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  • Contents

I Description

LM1875 is a power amplifier integrated block chip. Its superior performance and attractive tone have been accepted by many enthusiasts, and it was all the rage in the 1990s.

LM1875 adopts a TO-220 package structure, which is shaped like a mid-power tube, small in size, simple in peripheral circuits, and large in output power. The integrated circuit introduced in this blog is equipped with overload, overheating and inductive load reverse potential safety protection, which is one of the ideal choices for high-end audio.

lm1875Figure 1. LM1875

Catalog

I Description

II LM1875 Parameters

III LM1875 Circuit Principle

IV LM1875 Circuit Assembly and Debugging

V In the End

FAQ

Ordering & Quantity

II LM1875 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=1kHz
Working Voltage ±25V
Conversion Rate 18V/μS

III LM1875 Circuit Principle

This circuit is composed of an attenuated tone control circuit controlled separately for high and low sounds, an LM1875 amplifier circuit, and a power supply circuit. Among them, the sound quality control part uses an attenuated tone circuit controlled separately for high and low sounds. The specific components are as follows:

  • R02, R03, C02, C01, W02 form a bass control circuit ;
  • C03, C04, W03 form a treble control circuit ;
  • R04 is the isolation resistance;
  • W01 is the volume controller, which adjusts the volume of the amplifier;
  • C05 is an isolation capacitor to prevent the downstream LM1875 DC potential element from affecting the previous tone circuit.

The amplifying circuit mainly adopts LM1875, which is composed of 1875, R08, R09, C066, etc. The magnification of the circuit is determined by the ratio of R08 to R09; C06 is used to stabilize the drift of the LM1875's 4th pin DC zero potential, but it has a certain impact on the sound quality; C07 and R10 function to prevent the amplifier from generating low-frequency self-excitation. In Addition, the load impedance of this amplifier is 4→16Ω.

lm1875 circuit

Figure 2. LM1875 Circuit

What about the power supply circuit of the power amplifier? Please take a look at the picture below. In order to ensure the sound quality of the power amplifier board, we need to pay attention to the following when designing the circuit:

  • The output power of the power transformer shall not be less than 80W;
  • The output voltage is 2*25V;
  • The filter capacitor uses two 4700UF/25V electrolytic capacitors in parallel;
  • Positive and negative power supplies share 4 4700UF/25V capacitors;
  • The two 104 base capacitors are high-frequency filter capacitors;

Figure 3. LM1875 Circuit

IV LM1875 Circuit Assembly and Debugging

This blog will introduce the necessary tools for circuit assembly, how to prepare for soldering, and finally how to debug the circuit. Of course, if you still feel like you are still cannot get enough, in the end, you can even follow our tips to try an extra interesting experiment with this circuit.

  • Tools That You Need
    • A 20W electric soldering iron, preferably with adjustable temperature;
    • A multimeter;
    • A pair of needle-nose pliers;
    • A screwdriver;
    • Some solder wire and pine perfume.
  • How to Prepare for Welding
    • Welding sequence:

① Welding jumpers;

② Welding resistance;

③ Welding capacitor;

④ Welded rectifier tube;

⑤ Welding potentiometer;

⑥ Weld LM1875.

  • Notes 
    • Fix the LM1875 with screws on the heat sink before welding LM1875, otherwise, the screws will be difficult to drive in when the heat sink is installed at the end;
    • The part of LM1875 in contact with the heat sink must be coated with a small amount of heat dissipation grease to facilitate heat dissipation;
    • Pay attention to the welding quality when welding. For beginners, you can practice a few more times on the old circuit board, and then formally solder.
  • How to Debug Correctly

The debugging of this power amplifier board is very simple. After the circuit board is soldered with electronic components, we must carefully check the circuit board for soldering errors. Special attention should be paid to electronic parts with polarity, such as electrolytic capacitors and bridge rectifiers. Once the welding is reversed, there is a risk of burning the components.

When the transformer is connected, the output terminal of the amplifier is not connected to the speaker, but connected to a multimeter (preferably digital display, and the multimeter is set to DC*2V).

In addition, pay attention to the reading of the multimeter when powering on the power amplifier board. Under normal circumstances, the reading should be within 30mV, otherwise we should immediately cut off the power to check the circuit board.

If the reading of the electric meter is within the normal range, it indicates that the function of the power amplifier board is basically normal. At this time, we connected the speaker, then input the music signal, and then power on the test machine. Under the correct procedures and specifications, turn the volume potentiometer, the volume should change, and turn the high and low knobs, the tone of the speaker will change.

lm1875Figure 4. LM1875

  • Experiment Worth Trying

First, we will short-circuit C6 and measure the DC potential at the output of LM1875 with a multimeter to see if it is within 30MV. Then, connect the speaker and test for two hours. Use a multimeter to measure the DC potential at the output of the LM1875 to see if the DC potential is within 30MV. If the DC potential is within 30MV, the capacitor C6 can be omitted. In this case, the amplifier board becomes a pure DC power amplifier.

V In the End

So far, the power amplifier board has been successfully installed and adjusted. Looking at this piece of work that you can be proud of and enjoying the wonderful music, are you satisfied?

FAQ

  • What package structure does LM1875 adopt?

TO-220

  • What is a LM1875?

A power amplifier integrated block chip.

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