The Portable Bluetooth Amp MK1 is the first iteration of a portable bluetooth amplifier.
The goal was to design a universal base platform for designing bluetooth speaker in a range of small to large power.
The amplifier is meant to be powered by LiPo batteries.
/pba:mk1
/docs # Documentation repository
/eagle # Contains eagle files
/gerber # Contains gerber board files
/pdf # Contains PDF versions of the schematics
The power supply provides different voltages for digital and analog purpose.
It provides the following rails:
D33- 3.3V 0.25A Digital - for digital logic and bluetooth moduleA75- 7.5V 0.10A Analog - for analog audio processingD120- 12V 1.00A Digital - Control voltage for Class-D amplifierVAMP- VBAT - 30V 3.00A - Amplification voltage for Class-D amplifier
The D33 rail is constantly enabled and provides power for the bluetooth module.
All other rails are enabled on demand by the bluetooth module, via the EN signal.
As regulators for all rails expect VAMP LMR50410 are used.
The VAMP rail can be varied between VBAT and 30V. This is provided by using a LM5122 boost converter, that will be enabled on demand. If not required, the LM5122 will go into passthrough mode and disable the boost action. The boost action is controlled via a separate LVL signal.
The power supply also provides the capability to charge and use a 4S LiPo battery. Please note: The power supply does not provide any balancing or protection circuitry. Those things must be included in your 4S LiPo battery pack.
As charging controller a LTC4006-6 is used, which incorporates all necessary functionality.
The power supply used to charge the LiPo battery is a 19V external power supply. The input current is limited to 3.07A.
Charging current is limited to 2.00A and the charging timer is configured to approx. 2 hours.
There is currently no NTC readout configured in the charging circuit.
As a charger supply, I recommend the use of generic laptop chargers with 19V and 3.54A - they are widely available, reliable and cheap.
As amplifiers there are two TPA3245 used in this project.
The first of them is driven by directly by the CSR8675 bluetooth chip DAC outputs, using a BTL configuration for two output channels.
The second amplifier is configured in a PBTL configuration and is driven by a PCM5102 DAC.
The first amplifier should be used for full-range or dedicated mid/migh-range speakers, whereas the second amplifier with its PBTL configuration is meant to drive low-range/subwoofer speakers, since due to its configuration it is able to deliver more current and drive speakers with low impedances.
All analog signals are used differentially to reduce noise and hum. Between the differential DAC output and amplifier inputs there are TS914 adding amplification to the signals for additional volume.
The amplifiers will automatically be enabled, if music is playing. This is controlled by the CSR8675 using the AMP_EN signal.
There is an additional level detection circuit that will switch the amplifier supply voltage by setting the LVL signal to high. This allows the amplifier chips to run at a lower voltage for moderate volumes and have enough headroom for high volume music playback. The usage of a higher voltage also allows for better use of higher impedance speakers, such as 8-16Ohms.
The threshold of the level detection can be configured via a potentiometer on the amplifier board.
As for bluetooth, the CSR8675 in form of BTM-875 modules is used.
That chip provides the following functionality:
- Bluetooth 4.0+ compliance
- Stereo differential analogue audio inputs (used as line-in)
- Stereo differential analogue audio outputs (driving
TPA3245in BTL configuration) - I²S output (driving
PCM5102for secondTPA3245in PBTL configuration) - LED and Button GPIOs (3 LEDs and 8 GPIOs are exposed)
- GPIO control for enabling external circuitry (enabling power with
ENand amplifiers withAMP_EN) - Kalimba DSP (used for crossover and DSP audio processing)
- True Wireless Stereo (can link two amplifiers together playing music in sync)
Software used on the CSR8675 is configured using the ADK in version 4.1.35.
| GPIO | Function | Description |
|---|---|---|
| LED0 | User interface LED | Can be used to display status messages. |
| LED1 | User interface LED | Can be used to display status messages. |
| LED2 | User interface LED | Can be used to display status messages. |
| PIO0 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO1 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO2 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO3 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO4 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO5 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO6 | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO7 | User interface GPIO | Can be used to connect a button for user interaction. |
| PWR | User interface GPIO | Can be used to connect a button for user interaction. |
| PIO10 | DETECT | Used to detect plugged in line-in source. |
| PIO11 | AMP_EN | Used to enable amplifiers when music is playing. |
| PIO12 | EN | Used to enable the power supply. |
| PIO15 | I²S DAC enable | Used to enable the external I²S dac if music is playing. |
Schematics and layouts are created using Eagle.
Gerber files provided have been used to create first batch of boards and working instances of the PBA MK1.