Blue DAC Rev 1.2 is a full-featured digital-to-analog converter platform built around the well-known PCM1794A and its close sibling PCM1798 from Burr-Brown (now Texas Instruments). Both DAC chips are pin-compatible, while offering slightly different sonic characteristics due to differences in internal architecture and performance grade.

The board supports direct USB interface mounting and OLED connection, includes an S/PDIF input, allows both AC and DC power supply options.

At its core, the PCM1794A has long been regarded in audiophile circles as one of the most "musical" delta-sigma DACs ever produced and praised for its natural tonal balance, low-level detail retrieval and fast presentation. Its advanced segment DAC architecture and true differential current output make it especially suitable for high-end discrete analog stages. The PCM1798, while based on a similar topology, is a more cost-optimized variant with slightly reduced dynamic range and relaxed specifications, but still delivers excellent performance and retains much of the family’s characteristic sound.

The I/V conversion stage is implemented using SOIC-8 dual op-amps, while the LPF stage uses a DIP-8 package. For the I/V stage, I personally prefer OPA1612 - a high-performance op-amp with bipolar inputs, offering very low noise and distortion in a compact SOIC-8 package. Slightly less optimal but still excellent alternatives include OPA1602 and LM4562.

Please note that PCM1798 has an output current of approximately 4 mA p-p, while PCM1794A provides around 7.8 mA p-p. Because of this, I/V resistor and capacitor values should be adjusted accordingly. For example, I typically use R = 560Ω and C = 330pF with OPA1612.

The I/V stage is followed by a low-pass filter based on a classic Butterworth topology, using DIP-8 dual op-amp. DIP-8 package allows easy op-amp swapping and experimentation with sound tuning. There is a wide range of compatible op-amps in this package, such as NE5532, LME49720, NJM2068D and so on, so you are free to experiment and choose what fits your preferences best.

NB: Due to the relatively high output current of PCM1794A, its recommended to use small heatsinks on both the DAC chip and the I/V op-amps. In my builds, I use a 9×9 mm heatsink for the PCM1794A and 6×6 mm heatsinks for OPA1612.

To further improve output performance, the analog signal path can include a discrete Class-A buffer stage based on BC547 and SK117 transistors. This stage provides better drive capability, improved linearity, and a characteristic output close with discrete designs. Of course, this stage can also be bypassed if not needed. A signal protection relay, controlled via MCU logic, provides safe startup and shutdown behavior, protecting downstream equipment from unwanted transients.

The board supports direct mounting of Amanero-style USB-to-I2S modules and also provides a coaxial S/PDIF input based on the CS8416 receiver. Source selection is handled by DD3 (74HC157 multiplexer) and DD7 (74HC00 logic). By default, USB input has priority. When a USB interface is connected and powered, the DAC receives I2S from USB. Otherwise, the S/PDIF input is used. S/PDIF can be forced regardless of USB presence using the XS7 jumper (see details below).

Both PCM1794A and PCM1798 operate in hardware mode by default, meaning no I2C microcontroller is required for operation. An optional STM32F070F6 microcontroller can be added for extended control features, i.e. OLED display information of sampling rate and filter mode, digital filter selection (Sharp or Slow roll-off), output relay control, power-off detection.

DAC modes are set using the XS7 jumper:

The DAC uses a linear power supply approach with onboard rectification and linear regulation. It accepts AC inputs, which are rectified on the PCB and regulated locally for each stage. It accepts:

It's recommended to use DAC PSU Rev 3.1 to supply the board, but you are free to use some custom external implementations. Further regulation is implemented using LM317 / LM7912 / LM1117 regulators followed by RC filtering.

The I/V, LPF, and buffer stages use separate supply lines, which helps reduce interaction between stages and minimizes noise coupling through shared power rails.

In subjective listening, the PCM1794A is often described as fast, detailed, and precise, with strong transient response and good microdetail retrieval. It provides a clear and open presentation without becoming harsh or fatiguing.

The overall sound can be tuned through op-amp selection. For example, LM4562 tends to sound more analytical, while NE5532 or NJM2068D provide a slightly warmer and more relaxed character.

The smooth yet resolving sound of the PCM1794A also makes it particularly enjoyable for ambient, atmospheric, and meditative music, where low-level detail and spatial cues play an important role.