# Car Radio Booster Schematic Circuit Diagram

One solution for increasing the power of an amplifier running on a low-voltage supply, like a car radio powered from at best 14 V, is to use a ‘bridge’ configuration, i.e. to connect the loudspeakers between the outputs of two identical amplifiers whose inputs receive the same signals, but in opposite phases. This doubles the apparent voltage applied to the loudspeaker, which in theory quadruples the maximum power available. In practice, because of the various losses in the power transistors, we can only triple it. The peak-to-peak voltage applied to the loudspeakers in the car radio example is 28 V, less the losses in the power transistors, i.e. around 24 V. So we have an RMS voltage of around 8.5 V (24 V / 2√2), which gives an RMS power — the  only one we hear — of 18 watts (8.5 V² / 4 Ω).

#### Impressive Power Output with Innovative Voltage Boosting

This booster stands out by delivering remarkable performance, capable of providing up to 55 watts rms into 4 Ω with distortion levels below 0.5%. Moreover, it can achieve 70 watts rms at the cost of slightly higher distortion (10%). This achievement is not a violation of physical laws but is attained through an ingenious method of boosting the supply voltage. The design incorporates integrated power switches and high-value electrolytic capacitors for this purpose.

#### Single IC Amplification and Voltage Enhancement

The circuit utilizes a single IC per channel, namely the TDA1562Q from NXP. This IC handles both power amplification in class H and the crucial task of voltage boosting. Specifically crafted for integration ‘behind’ a car radio, this circuit lacks a volume control. Its high-impedance input allows seamless connection to either the car radio’s loudspeaker output or, preferably, the line output present in modern car radios.

#### Ingenious Voltage Boosting and Control Mechanisms

Capacitors C3 and C6 play a pivotal role in voltage boosting. These capacitors are alternately charged to the circuit’s supply voltage through the TDA1562Q’s integrated electronic power switches. Once charged, they are placed in series with the supply, effectively doubling it to power the output stages. To handle the substantial currents during this process, effective decoupling is ensured with the use of capacitor C2.

#### Diagnostic and Mute Control Features

Transistor T1 interfaces with a ‘diagnostic’ LED, providing feedback from IC1 pin 8. This LED remains off during normal operation but flashes when the IC detects output distortion (clipping) above 10%. It stays steadily lit if the IC identifies a short-circuited output, lack of an output load, or when thermal protection activates. Additionally, a mute control puts the circuit into standby mode when grounded, ceasing output signal production and minimizing power consumption.

#### Practical Considerations and PCB Layout

The circuit components are compactly arranged on the PCB [1], with two of these PCBs required for stereo applications. Given the substantial currents involved, robust conductors with a minimum cross-sectional area of 2.5 mm² are necessary for power supply and loudspeaker connections. Ensuring optimal performance, the TDA1562Q must be securely mounted on a heatsink, the efficiency of which dictates the maximum duration the circuit can operate at full power.

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