In the previous tutorial, a Bass Boost power amplifier was designed. Now it's time to start designing power amplifiers suitable for specific applications. In this tutorial, a car audio amplifier will be designed.
Cars have come with built-in audio systems for years. A car's audio system is one of the important features that shines as a unique selling point (USP) for any car in the market. Audio systems have become a vital accessory for any car. Consumers often replace the standard audio systems that come in their cars with new ones for a better driving experience.
Any car audio system has the following basic components –
1) Main Unit
2) Amplifier
3) Speakers
The head unit is the heart of the car audio system. The head unit provides audio signals (to be reconstructed as sound) from cassette tapes, CD players, satellite radio, Internet radio, USB drive, or other audio source. Nowadays, head units in high-end cars also come with video capability . These head units have a display source connected to the head unit so that a video from a CD, DVD, or USB drive can be played on it. Even head units are today equipped with automated voice assist systems for the driver.
Audio signals from the main unit are weakened due to the resistive nature of the connecting wires. Therefore, a power amplifier is needed at the output stage of the head units to boost the signals so that they can reach the speakers without significant losses. Generally, due to car size restriction, amplifiers are only matched with the head unit. Therefore, there are amplifiers built into the main unit itself. You can also connect external amplifiers to the main unit to further enhance the sound quality.
With speakers, a car's audio system is complete. There are three or four speakers in a car placed in different positions inside. The speakers pick up the audio signals from the amplifier and reproduce the sound. A car requires a high power amplifier as the sound needs to be louder in it. Generally, 10 Watt amplifiers are standard and sound quite loud. For descending sound quality, a 6 Watt amplifier is sufficient.
In this tutorial, a 6 Watt car audio amplifier has been designed using IC TDA2003. TDA2003 is a 10 Watt car radio power amplifier. This IC can sustain a peak voltage of up to 40 V and has output current capability of up to 3.5 A. The IC can be operated by supplying a voltage between 8 V and 18 V. The audio amplifier designed in this tutorial is also a power amplifier. Based on the application, audio amplifiers can be categorized into two classes –
1) Preamplifier
2) Power amplifier
Preamplifiers are used to level audio signals from a microphone or audio source to standard voltage levels, while power amplifiers are generally used in the output stage of audio systems to boost audio signals. audio before being played through the speakers.
In the introductory article of this series, various design parameters of audio amplifier circuits were discussed such as gain, volume, slope rate, linearity, bandwidth, clipping effect, stability, efficiency, SNR, output power, THD and grounding. loop. This amplifier circuit will be designed considering the following design parameters –
Gain (voltage) – 34 dB
Bandwidth – 40 Hz to 15 KHz
Output power – 6 W
The amplifier will be designed to deliver audio to a 10 Watt speaker with 4 ohm impedance. The circuit will have the following additional features –
– No clipping effect
– Volume control
Circuit design will be followed by circuit testing to verify the intended design factors.
Required components –
Fig. 1: List of components required for 6 Watt car audio amplifier
Block diagram –
Fig. 2: Car Audio Amplifier Block Diagram
Circuit Connections –
The amplifier circuit is constructed by assembling the following components together –
1) DC Source – A 12 V battery is used to power the circuit. This DC source also supplies the bias voltage to the amplifier.
2) Audio Source – Audio input is provided by a smartphone. To receive audio from the smartphone, a 3.5 mm audio jack is connected to the phone. The 3.5mm audio jack has three wires – one for ground and two wires for the left and right channels. Since the amplifier is designed for single channel, only one of the channel wires will be connected to the amplifier as audio input. The connector's ground wire will be connected to the circuit's common ground.
