DIY Audio Boost: Mini Bluetooth Amp M02

If you find desktop power supply too complicated with a 12V integrated power supply for over a dozen devices, extracting audio signals from HDMI leads to squeaky sounds when connecting external active speakers. In the noise, faint useful sounds are barely audible, making it impossible to hear. In that case, you can consider DIYing a solution.

Introducing the Mini Bluetooth Amplifier M02, equipped with built-in audio isolation to eliminate concerns about external power interference during LINE input. It features a simple pre-stage amplification, pre-stage amplification with isolated power supply, and a Bluetooth receiving module (always on).

Input Voltage: 12-20V-DCIN TYPEC, built-in 12V deception (tentatively set at 12V deception)

Output Power: TPA3116 - 2x30W, powered by 15V with copper foil for heat dissipation, Class D (tested to limit power to 10W)

Audio Source Connection: Default Bluetooth audio source connection on startup, switchable to 3.5mm LINE input with a button.

Dimensions: PCB=43*60mm, whole device=Length 62.4 (including baffle)Width 45Height 18.5mm

Built-in Bluetooth 5.0, The project still uses the first-generation AC22BP from JL for listening purposes.

EPC-13S - 1:1 isolation transformer

Includes a circuit to eliminate power-on and power-off pops, as well as pops during Bluetooth/LINE switching. (Tested, there may be a slight pop when turning off under Bluetooth connection, possibly due to Bluetooth power supply issues.) The main power is turned on, and Q5 is briefly turned on to pull down SDZ to achieve startup mute. When turning off, Q5 is turned on through the residual voltage of C73 to pull down SDZ to achieve shutdown mute. The source switch is controlled by an optocoupler, pulling down Q4 first and then allowing Q5 to pull down SDZ for mute. Due to the presence of C34, the relay lags behind the amplifier chip mute to avoid noise from the contacts.

The pre-stage amplification power supply uses 78L12+7660 negative pressure to provide power. Amplification 3 times, and even a slight turn of the actual potentiometer results in loud sound. 

External LINE audio input uses an isolation PCB to eliminate noise. As mentioned before about current noise, this sound is clean. The loss is imperceptible. Due to the height of the casing, a separate isolation PCB is made and pasted on the mainboard.