The Green DAC Rev 1.1 is a compact, modular DIY audio DAC built around Texas Instruments Burr-Brown PCM5121/PCM5122 delta-sigma chips within a 100×100 mm footprint. Unlike high-performance DAC chips with differential current outputs, the PCM512x family features integrated voltage outputs with an internal charge pump. This project serves as an engineering experiment to extract the absolute maximum performance from an all-in-one budget DAC architecture by surrounding it with premium digital control, galvanic isolation, and an advanced analog buffer.

As a Revision 1 design, the board features direct AC power inputs designed to be fed straight from a power transformer.

The system is fully managed by an onboard STM32F103C6 microcontroller operating over software I2C to handle configuration control, 128x64 OLED display output, custom user inputs, digital filters selection, and VCOM or VREF mode selection. It provides automatic detection of SSD1306 or SH1106 OLED modules (128x64, 4-pin I2C) and offers dual software brightness levels (full brightness and dimmed modes). The firmware allows software configuration of the DAC's internal registers, including digital filter profile selection and real-time switching between VCOM or VREF operating modes. It utilizes the MCU's digital monitoring routines to track the incoming rail state, instantly triggering the output mute relay during power-down sequences to guarantee a click-free shutdown. Finally, a 3-second Independent Watchdog timer (IWDG) is implemented for system automated recovery.

NB: To prevent harsh hard-clipping and waveform distortion on maximum amplitude 0dBFS sine waves, the firmware initializes the DAC by automatically cutting the digital volume output level to -1dB via the registers command "pcm512x_Set_Volume(0x33, 0x33)". This intersample peak clipping issue is a well-known architectural limitation of the PCM512x internal output stage, see [1], [2] and [3].

Crucially, because the internal miniDSP of the PCM512x architecture processes volume attenuation using 32-bit high-precision floating-point arithmetic rather than integer truncation, this initial -1dB safety headroom adjustment maintains perfect signal fidelity and prevents any degradation of the DAC's dynamic range [4].

To overcome the sonic limitations of the PCM512x built-in output stage, a dedicated external analog section was implemented to provide a standard 2.1V RMS line-level output. The DAC outputs feed into a DIP8 dual op-amp low-pass filter (LPF) and then to a Class-A discrete buffer follower utilizing BC547 NPN transistors and J-FET loads. In this specific project, a 4-channel digital isolator is used for the USB interface, meaning only the I2S stream along with the MCLK signal is passed through the isolation barrier, while auxiliary control signals (such as MUTE, DSDON, and USB_PLUG) are omitted. The board tracks the presence of the incoming MCLK signal; once detected, the firmware assumes the USB host is connected, causing the DAC to instantly wake up from sleep mode. Because the PCM512x architecture itself features excellent inherent pop-free performance, no external hardware MUTE tracking from the transport is required. An MCU-controlled high-quality signal relay completely isolates the outputs during shutdown and startup sequences while the analog circuits stabilize to their optimal operating parameters.

The board features onboard rectification and linear regulation, allowing direct interfacing with external transformers:

The native compatible power supply unit is the Energy Classic. However, either the Energy Ultra-Prime or the Energy Ultra-Quad linear power supply unit can be utilized as well, using their raw AC outputs only.