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     

Miscellaneous:
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:
[wpdm_file id=37]
Download LM1875 Datasheet:
[wpdm_file id=38]

This is 20 watt audio amplifier circuit design which use IC LM1875 as main component. This circuit is simple and very easy to built. For stereo channel, you need to build the two same circuits which will amplify the both of audio channels (right and left channel). Dual polarity / symmetrical power supply required for this kind of 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 4 ohm or 8 ohm load on 25V supplies. By using an 8 ohm 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.

Download LM1875 datasheet for 20 Watt audio amplifier reference project:

LM1875 Datasheet

1 file(s) 12.00 KB

Download Datasheet