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Mr White's "Opus", designing a simple balanced DAC

Hello All,

Some of you may recall that a while back I was designing a balanced DAC around a Wavefront chip. I still have that design and I still like it. But in some ways it just was not really what I wanted. I wanted to be able to accept I2S in and I wanted the higher bandwidth of a better chip. Someone, I think Craig, suggested a look at the Wolfson line of DACs. That was a good suggestion.

After a bit of googling I could see that Wolfson made some very highly regarded DACs. They are used in products such as Arcam with very good reputations.

I looked in particular at the WM8740 which has everything I am looking for. It is a 24-bit 192KHz DAC with balanced stereo output and when used in stereo mode (to keep it simple) it has a 117db SNR. It has a pretty simple hardware interface with no need for a micro-controller to get the thing to work. Nice and simple, just the way I like it.

This is really a continuation of my efforts from the Wavefront DAC, so it is in reality my very first DAC project. I want it to be a community project. I want to share what I am learning with all of the others on the forum who may have a chance to apply the techniques to their own projects.

I will be publishing schematics as I go so that I can get critiques on the ideas I propose and I can get a clue on whether what I propose will even work in the first place.

Now let me lay out some design goals so that we donot go off on too many rabbit trails.

1)The DAC PCB will be just a DAC with a Receiver and an optional TORX Receiver which can be omitted.
2)Digital input will be S/PDIF or I2S.
3)There will be no output stage on the DAC PCB itself as I want that to be separate since in practice there are many ways to execute it. My idea is to design one filtering output circuit which will stack with the DAC. You could actually go directly from the DAC circuit to a balanced preamp, but without any analog output signal filtering.
4)The PCB for the circuit will be designed to be small so that stacking modules will be very easy and integration with other projects will be simpler. One module idea I have for the future is to create a USB input circuit which would feed an I2S stream direct to the DAC with no S/PDIF conversion in the process.
5)I want to keep the circuit SIMPLE. That is key. I want to avoid using external clocks and such, at least for the first version. There will be future revisions which may include such features. But it is an explicit design goal of the first version to only utilize the WM8740s internal PLL.

Now some requests for help on areas I am not sure of.

1)Does my S/PDIF input scheme look sane? Can I do it better?
2)I wanted to keep the digital power supply simple at 5V only, so I chose the CS8414 but would it better to use the CS8416? If so why?
3)I would be very grateful if someone could take a closer look at my reset scheme for the receiver and the DAC. Does it look workable? Particularly I wonder about the choice of the timing capacitor for the voltage supervisor (TLC775).

I have attached my first cut at the schematic for the DAC circuit. Please forgive that it is not very polished. Some value are missing or have yet to be determined, but it should give you insight into what I have planned.

Thanks for looking and for all who have expressed interest so far. I hope everyone can use the information that comes from the project.

Cheers!
Russ
 

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The SPDIF front-end is crummy...check the multitude of other designs out there for more info. I would also stay away from optical...it's not going to do you any favors in terms of jitter. You also may want to consider putting something better than just caps between your SPDIF input and the CS8414 as well...
 
ble0t said:
The SPDIF front-end is crummy...check the multitude of other designs out there for more info. I would also stay away from optical...it's not going to do you any favors in terms of jitter. You also may want to consider putting something better than just caps between your SPDIF input and the CS8414 as well...


Ok Will will do some searching, but even just one link woud be nice. :)
 
ble0t said:
The SPDIF front-end is crummy...check the multitude of other designs out there for more info. I would also stay away from optical...it's not going to do you any favors in terms of jitter. You also may want to consider putting something better than just caps between your SPDIF input and the CS8414 as well...


You have a design in the works over at headfi, right? That one uses CS8416, but it's in software mode and uses a PIC controller. I'm working on a design using the same chip, but in HW mode. I'm not sophisticated enough to use a microcontroller. Anyway, I've attached a preliminary schematic, which also includes AD1896 for re-clocking and PCM1794 for the DAC. I need to work on schematics for the power supply, clock, I/V and reset switch.
 

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Hi Guys

Sorry for crashing what is obviously a well-informed conversation with a newbie question. I'm hoping to learn something about these CD spinners.

The question is: how the heck does one identify the signals coming from the transport? I have been a computer tech for almost 30 years, so am familliar with the bits-n-bytes universe, but when I look at a candidate CDP I am stumped over the meansd to identify the traces on the ribbon running to the transport.

My current project is an old Pioneer PD-X303. This has two major plusses as a donor - 1) it looks like a very simple device and 2) I can live without it if I somehow destroy it :cannotbe:

I have looked on the web for a schematic with no success (no big surprise). I think I just need a starting point. The silkscreen on the board has a number of identifiers, but I cannot make head ort tail from them.

Any hints or suggestions woul be a great help.


Jess
 
ezkcdude said:



You have a design in the works over at headfi, right? That one uses CS8416, but it's in software mode and uses a PIC controller. I'm working on a design using the same chip, but in HW mode. I'm not sophisticated enough to use a microcontroller. Anyway, I've attached a preliminary schematic, which also includes AD1896 for re-clocking and PCM1794 for the DAC. I need to work on schematics for the power supply, clock, I/V and reset switch.


I think the TI/BB1792 and 1794 is a better chip.

-David
 
pinkmouse said:
Russ, I'm with you.

HI Al,

I read through the datasheets of all the Wolfson DACs and just thought the WM8740 was the best match for my design goals. Differential output was a critical factor, and voltage output was a big plus. You also just can't argue with the specs and the number of very high end products which utilize the chip. You should be able to build a very good DAC with the chip with a minimum of auxilary "stuff" hanging off the PCB.

This is my first DAC design, it is not intended to be the "best ever" just "really damn good". :)

To all, Still looking for input on my reset scheme. Does it look reasonable?

Cheers!
Russ
 
Russ,

I completely agree with your Wolfson DAC choice. There are many pitfalls awaiting for a first foray into the digital world, and keeping your design simple will save a great deal of time in the development by limiting the permutations of troubleshooting that may be encountered. The Wolfson DAC is highly regarded and gives up little if any in performance when compared to other "Top of the Line" DAC chips, and as you have said greatly simplifies potential issues that need to be addressed.

Great idea, and one in which I'll heartily participate.

Terry
 
Terry,

Thanks for the input. It really helps to know that I am not in this alone. :) I am sure there will be a lot to learn along the road.

The K.I.S.S rule is certainly in full effect for me here. Having guys like you around helps keep me motivated. BTW I nearly have a couple of SuSy Channels ready to be fed from my XBOSOZ which I hope will soon be fed by this DAC.

Cheers!
Russ