the speaker first, but the multimeter is connected. It is best to display the digital display. The multimeter is placed in the DC*2V file. On the power amplifier board, pay attention to observe the reading of the universal meter. Under normal circumstances, the reading should be within 30mV. Otherwise, the board should be powered off immediately. If the reading of the meter is within the normal range, it indicates that the function of the power amplifier board is basically normal. Finally, the speaker is connected, the music signal is input, the test machine is powered on, the volume potentiometer is rotated, the volume should be changed, and the high and low sound knobs are rotated. The pitch has changed.
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