Kit contains the essential components for a high quality amplifier kit using the LM1875.  Heatsink, speaker and cables are not included.  Up to 30 Watt output at 8 ohms. 

On-board 2 x 2A fuse protection.

With the addition of a handful of parts and a suitable power supply, this module will deliver 25W RMS into 8 ohms @ 1% THD.

PCB dimensions are a tiny 53 x 50mm (2.1" x 2").

Heatsink, power transformer, speaker and cables are not included. This is not only to save weight in the kit but also because such items are often a matter of personal preference by the user. Some users may choose to spend more money to buy top quality cables, connectors and speakers, while others may have all these items already in their junk box.

Mains power supplies vary from country to country, making the supply of a power transformer difficult.

The kit is constructed on single-sided printed circuit board. Protel Autotrax & Schematic were used in the design.

Short circuit protection

94dB supply rejection ratio

Built-in thermal protection

S/N ratio in excess of 100dB

Open loop gain typically 90dB

70mA quiescent current (typical)

The following items have to be supplied by you:

- a heatsink with a thermal resistance of 1.4

- Use heatsink compound & screw to attach the IC.

- Mains power transformer

- 8 ohm speaker

- Suitable cables. 

Solder the power and audio cables directly to the pads provided on the PCB.

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 non-inverting 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 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.

The module is powered by a dual +/- 25V supply. These rails are supplied by a separate power supply board. We have one available for use with this kit - K114. 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.

Before applying power, check that all parts are inserted in the correct locations. Make sure that the electrolytic capacitors are the right way round. Connect the power supply leads with a multimeter (set to read amps) in series with the positive rail. Do not connect a speaker or audio input yet. Switch on the power and check that the current settles down to 50-70mA. There will be a brief suge as the main filter capacitor (C7) charges.

Note: You must have the heatsink fitted, otherwise the thermal overload protection circuit may cut in and switch the device off. If the quiescent current is correct, check the DC offset volrage across the speaker terminals. It should be less than +/-  50mV. If all is well, switch off the power and connect a speaker and an audio input signal.

Maximum output power into 4W loads is 20W. It is recommended to use an 8W speaker. To minimize noise and distortion, keep the input signal leads away from the power supply leads.

Check out these related products:
Dual Unregulated Power Supply Kit