Sunday, January 22, 2012

LM1875 Hi-Fi 50-watt power amplifier

LM1875 50 watt Hi Fi power amplifier

 The LM1875 is a monolithic power amplifier offering very low distortion and high quality performance for consumer audio applications.

The LM1875 delivers 20 watts into a 4Ω or 8Ω load on ±25V supplies. Using an 8Ω load and ±30V supplies, over 30 watts of power may be delivered. The amplifier is designed to operate with a minimum of external components. Device overload protection consists of both internal current limit and thermal shutdown.
The LM1875 design takes advantage of advanced circuit techniques and processing to achieve extremely low distortion levels even at high output power levels. Other outstanding features include high gain, fast slew rate and a wide power bandwidth, large output voltage swing, high current capability, and a very wide supply range. The amplifier is internally compensated and stable for gains of 10 or greater.

This circuit diagram 50Watt Hi-Fi Power Amplifier based on LM1875 ICs and 2SA1943 – 2SC5200 bridge transistors. It has good input sensitivity, low distortion, good operating stability and full protection against overloads and output short-circuits. It can be used as a booster amplifier for existing small systems or to drive a second pair of speakers besides the ones already connected to the system. The board needs a symmetrical power supply of ±18V – 25V dc/4A and can be connected to 8 or 4 Ohm speakers. Large heat sink is required for this circuit. Diagram shown below indicates only the left channel. Make two circuits for for stereo version.
The output coil L1 using φlmm enameled wire wound resistor in R10 3W without being tightly wound 10 turns on. Power section using dual-18V 150W high-quality toroidal transformer, 35V/4700μF the filter capacitor 4, 35V/100μF two, 100V/0.1μF two, insurance management FUSE1, FUSE2 use 4A.

Technical Specifications:
  • Supply voltage = ±25Vdc/4A symmetrical (see text)
  • Current consumption = 3A maximum
  • Input impedance = 47K Ohms
  • Input sensitivity = 650 mV
  • Signal to noise ratio = 90 dB
  • Frequency response = 20 - 20,000 Hz ± 1 dB
  • Distortion = 0.5 % maximum
  • Load impedance = 4 - 8 ohm
R1 = 470R – 1/2w
R2 = 47k – 1/2w
R3 = 1,2K – 1/2w
R4 = 1R – 1,4R - 4w
R5 = 1R – 1,4R - 4w
R6 = 0.22R - 4w
R7 = 0.22R - 4w
R8 = 47k – 1/2w
R9=22R – 3w
R10=10R – 3w

C1 = 4,7uf-25V
C2 = 100uF-25V
C3 = 1000uF -62V
C4 = 1000uF -62V
C5 = 100nF-62V
C6 = 100nF-62V

L1 = (see text)

Q1 = 2SC5200
Q2 = 2SA1943

IC = LM1875


  • geo says:
    January 6, 2016 at 9:47 PM

    Place transistors Q1 to Q2
    Other way is not working
    With respect

  • Ramon Schepers says:
    September 7, 2016 at 8:40 AM

    The pin outs of the transistors is incorrect on the PCB. Please be aware of this when building it.

    It is: base, collector, emitter.
    Not: base, emitter, collector. (Which the PCB has been designed for)

    I am going to order a few (corrected) PCBs soon and test it once I gave time.

  • Ramon Schepers says:
    September 7, 2016 at 8:42 AM

    I also miss the 2 100nf capacitors near the lm1875 ic, this could add lots of high frequency noise!

  • Ramon Schepers says:
    September 7, 2016 at 8:43 AM

    These caps should be between the voltage rails to ground

  • Waqar Ahmed says:
    April 7, 2017 at 9:18 PM

    This amplifier work to increase output by adding another transistor Q1x2 and Q2x2 with extra voltage supply

  • Waqar Ahmed says:
    April 7, 2017 at 9:18 PM

    This amplifier work to increase output by adding another transistor Q1x2 and Q2x2 with extra voltage supply

  • Areeb Ahmed says:
    May 25, 2017 at 9:31 PM

    By using tip 147 &TIP142 WORKS WELL

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