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Russ White 12th May 2006 03:53 PM

Mr White's "Opus", designing a simple balanced DAC
 
2 Attachment(s)
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

hifimaker 12th May 2006 04:10 PM

Did you consider the TI 1794 DAC Chip? I've heard great things about this chip.

Also what about adding the Pass D1 I/V stage?

There is a serious shortage of balanced DACs available.

-David

Russ White 12th May 2006 04:14 PM

Yes I considered quite a lot of DACs. But I really like this one.

To your question one of the reasons I like this DAC is that the output is balanced voltage. No I/V stage required at all. :) Remember simple is the key word for the design. :)

Cheers!
Russ

hifimaker 12th May 2006 04:27 PM

Hi Russ,

I'd imagine that a discrete component approach would be better than an on chip I/V. it would allow the use of premium components for best sound.

-David

Russ White 12th May 2006 04:30 PM

Why would you imagine that? ;)

In any case this chip fits the design goal. That is why is was chosen. I don't want to have to add an I/V stage. And this DAC has a very solid audiophile reputation so I am quite confident in its on chip "I/V stage". I think it has voltage output due to its Delta-Sigma design.

Cheers!
Russ

ezkcdude 12th May 2006 05:06 PM

Can you post a bigger (higher resolution) image? The font size is way too small to read on my monitor.

Russ White 12th May 2006 05:10 PM

2 Attachment(s)
Schematic as PDF:

ble0t 12th May 2006 05:56 PM

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...

Russ White 12th May 2006 06:01 PM

Quote:

Originally posted by ble0t
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. :)

ezkcdude 12th May 2006 06:57 PM

2 Attachment(s)
Quote:

Originally posted by ble0t
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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