DAC Schematic Review and Layout Suggestions - WM8804 and PCM5102A

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I'm looking at the Subbu design right now; it looks quite interesting. Maybe I'll make some changes... :D

You are welcome although I learnt that native english speakers possibly mean something totally different when they say "quite interesting" ;). IMHO I think/know it will be hard to have the same or better quality certainly when using PCM5102A. There are more than enough mediocre performing DACs out there so please don't burn your fingers at "yet another DAC" by making the same design errors as others have done by omitting factors that define "quality". Quality sometimes can be determined in a few minutes extra thinking and a few extra pennies....

I hereby encourage and challenge you to surpass the Subbu V3 :D Of course I am willing to beta test your design if you like.
 
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You are welcome although I learnt that native english speakers possibly mean something totally different when they say "quite interesting" ;). IMHO I think/know it will be hard to have the same or better quality certainly when using PCM5102A. There are more than enough mediocre performing DACs out there so please don't burn your fingers at "yet another DAC" by making the same design errors as others have done by omitting factors that define "quality". Quality sometimes can be determined in a few minutes extra thinking and a few extra pennies....

I hereby encourage and challenge you to surpass the Subbu V3 :D Of course I am willing to beta test your design if you like.
Jean-Paul, i try to locate the initial thread for Subbu (any version) DAC without success. The only i found so far are 2 threads, a "Subbu v3 DAC building thread" and a "Modifying the Subbu v3 DAC" as well a group buy for PCBs. I came accross these threads but i could not locate any schematic. To be sincere, i am mostly interested for the WM8804 circuitry you use as in our project i use it but with the WM8741 DAC since i try to hit a total DNR close to 120dB. Could you please drop me a link where are the initial schematics? You should know that i am not so experienced with digital audio, simply it helps me a little my knowledge about digital circuits and microcontrollers.
Thank you!:)
 
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It's OK. Don't know who did that as we are a bit reluctant posting our proven and tested schematics on line (just to avoid useless pages full of "why didn't you ..." ;) ) Since it is there feel free to use it for inspiration, then reread post #3...
 
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Disabled Account
Joined 2014
Well the schematic is shared under Creative Commons, so someone with a copy could have legally posted it online.

For now, my plan is to stick with USB but leave a header on the board for connections to a seperate power supply. I read somewhere that it isn't good to place a transformer on the same board as everything else (due to EMI and similar), and having two boxes on my desk isn't much of a hassle anyways. An annoying thing with USB is the inrush current limit (post #4) that prevents me from using large caps - so if all goes well I might just switch to an AC supply altogether.

To be honest, my main concern with building an AC supply is the fact that I'll be working with transformers and high voltage from the power outlets. My parents just don't really want me to take the risk (I'm just a high school student) for safety's sake.

I suppose I could use a pre-built power supply, and as stated earlier just plug it in to the header on my board. However, most of the AC adapters on the market probably suck for audio use too, and won't really be much better than USB.
 
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Mmm, I am not in to legal stuff as I am not a lawyer (I have a soul, that is why). I only worked together with Subbu to make it a nice device ;)

No fear for AC 230 or 115 V provided you have the knowledge and if you are not a teenager (you probably are, just checked on Google what high school is). Be aware of the risks, work tidy/safely and think + check twice and definitely have a fellow DIYer assist or guide you. I was working with mains connected stuff when I was 12 so it must be possible. Fear is your worst enemy, just check people that fear electricity. They get most shocks :) I would like to suggest a healthy respect for high voltages and an awareness to think of possible risks before you start on the job even if it is only connecting L and N.

Our PSU has been measured and it has 10 µV noise. I am very sure you won't get that result with USB adapter type power supplies. As you can see I tried to keep wiring to an absolute minimum to avoid risks for builders. If you use the recommended Phoenix connectors and work tidy it will be OK certainly when you have it checked by an experienced builder (before switching it on that is). When you will go on the path of designing good quality stuff you will have to make the switch to good power supplies one day anyhow. A good power supply is the heart of any audio device. The best DAC chip will not perform optimally with a mediocre power supply. In my personal opinion most care should go to the PSU... so in the light of audio fundamentalism you are already on thin ice. I hope you try to make your designs as good as possible. It can be very frustrating afterwards if corners are cut and your design will not perform as intended. It is bad enough that such is done is consumer gear so don't step into that trap when DIYing. See your designs as a kind of art form and every product as a "child" and you'll get the hang of it ;)

Even when you still have doubts regarding AC I would include pads for a large filter cap and rectifier on the board. You can connect low voltage AC from a transformer in that case later of have it connected by an experienced person. No pads = no possibility.
 
