My new SPDIF + SRC + DAC design platform

[AAK]

Hi Everyone,

The pics below are from my latest project. Finally after years of building amps I got around to designing my own Sample Rate Converter Board (SRCB) and DAC board. The objective from the start was to design a platform where I can easily plug in or stack different DACs onto the SRCB to evaluate and compare performance. I started with Wolfson’s WM8741 DAC, with TI’s PCM1794 completed but not yet tested. I’m open to suggestions if anybody is interested in other DACs.

I designed the SRC and DAC board to be very flexible providing all the bells and whistles I can think of while keeping the boards as compact as possible to keep signal paths short to help reduce noise. Both PCBs measure 2.6” x 2.6” with only two layers.

I’ve been listening to the WM8741 for about a week now and it sounds beautiful. The final results have far exceeded my expectations. Here’s some more info.

The SRCB features the following:

*Wolfson’s WM8804 transceiver into TI’s SRC 4192/4193 Sample Rate Converter

*Two tri-state octal buffers for easy switching between the WM8804 and SRC4192/4193 I2S signals via a header with two LED select indicators.

*Transformer isolation of SPDIF coaxial input and output signals.

*Crystek 24.567Mhz, and 22.5792Mhz (mounted on the bottom) low jitter crystals. Can be switched in hardware or software with LED select indicators.

*Two 10-position tri-state switches for WM8804/SRC4192 on-board hardware control, SRC clock input select, and an MCU auxiliary input. Critical hardware control signals for both devices come out to headers for easy access.

*An on-board 20 pin Microchip PIC16F690 MCU provides software control via I2C/SPI for the WM8804, SRC4193, and plugged in DAC. The MCU is powerful enough to add an LED/LCD display for displaying volume and configuration settings. An IR remote RC5 decoder can be programmed if desired. Each pin comes out to a header for easy access. The MCU has it’s own 1A LDO regulator.

*Each power stage has it’s own low noise LDO regulator and power on LEDs.

The DAC features the following:

*Wolfson’s WM8741 DAC

*On-board balance to single-ended converter using a single dual op-amp. AC coupled stereo balanced outputs are available on the output terminal block.

*Leaded resistor for the output amplifier section for improved sound quality.. Accepts 1/8 or 1/4 watt resistors.

*One 9-position tri-state switch for on-board hardware control. All control signals come out to headers for easy access.

*On-board 8 pin MCU for stand-alone software control.

*Separate analog, digital, and MCU power stages using low noise LDO regulators. Each stage has a power on LED.

*Two WM8741 boards can be stacked when operating in dual-mono mode and controlled in either hardware or software mode.

I’m sure I left something out. I hope you all like it.

Cheers,

Al

[SoNic_real_one]

Other DAC that worth tested is AD1955.

Also, you might want to try their SRC: AD1896. Pin-compatible with SRC4192 (AD1896 actually it was first on market), it it said to have better jitter reduction(Benchmark Media Systems):

In both cases, the D/A clock is clean and jitter‐free. The difference is that the AD1896 produces a clean output spectrum while the SRC4192 introduces an output spectrum that has jitter‐induced phase modulation (jitter‐induced distortion). The jitter‐induced distortion produced by the SRC4192 is identical in amplitude and frequency to the distortion that would have been produced in the D/A circuit if it was driven directly from the high‐jitter interface clock. The SRC4192 simply converts jitter into distortion – giving fuel to the armchair critics of ASRC‐based jitter attenuation. In contrast, the AD1896 removes the jitter without adding jitter‐induced distortion. The AD1896 is so effective that jitter‐induced distortion never exceeds ‐135 dB FS when the input of the DAC1 is subjected to extreme levels of jitter.

Truly thou, those cip-based SRC are not all that great sounding. Not enough computing power IMO. A full-blown DSP (running SRC software) is really necessary for a good conversion.

[AAK]

Hi Sonic,

Thanks for the suggestion on the DAC and SRC. The SRC4192 according to the data sheets outperforms the AD1896 on S/N, DR, and THD and that's why I selected it. Your point on the SRC4192 jitter‐induced distortion is interesting. I'll look into it.

I agree with your point on the way the SRC sounds. That's why I added the two tri-state buffers to be able to select between the SRC4192 or WM8804 I2S outputs. Thanks again.

Best Regards,

Al

[SoNic_real_one]

You use that for jitter reduction primarily. Otherwise, why bother to change samplerate (especially if you don't change the bitdepth too)? DAC does not care about samplerate, but it does care about jitter.
Read the Benchmark pdf document linked above.

I was able to compare a SRC done with TI part and with a BlackFin AD DSP (programmed the Denon AL24 Processing Plus software) and the difference is clear.

[AAK]

I added SRC for flexibility and jitter reduction. Some a may prefer the SRC and some may not, either way the option is there to bypass it.

[AAK]

I added SRC for flexibility and jitter reduction. Some a may prefer the SRC and some may not, either way the option is there to bypass it.

[SoNic_real_one]

Well, it's not a bad ideea, especially if you can test the AD SRC too.
Hmmm, I would be interested to test the AD SRC (not the TI one) in comparation with the Denon DSP.

[AAK]

SoNic thanks for benmark pdf document. After reading it and searching the web I'm convinced the AD1896 does a better job at jitter reduction than the SRC4192. I have a spare board, I'll try the AD1896 and compare. I'll report the results in a week or two. Thanks!

[Floche]

Hi everyone,
I am wondering why no one use the SRC4392 which gather receiver, transmitter and sample rate converter with low jitter in this kind of project ? I did my dac project with and it work pretty fine !

[AAK]

Hi Floche,

I didn't use it because the SRC4392 is strictly software controlled. It's probably one of the reasons why others haven't used it. I wanted my design to provide hardware control as well.

Plus, why spend the time on the SRC4392 when today you have ESS Tech's ES9018 that includes everything.

Best Regards,

Al