25W Hi-Fi Audio Amplifier based LM1875

Here the circuit diagram of 25 watt Hi-Fi (high fidelity) audio amplifier that build based on single power amplifier IC LM1875 that should be run using heatsink attached on its.

25W Hi-Fi Audio Amplifier based LM1875 circuit diagram

Components List:

R1 _____________ 1K
R2 _____________ 1M
R3 _____________ 22K
R4 _____________ 10K
R5 _____________ 180K
R6 _____________ 1R

IC1 ____________ LM1875
C1 _____________ 1uF 50V
C2,6 ___________ 100nF
C3 _____________ 22uF 63V
C4 _____________ 0.22uF
C7,5 ___________ 220uF 50V     

Fuse holder & cover
2 amp fuses

Circuit Description:
The IC can be used with two power supply arrangements single supply and dual supply. We have decided to use the dual supply version. The single supply version would require the use of 63V electrolytic capacitors, thus increasing the cost and overall size of the module.

The input signal is coupled via R1 and C1 to the noninverting input (pin 1) of the IC. The closed loop gain of the amplifier is set to 19 by resistors R5 (180K) and R4 (10K). R4 and C3 (22uF) set the lower 3dB frequency point to 7Hz.

The output appears at pin 4 of the IC and drives a loudspeaker directly. Long speaker leads can produce enough capacitance to drive amplifiers into VHF oscillation. Resistor R6 (1W) and capacitor C4 (0.22uF) help prevent this. They are connected across the output and form a Zobel network which provides high-frequency stability when driving capacitive loads.

The IC power supply is connected via 2A fuses to protect against any external shorts to ground. Capacitors C5 and C7 provide further on-board supply decoupling.

Power Supply
The module is powered by a dual +/- 25V supply. These rails are supplied by a separate power supply board.

We have one kit available for this purpose – Kit 114. It consists of four diodes (1N5401) connected as a bridge rectifier and four electrolytic capacitors (2200uF) to smooth out the ripple.

The mains transformer used to power the module should be rated at 60VA with a 35V centre-tapped secondary winding. A 30V secondary winding transformer could also be used but the lower DC output would result in reduced power output.

If you want to run two modules in a stereo amplifier you can use a common power supply. In this case the transformer should be rated at 80VA

LM1875 Specs:

  • Short circuit protection
  • 94dB supply rejection ratio
  • Up to 30W output power into 8 ohms
  • Typical THD of 0.015% @ 1kHz, 20W output
  • In-built thermal protection
  • S/N ratio in excess of 100dB
  • Open loop gain typically 90dB
  • 70mA quiescent current (typical)

Download 25W Hi-Fi Audio Amplifier based LM1875 circuit kit manual:

Download LM1875 Datasheet:

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