Premium Sound
The later BMW E30 may be equipped with the 4-channel Premium Sound System.
The audio head unit is the Alpine CM5908 with the 4 audio preamp outputs: Front Left, Front Right, Rear Left, Rear Right. The 4 preamp outputs are connected to the Blaupunkt amplifier (called Quadro Booster), but the Ground output wires of each channel are connected and form a single Ground wire input to the amplifier. This amplifier connection is called “Single-Ended Input”. There is nothing bad about it. If you have a newer head unit , connect the grounds of all preamp outputs (usually, RCA connectors) together. The Alpine CM5908 uses 2-pin DIN connectors for preamp outputs, they are covered with silver, which has tarnished by now: DIN 41529 connector.
4 RCA outputs.
Preamp output signals are measured in volts, usually in the range of 2-5 Volts. Older head units provided the voltage close to 2V. Newer head units provide 4V preamp outputs, which require lower gains in an amplifier; otherwise, audio from an amplifier will be too loud even at the lowest loudness levels.
Blaupunkt Quadro Booster Amplifier
The default amplifier is pretty simple. It uses 8 venerable SGS TDA2003 chips. They are still produced by Chinese companies like Unisoc.
The TDA2003 is a Class-AB amplifier chip. Each two TDA2003s are connected in the bridge configuration providing 14 Watts into each of the 4 channels.
The Remote Enable (Auto Antenna) input enables the relay that provides power to the circuit. Two inductors filter the power supply line. The TDA2003s are mounted on the heatsink.
The vertical and horizontal panels are connected using wires, which break easily. Also, there are many electrolytic capacitors that should be replaced.
Class-D Amplifier
Schematics
Vertical panel
The vertical panel runs traces from the main connector to the 2 board-to-board connectors.
Horizontal panel
The horizontal panel contains all components.
Protection Circuit
The protection circuit with the LM76202 protects against reverse power connections, when a car battery is connected incorrectly. It also protects from undervoltage (< 10 V) and overvoltage (> 16 V) power supply. Its Shutdown pin is connected to the Remote Enable (Auto Antenna) output from a head unit; when the signal is High, the chip provides power to the circuit.
The chip TPS3711 monitors the power voltage and sets the fault line Low when the voltage is under 11 V. It allows the microprocessor to run fault routines.
Microprocessor Circuit
The microprocessor circuit uses the chip STM32L412KBT3, which has only 32 pins. The amplifier gain is set by the switch, the selected value is read by the microprocessor at the initialization stage and the gain register is set in the amplifier chip.
TAS5414C Circuit
TAS5414C is a 4-channel D-class automotive amplifier. It’s suitable for setups with single ended inputs. Its registers can be set using I2C. During the initialization step, tests can be run to test each channel for faults. It doesn’t consume much power, so a small chip-mounted heatsink can transfer all excess heat to the existing amplifier body.
It sounds great.
Companies rarely provide full documentation for later chips. The TAS5414C is still produced with full documentation available. There are inexpensive test boards from Microe.
TAS5414 Zephyr Driver
The microprocessor does the initialization of the amplifier chip TAS5414C. It sets the registers, runs tests to find out if any of the outputs has faults, especially when speakers are not connected. When the power goes down, its fault interrupt routine sends a mute message to the amplifier chip.
The driver is written for the Zephyr RTOS, so it can be adapted to run on different processors.