DDDAC 1794 Build – Part 2

In the article DDDAC 1794 Build I describe my personal build of the DDDAC1794 NOS DIY DAC. Because it is a DIY project it is always on the move. Part 2 contains the updates since the previous article.

Changes are:

  • Add third DAC Module PBT
  • Used a LT3045 from LDOVR to replace the LF50 on the mainboard.
  • Power Distribution PCB
  • Used Ian Canada FifoPi Q3 to replace the combo Hifiberry Digi+ Pro and Allo Isolator.

The schematics and tweaks of the previous build are archived on in my Github repo under the tag build_hf_alloisolator.

Third DAC Modules PBT

With a third DAC module the maximum capacity is reached in my enclosure.

As always Audio Creative delivers fast and complete. Soldering only requires through-hole components so it is easy to do.

Within 30 minutes the DAC board is finished.

Just put this on the top one existing one.. Instead replacing the silver wire of the already existing boards, I just solder the silver wires with silver solder to top of the existing connections.

Next the value of the V/I Rload resistors needs to be correct. The value is 134Ω divided by the number of boards. For the 2 boards boards 68Ω is already used in place. Just putting a 133Ω in parallel with it will make an Rload of 45Ω.

After power the leds of the onboard tent regulator turn on so it looks like the mission succeed.

Next is calibration of trimmers. Both need to be adjust to 40mV on the test point. Which the voltage between the leg of the resistor next to the trimmer and common.

LT3045

The LT3045 from LDOVR is a 78xx/LFxx pin compatible power regulator.

Ultra Low Noise Low Dropout Voltage Regulator, 78XX replacement

We gonna use this regulator to replace the mainboard 5V regulator. There it is used to feed the logic for the I2S delayline, buffer and I2S/SPDIF selector. The SPDIF electronics and Tent clock are fed by separate regulators.

To fit the LT3045 you first have to desolder you current LF50, it sit just next to power terminal of the mainboard.

AC Power Distribution PCB

To cleanup the wiring for the power distribution and remove the current relay board, I created a small PCB. You can read about it in article below:

FifoPi Q3

FifoPi Q3

Apart from the hopefully soundwise improvements, replace the Hifiberry + Isolator with the FifoPi also reduces the building complexity a little bit:

  • Normal size screw terminals for the dirty and clean power.
  • Isolated and non isolated GPIO header, no need a GPIO for breakout between the Pi and Isolator anymore.
  • Nice F.UL connector for the I2S signals.

Downside is that the FifoPi form factor isn’t exactly the size of a regular Pi hat. Especially on the side of the SD card makes this it harder to replace you SD card.

The new FifoPi has stripped the LDO regulators from the PCB to allow your own PS solutions. The isolated part requires a clean 3.3V.

Th e DIYIHNK 4.7uV Ultra low noise regulator.

The voltage regulator I used to feed the Allo Isolator was a DIYHNK 4.7uV ultra low noise regulator based on a TPS7A4700. Luckily the TPS7A4700 is a programmable regulator and default a jumper is present to select between 5V and 3.3V. Other voltages can be made by adding jumpers to the available solder pads.

For connecting the I2S with F.UL connectors I ordered three (MCLK isn’t used) 6″ premade cables. Each cable contains a signal and GND as shield. On one side just cut of the connectors added a header for the I2S connector.

Because the power supplies between the FifoPi and the DDDAC are completely separated, it also requires that the ground is connected. By using the shield of one of the F.UL cables this is a very clean solution, very small GND path. In the image above this the white/green wire.

Use some hot glue to provide some additional support to the wires.

As probably known I’m moOde fan, which uses a regular Linux kernel for the Raspberry Pi. The used driver for FifoPi is the generic rpi-dac, which is actually for a PCM1794 (talking about coincidence ). Only the codec of this driver has a software limit of 192kHz, while the Pi supports 384kHz and also the FifoPi/ DDDAC1794.

I have created a small patch to increase the max frequency to 384kHz in the article below:

Updated schematics

All updated schematics can be found in the matching github repo https://github.com/bitkeeper/dddac1794build.

For completeness below are the updated schematics shown:

The blocks:

The logical connections:

And the electronic schematics:

We reached the end of the changes to the build. And maybe Santa send me some clock upgrades for the FifoPo …

For now, it is time to enjoy listening to music again!

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