The LM1875 is a Class AB audio power amplifier IC designed to increase a low-level audio signal so it can drive a speaker. The device can deliver about 20 W of output power and may reach higher levels when the power supply, speaker load, and cooling system are properly designed. It supports 4 Ω and 8 Ω speakers and includes useful protection features such as thermal shutdown, short-circuit protection, and internal output protection diodes. This article explains the LM1875 pinout, circuit operation, specifications, real output power, and more.

Catalog

LM1875 Features and Specifications

LM1875 Pinout and Pin Functions

LM1875 Packaging Information

LM1875 Amplifier Circuit

This diagram shows a single-channel <a href="https://www.y-ic.com/pdf/TI/LM1875-IC.html" target="_blank" "="" style="cursor: pointer; color: rgb(0, 0, 238); font-weight: bold;">LM1875 Class AB audio power amplifier operating from a dual power supply, marked as +VCC and −VEE. The audio signal enters through VIN, is amplified by the LM1875, and then drives a 4 Ω to 8 Ω speaker from pin 4. Because the circuit uses positive and negative supply rails, the output can remain close to 0 V when no audio signal is present. This allows the speaker to connect directly to the amplifier output without a large output coupling capacitor.

The input audio signal first passes through C1, a 2.2 µF coupling capacitor. C1 blocks any DC voltage coming from the audio source while allowing the AC audio signal to enter the amplifier. R1, rated at 1 MΩ, gives C1 a discharge path and helps prevent unwanted popping when the source is connected or disconnected. R2, rated at 22 kΩ, keeps the non-inverting input at ground potential and sets the approximate input impedance of the amplifier. The audio signal then enters pin 1, which is the non-inverting input of the LM1875.

The LM1875 amplifies the difference between the signals at its non-inverting and inverting inputs. The feedback network formed by R4, R3, and C2 controls the amplifier gain. R4 is a 20 kΩ feedback resistor connected from the output to pin 2, while R3 is a 1 kΩ resistor connected from pin 2 toward ground through C2. At normal audio frequencies, the closed-loop voltage gain is approximately:

Using the values shown:

This means the output voltage is approximately 21 times greater than the input voltage, equal to about 26.4 dB of voltage gain. For example, an input signal of 0.5 V RMS could theoretically produce about 10.5 V RMS at the output before clipping, provided the power supply and speaker load can support it.

C2, rated at 22 µF, works with R3 to reduce the amplifier gain at very low frequencies and DC. This prevents the circuit from amplifying DC offset and unnecessary subsonic signals. The approximate low-frequency cutoff formed by R3 and C2 is about 7 Hz, which is below the normal audible range. Therefore, the circuit can reproduce normal bass frequencies while maintaining stable DC operation.

Pins 5 and 3 provide power to the LM1875. Pin 5 connects to +VCC, while pin 3 connects to −VEE. Capacitors C3 and C4, both 0.1 µF, provide high-frequency supply decoupling. They remove fast voltage spikes and help prevent oscillation. Capacitors C7 and C6, both 100 µF, provide additional energy storage and smooth lower-frequency supply variations. These capacitors should be installed close to the LM1875 power pins, with short and wide PCB connections.

The amplified audio signal leaves the IC through pin 4 and goes directly to the speaker. The speaker should have an impedance of 4 Ω or 8 Ω, but an 8 Ω load is generally easier for the LM1875 to drive because it requires less current and produces less heat. A 4 Ω speaker places a heavier load on the amplifier, so the power supply and heatsink must be designed carefully.

R5 and C5 form a Zobel or output-stability network. R5 is a 1 Ω resistor, and C5 is a 0.22 µF capacitor. This network provides a controlled high-frequency load at the amplifier output. It helps prevent instability caused by the speaker cable, the loudspeaker voice coil, or other reactive loads. R5 should be a suitable non-inductive resistor, and both parts should be placed close to pin 4 and the output ground connection.

How the LM1875 Amplifier Works

When an audio signal enters pin 1, the LM1875 compares it with the feedback signal at pin 2. The IC then adjusts its output until the difference between the two inputs becomes very small. This negative-feedback process controls the gain, lowers distortion, improves frequency response, and keeps the amplified output close to the shape of the original audio signal.

The internal schematic shows the main stages built inside the IC. The input section receives the audio and feedback signals, while the voltage-gain stages increase the signal level. The output section then supplies enough current through pin 4 to drive the connected speaker. The internal circuit also includes current limiting, protection diodes, short-circuit protection, and thermal shutdown to improve reliability during overload or overheating.

The LM1875 uses a Class AB output stage. Its internal output transistors share the positive and negative halves of the audio waveform, which provides a good balance between efficiency and low distortion. However, some electrical power is still converted into heat, especially at higher output levels. For this reason, the LM1875 must be mounted on a suitable heatsink to prevent thermal shutdown and maintain stable performance.

LM1875 Output Power in Real Conditions

The LM1875 is commonly rated as a 20 W amplifier, but the actual output depends on the supply voltage, speaker impedance, heatsink, and power supply quality. With an 8 Ω speaker, about 16–20 W is practical from a ±22 V to ±25 V supply. A 4 Ω speaker draws more current and creates more heat, so strong cooling is required.