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Disabled Account
Joined 2014
Hmmm... I've looked at the ODAC schematic and realised that while it is powered by USB, the performance is quite high. The input caps aren't that large, and basic filtering is provided using ferrites and inductors.

However, a set of low-value caps in parallel sit between the regulators and chips. See below for an example:

YwhzPj8.png


Maybe that's a solution to the USB noise problem? Otherwise, I'll just stick with an AC-AC adapter and power my board off that.
 
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Joined 2002
Mmm, I better not comment on ODAC except that V3 sounds better. I think that is what its creators also like to hear ;)

All that caps are the caps that are used for decoupling power supply pins of chips etc. They are not simply connected in parallel, technically they are but in practice they are scattered over the board. Just check the board layout. Indeed decoupling is very important, just like the beads, coils and series resistors etc. That is if you want to have a good final product that can compete with existing designs.

Still it is better to feed the DAC clean power and not USB/SMPS polluted power. Any manufacturer that says he uses SMPS for low power audio devices because they are supposedly better than linear power supplies is doubtful. They can be very good but then one needs to spend quite some time and money to get it right (Linn, Nuforce etc.). Have it your way, making compromises on simple things will never get your design to the next level. I have made many mistakes in the past (and still make them) and I am now convinced that you have to think to make it the best device in the world to make it happen. Even if it ends up to be an acceptable performing device you have done the best you could and then the only thing you can do is trying try to surpass your own level with a next product that again needs time and money. It has no use making compromises as you will put your time and your money in this. More time and money than when buying a good ready made DAC. Some will disagree with me but IMO it is fully acceptable at your age that you borrow knowledge from other designs just to have better results as long as you try to understand why things are done like that.

My best advice is: always try to improve the product while you are designing it, not when it is ready.
 
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Disabled Account
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Thanks for your offer, but I've already started designing and simulating with an AC-AC adapter (basically a wall-wart containing only a transformer). It's safer than directly connecting mains power to my board, and lets me work with a linear power supply and its merits.
 
Disabled Account
Joined 2002
You're on the right track Padawan :) Just be aware that Ac wiring picks up nasties easier than DC wiring so an AC to DC adapter would be more optimal. Try to keep voltages as low as possible but with a margin to allow for lower mains voltages etc. For instance 12 V be it AC or DC will reult in hot and a few seconds after that defective regs. LP5907 has a mx. input voltage of 5.5 V. So you need either a 5 V DC adapter or a very low voltage AC adapter with th erisks of spikes destroying your LP regs. In your specific design it would be wise to include a 5 V prereg. A nice low noise LDO is sufficient, otherwise a 7805 is possible to but be aware that that one needs at least 2 to 3 V higher voltage.
 
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Disabled Account
Joined 2002
:D There are some quite basic/universal reasons why the V3 is a good DAC. There is nothing wrong with adjusting your design (with only 2 chips designs quickly resemble eachother) but if you don't want that you can do whatever you like. Keep in mind that there are basic rules that always apply: the quality of the power supply defines the quality of your device is such a rule. It is not a question of selecting the "best" chips only.

Best would be to build a version of the first schematic and a version of the last schematic and then compare. After some years one knows what to choose and where the differences are. You simply can't have such experience due to age. See post #35: I am quite sure you would have had a just 1 second good functioning DAC. Agreed, it is better to make your own mistakes and learn from them instead of borrowing others experiences (but it takes way more time...). As I followed that path I know how many disappointments I had and learned a bit from them which translates to the guidelines I am giving you.

Good luck.
 
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Disabled Account
Joined 2014
It's a bit late (I've had a busy summer), but here is the layout of my DAC. I've tried to minimize tracks on the bottom layer, and tried to connect the grounds of the decoupling caps with their chips. However, the design of the PCM5102A makes it hard for me to do so without routing in huge loops.

Anyways, the layout is downloadable as high-res PDFs here or as an attachment. Suggestions for improvement are welcome and encouraged.

mU4Vyla.png


If all goes well, I'll have boards ordered by the end of this week;).
 

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