New Posts  All Forums:Forum Nav:

The SkeletonDAC

post #1 of 215
Thread Starter 

Back in the days when tomb and I were working on bringing the grubDAC to production, we were looking at the kitting process and lamenting the fact that the PCM2706/7 were in such short supply and that the PCM2704/5 had huge quantities in stock.  We discussed the fact that it would be convenient to have a DAC built on one of those chips.  Spring and Summer came and being outside mountain biking seemed like a much better idea than sitting inside designing a DAC.

 

However, it has been winter.  I'm not as young as I used to be and riding in the snow is not quite my thing.  So I designed the SkeletonDAC:

SkeletonDAC top 1-small.jpg

 

The idea was to build a "bare-bones" (i.e skeleton) DAC based upon the PCM2704/5.  No external regulator and low part count.  However it did morph a little bit due the fact that after laid the PCB out, I kept looking at the fact that there was this expanse of board with nothing on it.  With a little bit of part shoving and some choices of parts, I managed to incorporate the SPDIF output of the PCM2704 onto the board, complete with output transformer.  As well, given that the PCM2704 is spec'ed at being able to drive headphones, there is the option to put larger output caps - basically anything with 2.5mm or 5mm lead spacing.  2.5mm caps positions are offset so that it is possible to use larger size electrolytics and drive headphones right off the dac.

 

The board dimensions are exactly the same size as the grubDAC.  The USB input and audio output are in exactly the same place.  The same Hammond case can be used (even with SPDIF), so it should be a great versatile little dac.  From a build perspective, the only challenge to this should be that most of the surface mount parts are 0805.  However, if you can get the PCM2704 down reasonable clean, you should have a very high chance of success building the DAC.

 

Board layers top and bottom:

SkeletonDAC r1 all layers.jpg

 

Top Layer

SkeletonDAC r1 top layers.jpg

 

Bottom Layer

SkeletonDAC r1 bottom layers.jpg

 

Schematic:

skeleton-dac-r1-schematic.png

 

The BOM is to come.  I need to clean it up to show the three versions that can be built - full, audio out only, SPDIF out only.

 

Since BatchPCB's shipping to Canada is really sucky, the boards took a detour through Beezar as it is a lot cheaper for me to have tomb ship them to me than to have BatchPCB ship them to me.  As a result tomb has built two already and has already provided me with some really useful feedback on what to do with future versions of the board.  He has reported to me quite reasonable results driving headphones direct, but I'll let him report on that.

 

At the moment, I'm looking into the cost of running a bit larger prototype run and was curious if there was anyone out interested in building some of these.

 

 

 

post #2 of 215

I am intrigued and would love to try... but I've never done any chip soldering before.

post #3 of 215

id be interested in building one.

post #4 of 215

I would definately be interested in doing one.  My goal this year is to build a couple of different DACs and try to get the design in my head for one on paper.


 

post #5 of 215

As cobaltmute said, the PCM2704/5 has a built-in headphone amplifier.  So, unlike previous DACs that were similar (Alien, Bantam), this one does a pretty good job powering small headphones directly.  Here's mine cased up in the familiar GrubDAC case with a pair of 47uf Elna RFS Silmic II's.  One has to be a bit creative in installing the caps.  They're too tall for the case lid to sit up vertically, but a little creative bending of the leads lets them sit horizontally, albeit a little staggered.wink.gif  Here's one I built awhile back with a pair of KSC-75's.  It drives them fine and sounds very good - at least as good as an Alien/Bantam.

 

SkelDAC-on-sm.jpg
SkelDACcased-sm.jpg
SkelDAC-1-sm.jpg

 


Edited by tomb - 3/2/11 at 4:45pm
post #6 of 215

Isn't being creative part of what DIY is all about?  That looks good in that case.

 

Would I be correct in assuming that the full version would be audio out and SPDIF out?  Is there room to mount SPDIF connector on the opposite end of the case from the headphone jack?

post #7 of 215

I'm interested in building one or two.

 

Ordered a few 2704's to much some time ago.

post #8 of 215

Have you thought about mounting the spdif pads 90* so that a pcb mount rca jack might fit?  I was looking how to incorporate a spdif input into the small hammond case, but it seems all the pcb mount rca jacks have pins that are inline (or tri pin) which differs from the pad layout on the board.  Is there any rca/spdif input part that will fit neatly for the hammond case?  This build definitely looks cool for maybe a friend or family member.


Edited by particleman14 - 3/3/11 at 4:21pm
post #9 of 215


 

Quote:
Originally Posted by tomb View Post


<snipped>


 

I must say....you've got a real thing for those blue cases, eh?

post #10 of 215
Quote:
Originally Posted by particleman14 View Post

Have you thought about mounting the spdif pads 90* so that a pcb mount rca jack might fit?  I was looking how to incorporate a spdif input into the small hammond case, but it seems all the pcb mount rca jacks have pins that are inline (or tri pin) which differs from the pad layout on the board.  Is there any rca/spdif input part that will fit neatly for the hammond case?  This build definitely looks cool for maybe a friend or family member.


I'm not sure it's that much of an issue.  Granted, I haven't done it yet for real, but it seems to me that it would be easy enough to loop the wiring over the transformer (choke) to have it exit with an RCA jack on the left end of the case.  Top mounted might be another possibility, although not as aesthetic.
 

 

post #11 of 215

ya , I was trying to go for minimum protrusion from the rca jack.  I guess I'll have to use a panel mount rca if I want spdif.

post #12 of 215

I'd be interested, I've actually never built a DAC before

post #13 of 215

After looking at this some more I have another question concerning SPDIF out.  Exactly what is it meant to connect to?  I don't see a lot of SPDIF inputs on the amp circuits that I look at or have built. 

 

I know that probably sounds like a dumb question but I have to ask.  What searching I've done has not given any answers.

 

In any case can an SPDIF input be added to an amp with the proper circuit in place?


Edited by c12mech - 3/3/11 at 7:11pm
post #14 of 215

 

Quote:
Originally Posted by c12mech View Post

After looking at this some more I have another question concerning SPDIF out.  Exactly what is it meant to connect to?  I don't see a lot of SPDIF inputs on the amp circuits that I look at or have built. 

 


I'm pretty sure its there so you can use this thing as an USB-spdif converter to feed a bigger and badder DAC. 

 

We put a DAC in your DAC so you can convert your digital stream while convert your digital stream. 

post #15 of 215
Thread Starter 
Quote:
Originally Posted by nikongod View Post
We put a DAC in your DAC so you can convert your digital stream while convert your digital stream. 

 

^ This is why we do it.  And for those of us who don't have SPDIF on our computers.

 

There was also inquiries about doing it on the grubDAC and since there was room on the board, why not?
 

As for mounting a SPDIF RCA jack on the board, I'll have to look, but I'm not seeing a lot of room right away.

 

New Posts  All Forums:Forum Nav:
  Return Home