[00940]

I'm affraid that the ceramic caps on the ground plane would be a real pain to solder without thermals.

Isolate up , certainly.

 

[dsavitsk]

Looking great. Any chance you could post a list of the parts you are using?

 

[halman]

I would suggest these changes.

1, put the decoupling caps on the same side as the dac (define and reduce the loop areas where the current flows whenever possible) and use 0805 or maybe even 0603 caps.

2, put som inductance on those long power traces

3, Use a bnc for the digital out. Sadly there are no 75ohm rca around..so these should be avoided

4, Galvanicaly isolate the electrical output by using a good spdif transfomer, for example use SC982-04 1:2 with 499/750R resistors for your divider

5, Use a better line driver ic is instead of the 74*04

6, There is enough room to use a DIL8 oscillator..but something in the back of my head says there where some troubles with starting the ic while using those.. But even so there is enought space to make room for both.

 

[00940]

Quote:

Originally Posted by halman 1, put the decoupling caps on the same side as the dac (define and reduce the loop areas where the current flows whenever possible) and use 0805 or maybe even 0603 caps.

0603 are excluded, to let the project rather easy (I know the pcm2902 is a pain, no need to push further) and 0805 are still a bit too small for most as we want a good solder joint there. I'll see for the caps location. I could simply replace those two caps by a big 1uf ceramic cap (X7R), as done on the pcm2902 demo board. Strangely, while they advocate 10uF decoupling caps for the analog section in the datasheet, the demo board uses 1uf.

Quote:

2, put som inductance on those long power traces

there's a crl filter still to come on the top trace

Quote:

3, Use a bnc for the digital out. Sadly there are no 75ohm rca around..so these should be avoided

possible, most people have rca cables and rca inputs though, I'd like more feedback on this.

Quote:

4, Galvanicaly isolate the electrical output by using a good spdif transfomer, for example use SC982-04 1:2 with 499/750R resistors for your divider

I would need more feedback from others people interested on this. It raises the price by almost 40% and the scientific conversion xformer aren't easy to come by in Europe.

Quote:

5, Use a better line driver ic is instead of the 74*04

I'm not sure we would get huge benefits out of that, I'm gonna see.

Quote:

6, There is enough room to use a DIL8 oscillator..but something in the back of my head says there where some troubles with starting the ic while using those.. But even so there is enought space to make room for both.

Considering the way the spact operates, there's no real point in putting a better clocking scheme.

 

[Nisbeth]

Quote:

Originally Posted by 00940 possible, most people have rca cables and rca inputs though, I'd like more feedback on this.

I'm with 00940 here, BNC's are outside the scope of this project IMO

If you must have 75 Ohm RCA's, the Canare RJ-R jacks are listed as being 75 Ohms and made with Video/SPDIF transmission in mind. But they're a little big for a Hammond 1455C801


Quote:

Originally Posted by 00940 I would need more feedback from others people interested on this. It raises the price by almost 40% and the scientific conversion xformer aren't easy to come by in Europe.

Elfa in Scandinavia sell SPDIF transformers from Pulse Engineering (Link to Datasheet The price is app. 5 EUR each in single quantities with a small quantity discount for more than 10 pcs. If these are suitable they would be a possibility, but I still think they are unnecessary for this project. Small size and easy asssembly are the key points of this project, it doesn't have to be the last word in audio quality. If 00940 ever gets his "reference USB-DAC" going again, then they are worth considering as far as I am concerned but for this application I simply don't see the need

(Incidentally, if anybody outside of Scandinavia needs one or more of these transformers for anything, let me know and I can order them

)

EDIT: Forgot to mention, they only have the through-hole version

/U.

 

[dsavitsk]

Quote:

Originally Posted by Nisbeth I'm with 00940 here, BNC's are outside the scope of this project IMO

I agree with this too.


How about adding a couple of holes for mounting screws -- perhaps one by the usb jack, and one near the rca?

 

[Glassman]

I wonder if those Pulse Engineering xformers are suitable for higher than 7Mbps rates.. the price is not that bad.. or do anyone know about such xformers meant for higher rates?

 

[Nisbeth]

Quote:

Originally Posted by Glassman I wonder if those Pulse Engineering xformers are suitable for higher than 7Mbps rates.. the price is not that bad.. or do anyone know about such xformers meant for higher rates?

Do you have something that uses 192 kHz sampling rate?

I would trust the manufacturer's specs in this case.

/U.

 

[Glassman]

oh yes I do.. I've done a bit of research and found:

1) a newer Pulse Eng. xformers
2) a few, rather old Schott xformers with exactly the same specs as those newer by Pulse
3) Scientific Conversion xformers - these guys look like they really know what they are doing, have many models, you can see 'jitter' stated everywhere and officially support 32-192kHz rates with some models having bandwidth of more than 200MHz.. really cute!

I've already contacted Spoerle which supply Pulse Eng., couldn't find any serious European distributor for Schott and yet have to contact Scientific Conversion, but I got a strong feeling those are the ones I'll pick..

 

[00940]

What do you need those xformers for, exactly ?

 

[Glassman]

exactly for up to 192kHz S/PDIF transmission.. (CD/DVD-A/SACD)

 

[dsavitsk]

Not sure where this project is.

I am still voting for simple rather than complex, particulary in light of the fact that guzzler has a bunch of his usb dac's in kitted form thus reducing the importance of analog out on this one as his should sound better. At any rate, I have picked up a couple (literally 2 extra) of pcm2902's from digikey that I can send out once boards are made.

-d

 

[guzzler]

I suggest a deadline perhaps, for people who are interested and order from there. Obviously, the more people get, the cheaper it is per board but the total cost to the buyer goes up, so it would be helpful if people registered a definite interest as opposed to maybe. We could say the first weekend in March? That would give one month for final revisions and orders...

Personally, simple is best for me on this project. The analog out isn't going to be too spectacular, and as there's not an amp built in, it's not even going to be useful for portable use. But, if we can fit it in without any problem, we might as well...

g

 

[00940]

I suppose we could put an AD8606 on the output of the analog out ?

The use of reg101 could be too much of a cost, I'm looking for cheaper low drop regulators. Any idea ?

I wondered if cut-out in the board wouldn't be a better alternative to pcb mounted output jacks ?

 

[Magsy]

I'll certainly be in for a board, assuming usual costs ($10-15?)

I'm not really sure what I want, I would like spdif out for sure, if the analog requires a handful of components/space (which it looks like) then it might be nice to have the option there. I hadn't realised earlier the spec was worse on the 2902, looking at the datasheets shows it going to be average :/

If the board is a perfect fit in the smallest hammond then the board mounted jacks, assuming availabilty to us all, are a great idea - it tends to look more professional