Fig. 3: Typical 3.5mm audio jack image
3) TDA2003 – TDA2003 is a monolithic audio power amplifier, therefore all components of the amplifier circuit are integrated on the chip itself. This IC can sustain a peak voltage of up to 40 V and has output current capability of up to 3.5 A. The IC can be operated by supplying a voltage between 8 V and 18 V. The IC has the following pin configuration –
Figure 4: Table listing the pin configuration of the TDA2003 IC
The TDA2003 has the following pin diagram –
Figure 5: TDA2003 IC Pin Diagram
The TDA2003 has the following internal circuits –
Fig. 6: Internal Circuit Diagram of IC TDA2003
TDA2003 is basically a high power operational amplifier. Can be used as a power amplifier for car audio systems. The car audio amplifier designed in this tutorial using TDA2003 has a non-inverting configuration as the audio input is provided on the non-inverting input pin of the IC. This IC can produce a power of 6 Watts to 12 Watts. If you try to extract more power from the amplifier (beyond its specifications), the output voltage (amplitude of the audio signal) may begin to clip. The clipping effect can seriously damage the load which is the speaker in this case.
Fig. 7: Typical image of TDA2003 car audio amplifier IC
According to the TDA2003 technical sheet, the output power of this amplifier depends on the load impedance. The IC has the following power outputs at 40 dB for different output load impedances –
Fig. 8: Table showing the dependence of the TDA2003 output power on the Load Impedance
Therefore, the higher the load impedance, the lower the output power of the IC. In this amplifier design, a 10 Watt 4 ohm speaker is used, so the output power should ideally be 6 Watts. This IC can withstand a permanent short circuit of 16 V. The IC not only has AC and DC voltage short circuit protection but is also immune to polarity reversal having a surge current of up to 5 A. It has internal diodes for protection against open earth and load inductance.
The circuit diagram for this amplifier is a typical application circuit as given in the datasheet. The typical application has additional components for different purposes. In the circuit, a smoothing capacitor (shown as C1 in the circuit diagram) of 10 uF is connected to the non-inverting input pin on which the audio input is provided to block any DC components from the circuit input. Another capacitor (shown as C2 in the circuit diagram) of 470 uF is connected to the inverting input pin for ripple rejection. There are capacitors (shown as C3 and C4 in the circuit diagram) of 100 uF and 0.1 uF to filter the input supply voltage. The high-value capacitor C3 is connected to shunt high-frequency signals, while the low-value capacitor C4 is connected to shunt low-frequency signals.
An RC circuit (shown as C5 and R1 in the circuit diagram) is connected to the output pin of the IC to set the upper cutoff frequency. A capacitor (shown as C6 in the circuit diagram) of 1000 uF is connected to the output pin to block any DC components from the amplifier to the speakers. It also sets the lowest cutoff frequency. The higher the value of this capacitor, the lower the cutoff frequency and vice versa. The cutoff frequency is inversely proportional to the capacitor value.
Another RC circuit (shown as C7 and R4 in the circuit diagram) is connected to the output to stabilize the output frequency. 0.1 uF capacitor and a resistor of 1 ohm or less is recommended for this RC circuit for frequency stabilization. Using a resistor with a value greater than 1 ohm in this RC circuit may result in high frequency oscillations/distortions. Therefore, the recommended resistor and capacitor values are used in the circuit. All components are grounded in a star topology to avoid distortions in the audio signal due to loop grounding.
For volume control, a variable resistor (shown as RV1 in the circuit diagram) is used at the non-inverting pin input. This variable resistor can be adjusted to change the amplitude (voltage level) of this input signal which in turn controls the amplitude (volume) of the output signal.
4) Speakers – A speaker of 10 Watt power rating and 4 ohm impedance is used as load on the output of the amplifier. The speaker is connected to pin 4 of the IC which is the output pin of the TDA2003 and the speaker ground wire is connected to the common ground. 10 Watt speaker instead of 6 Watt is used as per availability.
Fig. 9: Typical image of a 10 Watt, 4 Ohm speaker
The following precautions should be taken while assembling the circuit –
1. Always place components as close together as possible to reduce noise in the circuit. Follow star topology when grounding, this will keep noise down.
2. Use high voltage rating capacitor instead of input signal.
3. Always use the filtering capacitor on the power supply input terminal to avoid unwanted ripple.
4. Use equivalent or high power speaker as amplifier output power.
5. Always use a capacitor in series across the amplifier output to block any DC components.
6. Always calculate the maximum power of the amplifier before connecting it to the speaker. The practical value may differ from the theoretical one.