Output power can be calculated using:

For reliable operation, an 8 Ω speaker with ±18 V to ±22 V provides a good balance of power, heat, and sound quality.

Choosing the LM1875 Power Supply

The LM1875 usually works best with a dual-rail DC supply. For an 8 Ω speaker, ±18 V to ±22 V provides a good balance of output power, heat, and reliability. A ±25 V supply can produce more power but requires a larger heatsink.

Use a well-filtered power supply with a bridge rectifier and large filter capacitors. A 15-0-15 VAC transformer typically produces about ±20 V DC, while an 18-0-18 VAC transformer can produce around ±25 V DC after rectification. Actual voltage depends on the transformer load and mains voltage.

For stereo operation, choose a transformer rated around 100 VA or higher, depending on the required output. Place 100 µF and 0.1 µF decoupling capacitors close to each LM1875 supply pin to reduce noise and prevent oscillation.

LM1875 Heatsink and Thermal Design

The LM1875 is a Class AB amplifier, so part of the input power becomes heat. A properly sized heatsink is required, especially when using high supply voltages or a 4 Ω speaker. Apply thermal compound between the IC and heatsink to improve heat transfer. If the metal tab must be electrically isolated, use an insulating pad and shoulder washer. Poor cooling can cause distortion, thermal shutdown, or permanent damage.

For most 20 W builds, use a substantial heatsink with good airflow rather than a small clip-on type. Stereo designs need more cooling because two amplifier ICs generate heat inside the same enclosure.

LM1875 PCB Layout and Grounding

Good PCB layout is important for low noise and stable operation. Keep the input traces short and away from the transformer, rectifier, speaker output, and high-current power tracks. Place the supply bypass capacitors close to pins 3 and 5, and keep the feedback components close to pins 2 and 4.

Use a star-ground arrangement so the input ground, speaker ground, and power-supply ground meet at one main point. Do not allow speaker current to flow through the sensitive input ground path. Poor grounding can cause hum, noise, feedback problems, and high-frequency oscillation.\

LM1875 vs Other Amplifiers

LM1875 vs TDA2030 and TDA2050

The TDA2030 is designed for lower-power and lower-cost amplifiers, while the LM1875 generally provides better output power and lower distortion. The TDA2050 offers higher potential output than the LM1875 but usually requires a stronger power supply and larger heatsink. These ICs are not always direct replacements because their supply limits, pin connections, and circuit requirements may differ.

LM1875 vs LM3886

The LM3886 is a much higher-power amplifier intended for larger speakers and louder audio systems. It can provide considerably more output than the LM1875 but needs a larger transformer, stronger power supply, larger heatsink, and higher project cost. The LM1875 is better for compact 15–25 W amplifiers, while the LM3886 suits higher-power home audio systems.

LM1875 vs Class-D Amplifiers

The LM1875 uses Class AB technology, which is simple and produces low distortion but generates considerable heat. Class-D amplifiers are more efficient, smaller, and require less cooling, making them better for portable and compact systems. However, the LM1875 is often easier to understand, repair, and build for traditional analog audio projects.

Best Applications for the LM1875

• Home audio amplifiers

• Bookshelf speaker amplifiers

• DIY stereo amplifiers

• Powered speaker systems

• Guitar practice amplifiers

• Small public address systems

• Active speaker crossovers

• Midrange and tweeter amplifiers

• Audio repair projects

• Electronics training projects

How to Choose an LM1875 Module

Choose an LM1875 module that matches your power supply and speaker impedance. For an 8 Ω speaker, a module designed for about ±18 V to ±25 V DC is usually suitable. For a 4 Ω speaker, use a lower supply voltage and make sure the board, wiring, and heatsink can handle the higher current.

Check the quality of the PCB before buying. A good module should have short feedback paths, wide output and power traces, local supply-decoupling capacitors near the IC, and a proper ground layout. Poorly designed boards may produce hum, noise, overheating, or high-frequency oscillation even when the components appear correct.

The module should also include enough space for a large heatsink or allow the LM1875 to be mounted directly to the enclosure. Avoid boards that use only a small clip-on heatsink for high-power operation. Also confirm whether the metal tab of the IC needs electrical insulation from the heatsink.

Review the component values and circuit diagram when available. The amplifier gain should normally remain at 10 or higher, and the board should include the input coupling capacitor, feedback network, supply bypass capacitors, and output Zobel network. Avoid modules with unrealistic power claims, because actual output depends on the supply voltage, speaker load, cooling, and distortion level.

Mechanical Dimensions

Manufacturer

Texas Instruments manufactures the LM1875 as a monolithic analog power-amplifier IC, integrating its input, gain, protection, and high-current output stages on a single semiconductor die. TI’s analog manufacturing capabilities include wafer fabrication, chip assembly, packaging, electrical testing, and quality control across a global network of manufacturing sites. After wafer processing, individual dies are separated, assembled into the LM1875’s five-pin TO-220 power package, and tested to verify electrical performance and protection functions. TI’s internal manufacturing and testing infrastructure supports consistent device quality, reliable high-volume production, and traceability, while the LM1875 design uses established analog processing techniques to provide low distortion, high current capability, thermal shutdown, and short-circuit protection.