This article is about a simple transistor power amplifier.

Figure 1: Device.

You can see how my circuit works in this video:

Step 1: Design the Circuit

I have drawn the circuit via PSpice software:

Figure 2: Design the Circuit.

The optional capacitor low pass filter circuit is useful when the amplifier is connected to mains powered 8 V power supply. The two 10 mF capacitors are needed to filter the possible 60 Hz or 50 Hz ripple. You do not need this circuit if the amplifier is battery-powered.

I have chosen two 10,000 uF capacitors for low pass frequency of:

Flp1 = 1 / (2 * pi * (2 * Cs1a) * (Rs1a / 2))

= 1 / (2 * pi * (2 * 10 mF) * (4.7 ohms / 2))

= 3.3863 Hz

The maximum current across the Cs1a and Cs1b capacitors is when the voltage across the capacitors is zero:

IcsMax = Vs / (Rs1a / 2) / 2

(divide by 2 because there are two capacitors)

The maximum voltage is 12 V.

= 12 V / (4.7 ohms / 2) / 2

= 2.5532 Amps

The maximum power is at half supply voltage:

PcsMax = VcsHalfSupply * IcsHalfSupply / 2

(divide by 2 because there are two capacitors)

= (Vs / 2) * ((Vs / 2) / (Rs1a / 2)) / 2

= 7.6596 Watts

Calculate the low pass frequency of the second power supply low pass filter:

Flp2 = 1 / (2 * pi * (3 * Cs2a) * (Rs2a / 2))

= 1 / (2 * pi * (3 * 470 uF) * (100 ohms / 2))

= 2.2575 Hz

The average current across the LEDs equals to:

Iled = (Vs - 2 * Vled) / (Rd1 + Rd2)

 = (12 V - 2 * 2 V) / (470 ohms + 470 ohms)

= 8 V / 940

= 8.5106 mA

The maximum current across the LED is equal to:

Iled = (Vs - Vled - Vbe) / Rd1

= (12 V - 2 V - 0.7 V) / 470 ohms

= 19.7872 mA

The 1 Meg potentiometer allows a clipping sound effect.

Step 2: Simulations

Time Domain:

Figure 3: Simulations Transient.

Frequency:

Figure 4: Simulations Frequency.

Step 3: Make the Circuit

I made the circuit on a small matrix board:

Figure 5: Make the Circuit.

Step 4: Encasement

I used a cheap $2 gift box to save money:

Figure 6: Encasement.

I used big 10 mF capacitors and 10-watt 4.7 ohms resistors because I connected the circuit to a 12 V battery.

Step 5: Testing

I used a Hantek 6022BE USB Oscilloscope to test my circuit.

The yellow plot is the input and the green plot is the output.

Figure 7: Testing 100 Hz Sine Wave Input.

Figure 8: Testing 1 kHz Sine Wave Input.

Figure 9: Testing 10 kHz Sine Wave Input.

Figure 10: Testing 20 kHz Sine Wave Input.

Figure 11: Testing Voice Input.

Talking:

Conclusion

The circuit can be produced without a 1 Megohm potentiometer and without the LEDs if you bias the output transistor emitters at half supply voltage.