Step-by-Step Guide: Building a Best Pure USB I2S/DSD Isolated Re-Clocker that Slaves to External Master Clocks

Step-by-Step Guide: Building a Best Pure USB I2S/DSD Isolated Re-Clocker that Slaves to External Master Clocks

USB has now become the most important digital music interface for audiophiles. But due to the limitations of the generic on-board clocks and the high USB EMI noise, to achieve the highest level sound quality potential, we not only need the galvanic isolation, but also have to run the whole USB interface in a mode that slaves to eternal higher grade audio clocks.

After discussions with the developer of Amanero, we realized that this solution is possible to be achieved. This guide walks you through the complete process, just hope it helps to the community members who want to create the best possible USB I2S/DSD re-clocker.

PART1: Flash the Combo768 firmware into Slave Clock Mode
1. Prepare a PC running Windows 11 or later.
2. Download the Combo768 ASIO driver and then install it. (You may skip this step if you already have this driver installed.)
3. Download the Combo768 Maintenance Tool from the same link above. Extract the downloaded files if required.
4. Connect the Combo768 to your PC via a USB cable.  Run the just downloaded M7Config15.exe, and click "OK" for Combo768 as Oemid. Then select 050126_fw63a6f8f_slave_dsdswp as the firmware.
5. Click "Start Programming". Once completed, your Combo768 will now be running in slave clock mode.
Note: Incase you want flash the FW of comb768 back to the default master Mode, please select 05126_NOVOLUME_24MHz_fw63a6f8f

Output connector signals in slave clock mode
1 XO select OUT 0 to select the 49.1520 MHz external MCLK
1 to select the 45.1584 MHz external MCLK
3 DATA/DSD1 OUT I2S DATA or DSD DATA1
4 SCK OUT BCK of I2S or DSD
5 LRCK/DSD2 OUT I2S LRCK or DSD DATA2
6 MCK IN Input of external MCLK, 45.1584 MHz or 49.1520 MHz depending on the PIN1 selection
7 DSDOE OUT 0: I2S,  1: DSD 
8,13,14,15 GND Power
9,10 3.3V OUT Power output, 50mA max
11 MUTE OUT 0: Play, 1: when mode or Fs changing
17,18,19,20 F0-F3 OUT Sample rate indicator
Other Pins NC
No connect

For other more information, Please download Combo768 User's manual 

PART2: How to connect this slave mode Combo768 USB to your own Isolated Re-clocker with the external master clocks

Some community members have developed excellent dedicated isolated re-clockers with nice external clocks. It would be very easy to integrate them with the slave mode Combo768. You just need to connect the following signals from the output connector:
PIN1 to the XO select input of the re-clocker
PIN3 to the DATA/DSD1 input of the re-clocker
PIN4 to the SCK/BCK input of the re-clocker
PIN5 to the LRCK/DSD2 input of the re-clocker
PIN6 to the MCLK output of the re-clocker, either 45.1584 MHz or 49.1520 MHz 
PIN8,13,14,15 to the GND pins of the re-clocker. 

However, U.FL cables would be highly recommended for this application for much better signal quality especially at higher Fs music signals. The #70K, Amanero U.FL adapter could be a great help in this case.

PART3: ComboMaster,  a design example of USB isolated re-clocker with External Master Clock to Upgrade Combo768 output quality

ComboMaster is a standard isolated re-clocker with swappable external clocks. It is specifically designed for the Combo768 (in slave mode) to enhances sound quality by overcoming the limitations of the onboard clock architecture.
The ComboMaster features galvanic isolation to completely isolate the USB data signals and ground, reducing noise and interference from the USB source. It also utilizes higher performance local master clocks to re-clock the output of the Combo768, allowing the USB audio stream to benefit from them instead of the Combo768's onboard default oscillators.
As a result, the Combo768 operates using the superior external clock source, delivering improved timing accuracy, lower jitter, and enhanced overall audio performance.
Please refer to the schematic diagram for detailed design reference.
Upgrading Combo768 itself using a ComboMaster
  1. Make sure your Combo768 has been flashed with the Slave Clock Mode firmware.
  2. Install the standoffs.
    Install two 12 mm M2.5 standoffs into the mounting holes on the USB connector side of the Combo768.
  3. Mount the ComboMaster.
    Insert the ComboMaster board-to-board socket into the output connector of the Combo768, then secure the ComboMaster to the standoffs.

  4. Install the master clocks.
    Install the clock modules as follows:

    • XO1: 45.1584 MHz
    • XO2: 49.1520 MHz

    Important: Be extremely careful to install the clock modules in the correct orientation. Reversing the orientation may damage the clocks or the board.

    For the best possible sound quality, SC-Pure clock modules (#80A and #80B) are highly recommended.

  5. Connect the clean power supply.
    Connect a high-quality 3.3 V power supply to the ComboMaster clean power input connector (J1).

    For optimum performance, an ultra-low-noise power supply is recommended, such as the LifePO4 Mini Pro 3.3 V, or another high-quality battery or ultracapacitor-based power supply.

  6. Connect the USB source.
    Connect your USB music source to the Combo768/ComboMaster assembly using a USB cable.

    The Power Good LED (D1) should illuminate, indicating that the board is powered correctly.

  7. Verify operation.
    Start USB audio playback. Depending on the sampling frequency (Fs) of the music being played, either the XO1 or XO2 LED will light to indicate the active master clock.

  8. Connect to your DAC or streamer.
    Your upgraded USB interface is now ready to connect to your streamer or DAC project.

