3, Power amplifier
Power amplifier, also known as signal amplifier, its basic function is to amplify audio signals to drive speakers to replay sound. Generally, the host has a built-in power amplifier, but its power amplification range is relatively small, so it cannot meet the high level of listening requirements, let alone be compared with an external power amplifier. Moreover, the bass unit must be driven by a power amplifier, and the host does not have the ability to drive such a large power of the bass unit. The signal amplifier is a crucial part of the entire car audio system.
The voltage of the car power amplifier is 12V DC, which is consistent with the power supply voltage inside the car. In order to expand the dynamic range of the signal and enhance the output power, the technology of inverter boost is generally used in power amplifiers, which means that the voltage is raised from 12V to 35-40V. Another benefit of inverter boost is that when there is a fluctuation in the power supply voltage, the power circuit of the power amplifier will automatically adjust the voltage to ensure the stability of the output power. At present, this technology is widely used in vehicle power amplifiers.
(1) Heat dissipation of power amplifiers
There are two types of heat dissipation for car mounted power amplifiers: natural heat dissipation and fan heat dissipation. Fan heat dissipation is generally used in high-end power amplifiers. The good heat dissipation has a significant impact on the output power.
(2) Classification of power amplifiers
Class A: Pure Class A power amplifier, also known as Class A power amplifier, is a completely linear amplification form of amplifier. When operating in a pure Class A power amplifier, the positive and negative channels of the transistor are in a normally open state with or without signals, which means that more power is consumed as heat, but the distortion rate is extremely low. Pure Class A power amplifiers are relatively rare in automotive audio applications because their efficiency is very low, usually only 20-30%. However, audio enthusiasts are passionate about their sound performance.
Class D: Unlike Class A, B, or AB amplifiers mentioned above, Class D amplifiers operate based on switching transistors and can fully conduct or cut off in an extremely short amount of time. Two transistors do not conduct at the same time, so they generate very little heat. This type of amplifier has extremely high efficiency (around 90%), reaching 100% under ideal conditions, while AB class amplifiers can only achieve 78.5% in comparison. On the other hand, the switch operating mode also increases the distortion of the output signal. The circuit of Class D amplifier is divided into three stages: input switching stage, power amplification stage, and output filtering stage. Class D amplifiers can operate in a pulse width modulation (PWM) mode when in on/off mode. By using PWM, the audio input signal can be converted into a high-frequency switching signal. The audio signal is compared with a high-frequency triangular wave through a comparator. When the voltage at the inverting end is higher than the voltage at the in-phase end, the output is at a low level; When the reverse terminal voltage is lower than the same phase terminal voltage, the output is at a high level.
In Class D amplifiers, the output of the comparator is connected to the power amplifier circuit, which uses metal oxide field-effect transistors (MOSFETs) instead of bipolar transistors (BJTs). This is because the former has a faster response time and is suitable for high-frequency operating modes. Class D amplifiers require two MOSFETs that can operate completely on or off in a very short amount of time. When a MOSFET is fully conductive, its voltage drop is very low; When the MOSFET is completely cut off, the current passing through the tube is zero. The switching speed of two MOSFETs working alternately in the on and off states is very fast, resulting in extremely high efficiency and low heat generation. Therefore, Class D amplifiers do not require a large heat sink.
