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AL5125 (I2S output firmware) + ES9018K2M + OPA4134 + TPA6120A2RG portable Bluetooth LDAC and USB decoding headphone amplifier

This headphone amplifier is based on the aluminum alloy casing of the ANMAZZ HP2 headphone amplifier. It supports two 3.5mm jacks for separate or simultaneous output, Bluetooth and USB modes, and the USB port supports Hiby Music's exclusive USB function. The total power consumption is approximately 130mA~150mA.

       The previous two projects, "Type-C Powered Portable 47mm Headphone Amplifier - JLCPCB EDA Open Source Hardware Platform (oshwhub.com)" and "Type-C Portable Decoding Headphone Amplifier - JLCPCB EDA Open Source Hardware Platform (oshwhub.com)," were built and open-sourced based on the aluminum alloy casing of the ANMAZZ HP2 headphone amplifier. Both amplifiers perform well overall. However, considering that the Type-C Powered Portable 47mm Headphone Amplifier - JLCPCB EDA Open Source Hardware Platform (oshwhub.com) lacks decoding capabilities, and the Type-C Portable Decoding Headphone Amplifier - ... The JLCPCB EDA open-source hardware platform (oshwhub.com) occasionally experiences issues such as USB disconnections and the inability to play content exclusively via USB on the HiBel player, and wired use is not very convenient. Therefore, I considered creating a headphone amplifier that supports LDAC Bluetooth decoding. After referencing various sources, I came up with a combination solution of AL5125 + ES9018K2M + OPA4134 + TPA6120A2RG. Later, to increase the fun of DIY, I designed and prototyped the casing myself, and the result is quite good. The casing file is attached. Please carefully read the attached documents before making the PCB and casing prototypes.
       The design references the high-fidelity portable Bluetooth headphone amplifier (PCM1794A + TPA6120 solution) - JLCPCB EDA open-source hardware platform (oshwhub.com) and the ES9018 DAC decoding board - JLCPCB EDA open-source hardware platform (oshwhub.com).
        After pairing with the AR5B22 Bluetooth adapter on a Xiaomi 6, Huawei Mate 30 Pro, Huawei Honor 10, Apple 13 Pro, Apple 5S, and a laptop, the performance was acceptable. The Xiaomi 6 and Huawei Mate 30 Pro support Bluetooth LDAC, the Apple phone supports Bluetooth AAC, and the Huawei Honor 10 supports Bluetooth APTX. When the computer's USB is connected and Bluetooth is turned on and connected, Bluetooth will be prioritized by default. When the computer's Bluetooth is turned off, it will automatically switch to USB decoding. Based on the past year of use, there may be a slight background noise when the headphone cable touches the Bluetooth antenna. I'm not sure if this is caused by the Bluetooth signal passing through the headphone cable and entering the internal circuitry. However, when the headphone cable touches the Bluetooth antenna and there is background noise, touching the Bluetooth antenna with your hand will make the background noise disappear. In actual use, I don't think this background noise is a big problem, since the background noise is masked when the music is playing. Alternatively, you can separate the headphone cable from the Bluetooth antenna. After testing with 16-ohm, 32-ohm, and 300-ohm small headphones, as well as 50-ohm large headphones, it felt okay. It can drive two 16-ohm headphones simultaneously. The AL5125 module has a 2.8V low-battery warning. It's said that an LED will flash and an audible warning will sound when the battery voltage drops to 2.8V. However
         , in practice, using the battery in the ANMAZZ HP2 headphone amplifier, it automatically shuts down when the voltage drops to 3V. The TPS65133DPDR's input voltage range is 2.9V~5V, so the 2.8V low-battery warning set in the AL5125 firmware is meaningless. The aluminum alloy casing in the picture is a modification of the original ANMAZZ HP2 headphone amplifier. For aesthetic purposes, I engraved some patterns myself. Another one is my own design. For personal DIY, the production cost is probably around 160 RMB.