    For the best signal integrity and clock performance, it is recommended to use the U.FL connectors (J2, J3, J4, and J5) whenever possible. If compatibility with the standard Combo768 output connector is required, you may instead use the DIP output connector M2.

Note:
If you plan to mount the Combo768/ComboMaster assembly inside an enclosure, you may use four 20 mm M2.5 standoffs to secure the complete assembly.

PART4: FifoPiUSB, a Higher Grade Solution to Build a Pure USB Re-clocked and Isolated streamer/DAC project

FifoPiUSB is a pure USB isolated re-clocker featuring sockets for swappable high-performance external clocks. It is fully compatible with the entire FifoPi ecosystem, making it easy to build a pure USB-based streamer/DAC project that delivers the best possible signal integrity and exceptional sound quality.

By integrating FifoPiUSB with other FifoPi-compatible modules and power supply, you can create a highly optimized pure USB audio system that takes audio performance to the next level. Here are the detailed steps to build a Pure USB Streamer:

1. Prepare the Power Supply

The LifePO4Mini Pro 3.3V is highly recommended as the power supply for this pure USB streamer due to its ultra-low output noise performance and compact size.

If you already have another high-quality 3.3V low-noise power supply, you may skip this step and proceed directly to Step 2.

For the LifePO4Mini Pro setup:

  1. Install four 25 mm M2.5 standoffs on the bottom side of the LifePO4Mini Pro.
  2. Screwing four 35 mm M2.5 standoffs on the top side through the same mounting holes.
  3. Install four 3.2V 18650 LiFePO4 batteries.
  4. Connect J6 to a USB power adapter using a USB-C cable.

High-quality USB power adapters such as Anker or UGREEN are recommended.

Fully charge the batteries until LED D7 lights up, then turn switch S1 to the ON position to verify that the power supply is operating properly.

Important Safety Notice
Please be extremely careful when working with the batteries, especially when using metal tools. The batteries remain energized even when switch S1 is in the OFF position. Avoid any accidental short circuits, as they may damage the batteries or the circuit board.

2. Install the Combo768

Make sure that the Combo768 has been flashed with the Slave Clock Mode firmware.

  • Install the Combo768 into the socket on the bottom side of the FifoPiUSB.
  • Secure it in position using two 12 mm M2.5 standoffs.

3. Install External Clocks

Install two external clocks into the XO1 and XO2 sockets of the FifoPiUSB.

Supported clock combinations:

  • 45.1584 MHz / 49.1520 MHz, or
  • 90.3168 MHz / 98.3040 MHz

The clock order/positions must not be swapped.

SC-Pure clocks are highly recommended for their ultra-low phase-noise performance.

The 90.3168 MHz / 98.3040 MHz clock combination enables re-clocking of audio streams up to PCM768 and DSD1024.

4. Install the FifoPiUSB Assembly

Install the FifoPiUSB + Combo768 assembly onto the standoffs installed on the LifePO4Mini Pro.

Connect two power wires from one terminal of each connector (J7 and J8) on the LifePO4Mini Pro to J5 on the FifoPiUSB.

Installing a MonitorPi Pro is optional but highly recommended.

  • Install the MonitorPi Pro onto the GPIO J2 connector of the FifoPiUSB.
  • The control cable from J5 of the MonitorPi Pro can also be connected to J11 or J12 of the LifePO4Mini Pro to enable the SYNC charging functions.

5. Power Up and Verify Operation

Reconnect the USB-C power adapter and turn switch S1 to the ON position.

One of the clock LEDs (XO1 or XO2) should light up.

Connect the Combo768 to the USB port of your music source. The Combo768 should be recognized by the USB Host.

The following indicators should become active:

  • D2 LED on the FifoPiUSB will light up.
  • The OLED display will turn on if the MonitorPi Pro is installed.

Start music playback.

The F0–F3 LEDs on the Combo768 (refer to the Combo768 User Manual) will indicate the current audio format.

The XO1 and XO2 LEDs will change according to the corresponding audio frequency family.

If the MonitorPi Pro is installed, the OLED display will show the real music format.

Normally, USB power is sufficient for the MonitorPi Pro. However, if additional power is required, you can connect the 5V output from J2 of the LifePO4Mini Pro to the AUX 5V input J4 of the FifoPiUSB by a 2P PH2.0 cable.

6. Install the Digital Output Interface

Install four 13 mm M2.5 standoffs onto the FifoPiUSB.

Install either the TransportPiAES or HdmiPi Pro onto the GPIO J7 connector of the FifoPiUSB, resting on the standoffs.

Remove all power-sharing jumpers.

Connect two separate power wires from the remaining terminals of J7 and J8 on the LifePO4Mini Pro to the independent 3.3V power input of the TransportPiAES or HdmiPi Pro.

Connect the master clock:

  • Connect MCLK1 or MCLK2 on the FifoPiUSB to the MCLK input of the TransportPiAES/HdmiPi Pro using a U.FL coaxial cable.
  • When using the new HdmiPi Pro III, the  2-inch short U.FL cable can be used for a slightly better clock signal quality.

If your DAC requires Mute and DSDEN signals, connect the Mute/DSDEN cable from J16 of the FifoPiUSB to J8 of the HdmiPi Pro.

7. Connect the DAC and Enjoy Music

Connect your DAC to the TransportPiAES or HdmiPi Pro.

Repeat the power-up procedure described in Step 5.

The system should now operate normally, and your music should play with the highest possible performance.

That is all you need to build an ultra-high-quality pure USB DDC streamer.

Congratulations and enjoy the music!



 
 


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