LM1875 Audio Amplifier: Various Power Amplifier Circuits

I Introduction

LM1875 is a power amplifier integrated block. It has few peripheral circuits, large distortion-free power, and can work with both single and dual power supplies. It also has a safe working area protection (inductive load) for overload, overheating and reverse potential suppression in the circuit, suitable for high-grade audio circuits. And LM1875 is also suitable for audio amplification, servo amplification, bridge amplification, and power amplification in test systems.

This Vedio Take LM1875 as an Example to Explain the Bridged Audio Amplifier

Catalog

I Introduction

II DC Negative Feedback BTL Power Amplifier Circuit

III 20W Single-power Amplifier Circuit

IV LM1875 Power Amplifier Circuit with High and Low Adjustment

V Current Feedback Power Amplifier Circuit

Ordering & Quantity

II DC Negative Feedback BTL Power Amplifier Circuit

The LM1875 amplifier circuit is simple, where the sound is beautiful, and has the timbre of the amplifier. The power amplifier produced by it can output power up to 25W under positive and negative 25V voltage. In order to output more power, it can be connected to BTL circuit. The output power of the following circuit exceeds 60W (8 ohm speaker), which is designed as a current negative feedback circuit, and the sound is more beautiful.

In addition, the main recommendation of this board is to cancel C11 and C21 in Figure 1, and add a capacitor at the input (change C11 here). Change the circuit to a DC amplifier, the sound effect will be better.

　

　Figure 1. LM1875 Cuircuit

III 20W Single-power Amplifier Circuit

The circuit is shown in Figure 3. The basic working principle of the LM1875 single power supply and the dual power supply is the same. The difference is that when the single power supply is used, R1 and R2 are used for voltage division, while 1/2VCC is taken as the bias voltage and added to R3 to pin 1, making the output voltage change up and down based on 1/2VCC. Therefore, the maximum dynamic range can be obtained.

 

　　Figure 2. LM1875 Cuircuit

IV LM1875 Power Amplifier Circuit with High and Low Adjustment

　　Figure 3. LM1875 Cuircuit

The circuit above is composed of three parts:

• An attenuated tone control circuit controlled by high and low sounds
• An LM1875 amplifier circuit
• Apower supply circuit.

The tone part uses attenuating tone circuits controlled by high and low bass respectively, among which 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, adjusting the volume of the amplifier; C05 is the DC blocking capacitor, to prevent the LM1875 DC potential of the subsequent stage from affecting the tone circuit of the preceding stage.

The amplifying circuit adopts LM1875, R08, R09, C06. Among them, the magnification of the circuit is determined by the ratio of R08 and R09; and C06 is used to stabilize the DC zero potential drift of the 4th pin of LM1875, but it has a certain impact on the sound quality; C07, R10 is to prevent the amplifier from generating low-frequency self-excitation .

The load impedance of this amplifier is 4→16Ω.

Figure 4. LM1875 Cuircuit

As for the power supply, in order to ensure the sound quality, the output power of the power transformer should not be less than 80W, and the output voltage is 2*25V.

The filter capacitor uses two 2200μF/25V electrolytic capacitors in parallel, and the positive and negative power supplies share four 2200μF/25V capacitors.

The two 104 monolithic capacitors are high-frequency filter capacitors, which are beneficial to the sound quality of the amplifier.

V Current Feedback Power Amplifier Circuit

The current mode power amplifier has better sound quality. The current mode power amplifier circuit using the power amplifier integrated circuit LM1875 is shown in Figure 5.

In the figure, the resistor R3 connected in series with the speaker RL has a very small value. The current flowing through the speaker flows through R3, and the sampling feedback voltage generated at both ends is proportional to the output current. The voltage formed on R3 is sent to the feedback input of the amplifier through R2 to form a current negative feedback, which increases the output impedance of the amplifier and reduces the damping coefficient. The so-called Ruber network was cancelled in the circuit, and only C4 was used as the lead compensation.

Although the setting of R2 prevents the output feedback current from being fed back 100% to the negative input terminal, it prevents the harmful effects of large feedback on the input terminal; At the same time, the bias of the positive and negative input terminals is balanced, and the DC offset of the output terminal is eliminated.

Figure 5. LM1875 Cuircuit

First solder two small 0.22uF capacitors to the pin ③ and ⑤, and weld the other ends of them together as the ground terminal. Regarding the resistance to grounding, all are bent and welded at this point. The ±24V power cord and output cord are directly soldered to the relevant pins. Short-circuit the input terminal to ground, solder a 10Ω resistor to the RL speaker, and test the voltage of the output terminal to ground after power-on.

If it is less than 100 mV and there is basically no deviation after half an hour, you can disconnect the input short circuit and connect the speaker.

 

After reading the blog, have you better understand LM1875? If you are also interested in detailed introduction to LM1875 , you may wish to browse right here right now!

Finally, if you have any questions about LM1875, please do not hesitate to leave a message in the comment section below!