7. Avoid clipping the output signal, as this may damage the speakers.
8. It is recommended to use a heatsink with the IC to provide cooling as the output power is high and this may heat up the IC.
Fig. 10: 6 Watt Car Audio Amplifier Prototype
How the circuit works –
The typical TDA2003 application circuit as indicated in its technical sheet is used for this amplifier. According to the datasheet, the open-loop gain of the IC when there is no output-to-input feedback is 80 dB, while the closed-loop gain when there is output-to-input feedback is 40 dB. The voltage gain of the amplifier can be set by the resistive divider network (shown as R2 and R3 in the circuit diagram) at the output pin of the IC. According to the datasheet, for a load of 4 ohms at the output, the IC provides power of 6 Watts. Therefore, for a load of 4 ohms and output power of 6 Watts, the maximum output voltage without clipping can be calculated as follows –
P = V2(pp)/2R
Where,
Output power, P = 6 W
Load resistance, R = 4 ohms
The peak to peak voltage at the output is then as follows –
P = V2( peak-peak )/2R
6 = V2( peak-peak )/(2*4)
Vp-p = 6.9 V
The maximum gain for a peak to peak voltage of 6.9V is as follows –
Gain = Vout/Vin
Considering Vin (input voltage) = 200 mV
Gain = 6.9/200
Gain = 34 (approx.)
To set the gain across resistors R2 and R3, the non-inverting OPAM gain equation for calculating gain can be used. The following is -
Gain = (R2/R3) + 1
Assuming R2 = 220 ohms
34 = (220/R3) + 1
R3 = 7 ohms (approx.)
Therefore, if R2 is assumed to be 220 ohms, R3 must be 7 ohms for the desired voltage gain.
In the circuit, capacitor C5 and resistor R1 are used to set the upper cutoff frequency. The recommended value for this capacitor is 39 nF for which a 33 nF capacitor is used depending on availability.
The value of resistor R1 can be calculated as follows –
R1 = 20*R3
R1 = 20*7
R1 = 140 ohms
The value of capacitor C5 can be calculated as follows –
C5 = 1/(2*Pi*B*R1)
Where, B is the frequency response. Each system responds differently to different frequencies. Some systems amplify the frequency of a certain band and attenuate the others. Therefore, the way any output of the system is related to the input signal at a different frequency is defined as Frequency Response of that system. In the datasheet of TDA2003, the following frequency response graph between capacitor value C5 and frequency response B is given below –
Fig. 11: Graph showing the frequency response curve of the TDA2003
Depending on availability, a 33 nF capacitor is used for C5 and a 150 ohm resistor is used for R1.
The output power of this amplifier should be 6 Watts for 4 ohm load.
Testing the circuit –
For testing the amplifier circuit, the function generator is used as the input source. The function generator is used to generate a sine wave of constant amplitude and frequency. Any audio signal is also basically a sine wave, so a function generator can be used instead of a microphone or actual audio source. Thus, the function generator can be used as an input source to test the audio amplifier circuit. During testing, also at the output, a speaker is not used as a load, as the speaker is resistive as well as inductive. At different frequencies, its inductance changes, which in turn changes the impedance (R and L combination) of the speaker. Therefore, using a speaker as a load on the amplifier output to derive its specifications may generate false or non-standard results. In place of the speaker, a purely resistive dummy load is used. Since the resistance does not change with frequency, it can be considered a reliable load regardless of the frequency of the input audio signal.
Ideally, the power of this amplifier should be 6 Watts. The load however had an impedance of 4.7 ohms instead of 4 ohms. So, in practice, the following observations were noted –
Fig. 12: Table listing the output characteristics of the 6 Watt car audio amplifier
So the amplifier output power was approximately 5.75 Watts instead of 6 Watts. This is a tolerable loss in power, so the amplifier circuit is working successfully. This amplifier circuit operates at a voltage of 12 V and has features such as Polarity Reversal, Thermal Shutdown and Short Circuit Protection.
In the next tutorial, an amplifier circuit for common headphones will be designed.
