LM1875 Datasheet – 25W HIFi audio amplifier circuit

Here is an LM1875 datasheet, used for a 20W audio amplifier on Class AB type. Voltage Gain is 90dB. Low Harmonic Distortion Noise is 0.015%, 1KHz, 8 ohms. and Wide supply range 16V-60V at 4A. Protection for short circuits to ground and thermal shutdown.

General Description

The LM1875 is used on a wide power amplifiers very low distortion and high-quality efficiency for audio applications. It provides power is 20 watts into a 4 ohms or 8 ohms load on 25V supplies.

Its amplifiers circuits are designed to working with a minimum of external parts.

When overload it has protection consists of both internal current limit and thermal shutdown.

Other features are high gain, fast slew rate and a wide bandwidth, large output voltage swing, high current capability, and a very wide supply range. See below.


In Figure 1 as Connection Diagram and Typical Applications of LM1875

The amplifier is internally compensated and stable for gains of 10 or greater.

Specification Value
– Up to 30 watts/channel output power
– Maximum Load Resistance: 4 to 8 Ohm
– Maximum Input Bias Current: [email protected]±25V
– Maximum Input Offset Voltage: [email protected]±25V
– Maximum Working Temp: 150°C
– Avo typically 90 dB.
– Low harmonic Distortion Noise : 0.015% @ 1KHz @ 8 ohms @ 20 watts.
– Wide power bandwidth: 70KHz
– High current capability: 4A.
– Maximum Supply Current: [email protected]±25V
– Wide supply range: 16V-60V

The Power supply type are single and dual, there is detail as follow.

-Minimum Single Supply Voltage: 16V
-Typical Single Supply Voltage: 18V|24V|28V
-Maximum Single Supply Voltage: 60V
-Minimum Dual Supply Voltage: ±8V
-Typical Dual Supply Voltage: ±9V|±12V|±15V|±18V|±24V|±28V
-Maximum Dual Supply Voltage: ±30V
– 94dB ripple rejection
– Typical Gain Bandwidth Product 5.5MHz
– Typical Supply Current: 70mA at ±25V
– Protection for AC and DC short circuits to ground
– Thermal protection with parole circuit.
– Internal output protection diodes.
– Plastic power package: TO-220

We use it’s on the High-performance audio systems, Bridge amplifiers, Stereo phonographs, Servo amplifiers, Instrument systems and more.

LM1875 Hi-Fi Audio Amplifier

This circuit is the audio amplifier circuit has a very popular one. This is a simple circuit. Small pieces of equipment. And Watt is high. The circuit uses IC number LM1875, which is under the protection circuit IC output or Too short circuit. And a significant distortion as low as 0.015% at a frequency of 1kHz.

25W HI-FI power amplifier using LM1875

This circuit uses positive, negative ground power supply to get the audio signal with both halves of the signal swing, positive and negative halves, The sound that came out crystal clear.Work is. When paying for the audio input. Audio is through R1, R2, C1, and R3 to limit the audio signal appropriately.And limit the noise to the input signal mixed down to the ground.Then sends a signal to noise to the input pin of IC 1.A noninverting pin, amplifier non-return phase.Out of the 4 output pins to access the speakers.The R6 and C4 eliminate noise mixed with the output down to the ground.And another part of the audio output pin 4 of the integrated circuit will be fed back into place through R5 to pin 2, which R4 and R5 determine the rate of boost up, can be calculated from R5/R4, will gain equal to 15 times, With a C2 to meet high frequencies better.

LM1875 amplifier circuit 30 watts

If you need to have a high power amplifier in small size. I suggest this project, It has powered drive output up 30 watts, by low noise and Low distortion. And easy to build because use a few devices. But is good sound energy worth the cheaper.

The circuit principle.

The LM1875T is Amplifier IC model of National Semiconductor.

  • A body on Supplier Package: TO-220, Pin Count to use is 5 pin.
  • This IC is built by High technology monolithic type makes very low Total Harmonic Distortion Noise less than 1% (0.015%@[email protected]) the quality of sound.

