40 watt Audio Amplifier
Amplifier Projects Electronics tutorial Opamp based project

Stereo Amplifier Circuit 20W + 20W

If you are looking for a first-class 40-watt music stereo amplifier, then look no further. High output efficiency, negligible hum, low harmonic distortion, clarity of the output sound, compact (modular) size, and low cost are some of the key features of this fully solid-state integrated Hi-Fi stereo amplifier which is capable of delivering an output of over twenty watts into each channel. The circuit uses a minimum of components and is built around just two power ICs. Upon adding just a simple preamplifier, the circuit can be converted into a 40-watt stereo cassette player.

Stereo Amplifier Circuit:

LM1875 Audio Amplifier

Working Of Amplifier:

The circuit is based on an LM1875 monolithic amplifier chip from National Semiconductors. The amplifier works in a way more or less similar to an op-amp. The 1 uF non-polar capacitors provide input coupling and the 2200 uF capacitors provide output coupling. The potentiometers RV1 and RV2 provide volume control.
The power supply consists of a conventional full-wave rectifier and filter. The LED provides an operation indication. Read working of operational amplifier to understand how an audio signal is amplified.
An array of psychedelic LEDs can also be added. Similarly, tone controls can also be introduced before volume controls.

Electronics Components List for Stereo Amplifier:

IC1, IC2 LM1875
D1-D4 1N4001 diodes
Resistors (all 1/2 watt, 5%)
R1-R8 18k
R9, R10 10k
RV1, RV2 100k with plastic shaft
R13, R14 2.2 ohms, 2 pieces for Zobel network
C1, C2 1uF (non-polar)
C3, C4 50uF (electrolytic)
C5, C6, C7 2mF or 2000uF
C8, C9 0.1uF ceramic
LS 4 ohms, 30 watts loudspeakers with cabinet, 2 pieces
Speaker cables


The circuit can be constructed on a PCB. The ICs should be bolted to the PCB and heat sink before they are soldered. The soldering should be done using a 40/60 type core. First, the resistors, then capacitors, and lastly the semiconductors should be soldered to the PCB. A 25-watt rod should be used for this purpose. The extra lead lengths should be cut off only after soldering is complete. The PCB should be cleaned with petrol and copper side painted over to prevent oxidation. The controls should be connected using shielded wires. Now recheck the entire wiring and if all is correct, switch on the unit, turn on the volume and watch your neighbors live up to its name!

Power Supply Circuit:

power supply unit 30V

Components List for Power Supply:

D1, D2 1N5408 diode
R1 1.5k (1/2 watt)
C1 2000uF (50v electrolytic)
X1 15 volts, 3.4-ampere transformer

psychedelic LEDs Circuit:

psychedelic LEDs

psychedelic LED Circuit

Components List for Psychedelic LEDs For Stereo Amplifier Board:

Q1, Q2 SL100 NPN Transistor
D1-D6 Red LEDs
D7, D8 1N4148
R1, R2 820 ohms
Hook up cables

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