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]

Component list:

Use this power supply circuit to handle the 25W HiFi audio amplifier.

Component list:

25W HiFi Audio Amplifier Circuit Notes:

• Can be directly connected to CD players, tuners and tape recorders. Simply add a 10K Log potentiometer (dual gang for stereo) and a switch to cope with the various sources you need.
• Q6 & Q7 must have a small U-shaped heatsink.
• Q8 & Q9 must be mounted on heatsink.
• Adjust R11 to set quiescent current at 100mA (best measured with an Avo-meter in series with Q8 Drain) with no input signal.
• A correct grounding is very important to eliminate hum and ground loops. Connect in the same point the ground sides of R1, R4, R9, C3 to C8. Connect C11 at output ground. Then connect separately the input and output grounds at power supply ground.

Technical data:

• Output power: well in excess of 25Watt RMS @ 8 Ohm (1KHz sinewave)
• Sensitivity: 200mV input for 25W output
• Frequency response: 30Hz to 20KHz -1dB
• Total harmonic distortion @ 1KHz: 0.1W 0.014% 1W 0.006% 10W 0.006% 20W 0.007% 25W 0.01%
• Total harmonic distortion @10KHz: 0.1W 0.024% 1W 0.016% 10W 0.02% 20W 0.045% 25W 0.07%
• Unconditionally stable on capacitive loads