Using a TIP35C-based 12V audio power amplifier to drive a speaker

Speakers are heavy loads, they usually need to be driven by high currents provided by an external circuit. This is because sometimes the sound output produced, such as from a microphone or guitar pickup coil, does not produce a high current high amplitude output and is therefore not suitable for driving a speaker. This is why we have something called an audio amplifier.

The difference between buffer amplifier, preamplifier and power amplifier:

The buffer amplifier produces the same signal of exactly the same amplitude from the weak sound source whereas the preamplifier amplifies the signal to a much higher voltage from the input source. The output of the preamplifier is further submitted to the power amplifier. The power amplifier provides current to the load as per the input signal amplitude. Hence, a power amplifier is an electronic device that provides the required power (voltage x current) to the speaker.

In this project, we will use a simple and low-cost power amplifier to drive the speaker. For the power amplifier circuit, we will use TIP35C power transistor.

Required Components

For this audio power amplifier project, the following components are required −

TIP35C power transistor.

Heat sink for TIP35C.

1k resistor.

470uF 25V capacitor.

Audio line-in jack (depending on desired input source connector).

Breadboard.

12V power supply unit

trumpet

Class A amplifier circuit diagram using TIP35C

The circuit diagram of the TIP35C audio power amplifier is shown below.

Working principle of TIP35C audio amplifier

The transistor acts as an amplifier by amplifying the input signal. If a DC bias voltage is applied across the emitter-base junction of the transistor, the transistor will remain in a forward biased state and will remain in this state regardless of the signal polarity. This is a Class A amplifier. Therefore the transistor is always biased in the on state. Therefore, the transistor produces minimum distortion at maximum amplitude of the output signal during a complete cycle of the input signal.

Since the Class A amplifier needs to drive a large amount of load current, the transistor rating must be sufficient to compensate for the high collector current. The load, i.e. the speaker is connected across the collector. Therefore, the transistor must have a high collector current. The TIP35C successfully achieves this as it is a 100V power transistor as well as a collector current of 25A. However, the major drawback of the above circuit is the overall efficiency of the power amplifier. Since this circuit is the basic structure of a Class A amplifier, there is almost a large amount of current lost as heat dissipation of the power transistor TIP35C. A large heat sink must be connected to accommodate the heat dissipation. The conversion efficiency of the circuit is low.

The detailed pin diagram of TIP35C is shown in the figure below

Resistor R1 acts as a base resistor, it provides enough base current to drive the transistor at the saturation point. The 470uF capacitor C1 is an important part of the circuit. This is because the capacitor serves two purposes. Firstly, the capacitor isolates the base from the input power supply so that the base voltage or current does not affect the audio source, the other purpose is to act as a DC blocking capacitor for the input source. Capacitors block DC and only pass AC. The 470uF capacitor effectively provides this, it only allows AC frequencies to pass.

The positive terminal of the power supply is in series with the speaker. The transistor powers the speaker through GND. Therefore, a small change in the base can manipulate the load, which is the speaker.

Testing 12V Power Audio Amplifiers

This circuit is built on a breadboard. My breadboard setup is shown below. As you can see, this circuit requires very few external components, making it easy to build.

The circuit was tested using a 9 watt speaker as shown below

Choosing the right speaker is important for any power amplifier. A poorly performing speaker can ruin a well-constructed amplifier. So for anyone who is building a board for an audio-related application that features a speaker as the main feature, make sure you have a good functioning speaker. For the testing of this amplifier circuit, the above speaker was used. This speaker is over 60 years old and was harvested from an old tube amplifier. However, this speaker was rebuilt by me about three years ago. It is a 4 ohm speaker that provides nearly 9 watts of power output and the diameter of the speaker is 6 inches.

Next is the audio input. The audio input is provided by the mobile phone. Since the mobile phone already has a built-in preamplifier, it can be considered that the basic preamplifier is used before the power amplifier during the test phase. The circuit works very well and the output performance is also very good. The full test video can be found at the bottom of this page.

in conclusion

It is a basic type of Class A power amplifier circuit, with a 12V input, and uses a minimum of components, only three. However, it is not as good as conventional power amplifiers on the market. Further improvements can be made, and the overall performance can be improved.