STK435 based Stereo Amplifier Circuit and PCB Design

This stereo AF amplifier uses the STK435 IC, a highly popular choice due to its simplicity. The STK435, with its 15-pin configuration, requires only a few external components, such as electrolytic and ceramic capacitors and resistors.

With a 12V DC power supply, the amplifier delivers approximately 7 watts to an 8-ohm speaker, offering excellent audio quality. The author’s prototype is shown in Fig. 1.

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Stereo AF Amplifier – Circuit and Working

The circuit diagram of the STK435-based stereo amplifier is shown in Fig. 2. The design includes the STK IC, resistors, and capacitors, with two audio inputs and corresponding amplified outputs for the right and left channels.

Start by completing the connections for the first amplifier section of IC1, referred to as IC1A. Pin 1 connects to two resistors (R2 and R3) in series, followed by a 100-ohm resistor (R5) leading to the +12V supply. A 10µF, 25V electrolytic capacitor (C3) is connected between the junction of R2 and R3 and the ground. A 0.47µF, 25V electrolytic capacitor (C2) connects to pin 1 of IC1, with its negative terminal linked to a 1-kilo-ohm resistor (R1). Additionally, a 470pF ceramic capacitor (C1) connects the negative terminal of C2 to ground.

Pin 2 connects to the ground through an electrolytic capacitor of 220µF, 25V (C4) in series with a 120-ohm resistor (R6). Pins 3, 4, and 8 are also grounded. The output from pin 5 drives an 8-ohm, 7-watt speaker (LS1) via a 1000µF, 25V electrolytic capacitor (C6). The negative polarity of C6 connects to ground through a series arrangement of a 0.1µF capacitor (C14) and a 4.7-ohm resistor (R7).

Pin 6 connects to pin 5 through a 47µF, 25V electrolytic capacitor (C5). Pin 7 is connected to the +12V supply, while pin 9 connects to +12V through a 100-ohm resistor (R5). A 12-kilo-ohm resistor (R8) bridges pins 2 and 5. A 100µF, 25V electrolytic capacitor (C15) connects pin 9 to ground.

The second amplifier section, IC1B, uses pins 10 through 15 and is connected as shown in Fig. 2.

This circuit operates simply: connect audio inputs (from devices such as laptops, desktops, or MP3 players) to CON1 and CON3, then connect a 12V power supply. The amplified output will be heard through the speakers.

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Construction and Testing

An actual-size single-sided PCB layout for the STK435 amplifier is shown in Fig. 3, with the component layout in Fig. 4. After assembling the circuit on the PCB, enclose it in a suitable cabinet, ensuring proper heat dissipation for the IC with an appropriate heatsink.

Alternatively, a 7cm x 9cm Veroboard can be used. Position a 15-pin female connector in the middle of the board, two 2-pin connectors on the far right and left sides, and a 4-pin connector at the bottom. Solder all components as indicated in the circuit diagram, using a low-watt soldering iron. The IC should be inserted after soldering. Connect two 8-ohm, 7-watt speakers to the 2-pin connectors, and supply power using a 12V battery. A clicking sound from the speakers may indicate proper connection.

After assembling on either the PCB or a Veroboard, the device is ready for testing. Gently touch a metal screwdriver to the 2-pin connectors to hear a humming sound from the speakers, confirming functionality. Once tested, connect an MP3 player, laptop, or desktop to the amplifier’s input and enjoy the audio output.

Raj K. Gorkhali is an electronics hobbyist and a regular contributor to EFY