Figure 1 Tiny 30 watts OTL amplifier using LM1875

They can use with DC power supply wide range between 16 volts to 60 volts and can use both dual and single voltage. In this project use, the single voltage supply is Positive and Ground. As Figure 1

The voltage of power supply will have effect to power of IC that is if we enter DC voltage supply of 50 volts, the IC has power up to 25 watts. When uses with impedance load between 4 ohms to 8 ohms.

But we use the maximum signal supply voltage up to 60 volts, and power output up to 30 watts. Which impedance load 8 ohms.

This IC model IC have the protection system with the Maximum Operating Temperature in a range -65 to 150°C, So widly use, but if you use fan to IC heatsink to reduce heat of them so very well.

In circuit diagram form in signal characteristics is non inverting input signals. in to the input. There are R4-resistor adjust input impedance and coupling through the C1-capacitor in to amplifier section inside the IC1.

Thus R1 and C1 will set the frequency of input signal, by R1, C2 filter the noise (D-coupling), from power supply to protect noise through the input.

Keep reading: ‘ 30W Amp OTL by TDA2030 transistor’ »

Both R5, R6-resistors are connected in feedback form, and act as adjusting gain of circuit. But R7 and C5 are used to bypass or through high noise frequency to ground. To To ensure the quality of sound output improves. And there is C6 coupling signal out to the output ( Load) The value that use to good sound quality even more. and also protect DC voltage leak to load.

Figure 2 the graph shows various features of the LM1875

Figure 2 is graph to show the comparin various feture of LM1875T-IC as table 2 (A) show compere between Distortion percent of signal (THD) and power watts at output. Which IC drive over 20 watts. The resistor load of 8 ohms. To the harmonic distortion less than load 4 ohms.

Table 2 (B) show compare between the values of harmonic distortion and frequency of signal at output. By in range frequency about 80 Hz up to about 10 KHz Harmonic distortion in the stable and have lower average between both load about 0.025%.

In table 2 (C) show compere output power and voltage of power supply at load 8 ohms. Which if we use voltage up to 30 volts will be watts power output about 32 watts.

This amplifier use high voltage power supply and highe current up to 3-4 Amperes. This helps improve the stability of the circuit. Resulting in better sound quality as well.

Buy components in this projects HERE!

How to build LM1875 project

The amplifier circuit as Figure 1 that is single side or mono system only. Thus if build on stereo system you have to builds two sets.

To begin with find all components before, so make an Actual-size,Single-sided copper PCB layout as Figure 3. Then assemble all parts to PCB as Figure 4 so easy.
Figure 3 Actual-size,Single-sided copper PCB layout

Figure 4 the components layout.

For who not have DC power supply voltage : 50-60 volts. Current around 3-4 amperes. To try circuit in Figure 5


Testing and aplication

First, apply good signal (CD, tuner) to input, If have the tone control and low volume with it better,Connected input signal is applied to the project.and speaker of 4-8 ohms sized about 30 watts to output.

The components list
Resistors size : 0.5 watts +5%
R7__________1 ohm s
C1__________1uF 50V___Electrolytic
C2, C3_______10uF 50V___ Electrolytic
C4__________100uF 50V___ Electrolytic
C5__________0.22uF 50V___Mylar
C6__________2,200uF 50V___Electrolytic
C7__________0.1uF 50V___Ceramic
The speaker size of 4-8 ohms 30W
PCB, terminal,wires, etc.

Keep reading: ‘ 60W RMS OTL audio amplifier’ »

This Post Has 4 Comments

  1. I was worried about the C4’s polatity…. I guess C4 in the schematics should be reversed. Correct me if i am wrong 🙂

    1. Hello, Lone Ranger
      Oops! I am very very sorry. I am so sad.
      You are right. I mistake.
      You are good. But I am too late. to check it.

      Thank you so much.

  2. lone ranger, you are right, + of c4 should be in the common terminal of the circuit since it’s a dual supply…

    1. Hi Princess,
      Yes, I mistake.
      Thank you so much.
      I hope you visit me again.

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