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pupDAC Step-by-Step Build Thread - Page 2

post #16 of 296
Thread Starter 

Once again, here are a couple of pics of the finished, trimmed, cleaned, and rinsed PCB.

 

Top:

pd47-sm.jpg

 

And bottom:

pd48-sm.jpg

Note that the tabs are neatly trimmed for the RCA jacks and stereo mini-jack.  The PCB will not fit into the case with these tabs remaining in place!  The mini-USB jack's tabs/leads are already short enough - they barely fit all the way through the PCB holes.  So, no trimming needed there.  The rest of the trimming was the capacitor leads, the LED leads, and the resistor leads.

 

 

Casework is next!

 

As mentioned, the pupDAC is designed specifically for the Hammond 1455C801 case (or 1455C802 with plastic endplates).  We have designed a special machined version from Hammond with Beezar-anodized blue and laser-etching.  Four screws are all that's needed for final assembly!

pd49-sm.jpg

Those are special blue-anodized aluminum socket-head cap screws.  If you build your own, remember that the Hammond case does not come tapped.  Aluminum screws such as these will strip in short order if you haven't tapped the case.  Hammond uses 6-32 screw sizes as a standard with their 1455 series cases.  The ones shown here from Fastener Express are 1/2" long and work quite well on the tiny pupDAC case, but it must be tapped.  Hammond supplies some inexpensive self-threading phillips head screws, but in silver, they're not nearly as pretty as these.wink.gif

 

Here is the finished pupDAC completely assembled.  It's your choice on whether to use the transparent-blue bezels.  The endplates will fit either way.  The tip of the mini-USB jack and the tip of the LED are flush with the outside surface of the endplate if you don't use the bezels - that's the only difference.

pd50-sm.jpg

pd51-sm.jpg

 

Yay!! I made it create a 2nd page! biggrin.gif


Edited by tomb - 11/21/12 at 6:35pm
post #17 of 296

Thanks.

post #18 of 296

Quick question, would swapping the op-amps from this list to the AD797BRZ be possible or would the entire system have to be redesigned?

 

 http://www.diyforums.org/PupDAC/bom/pupDAC-BOM.htm

post #19 of 296
Thread Starter 
Quote:
Originally Posted by Angelbelow View Post

Quick question, would swapping the op-amps from this list to the AD797BRZ be possible or would the entire system have to be redesigned?

 

 http://www.diyforums.org/PupDAC/bom/pupDAC-BOM.htm

1. The specs are better with the OPA2836 and at a quick glance, I think they may be better with the OPA2835, too - bandwidth, distortion, current output and slew rate are all better.  The LM6643 may be a bit worse, but that's only 1 out of 3 that all work well in the pupDAC.

2. The AD797 is not a low voltage opamp.  It can go down to +or- 5V, but you're not going to get that with any kind of quality using USB power.  The pupDAC works on a totally linear-regulated +or- 2.5V supply on the output opamp and it burns a volt or two of the USB power to get that.

3. The AD797 is a single.  The pupDAC uses dual opamps.  You could probably rig up a two-single-to-dual adapter, but you wouldn't want to do that on a DAC.  The trace lengths and layouts are way too sensitive.

EDIT: I think that's three strikes.wink.gif

 

Yes, you're looking at a different DAC altogether if you want to use that opamp.


Edited by tomb - 12/30/12 at 2:12am
post #20 of 296

being the SMD soldering noob i am, how does 1 ensure U1 is properly soldered? can i also run across each leg with a knife and make sure they dont connect?

post #21 of 296
Thread Starter 
Quote:
Originally Posted by kyoshiro View Post

being the SMD soldering noob i am, how does 1 ensure U1 is properly soldered? can i also run across each leg with a knife and make sure they dont connect?

Yes, I often use an X-acto knife to clear the pins.  It's hard sometimes to tell whether you've got a real bridge, or whether it's just some excess flux reflecting in the light.

 

That said - short of an actual test by plugging it in, which really can't be done until the DAC is finished - there are two methods:

 

1. Visual inspection.

This needs to be very rigorous.  Hold it up to the light as I've shown in this thread, use a magnifying glass, take a macro photo and blow it up, or scan it and blow it up.  If some methods are un-revealing (glare, no contrast in a photo), then use another.  I can't remember if I mentioned it in this build thread, but I also do a preliminary cleaning after doing the two PCM chips in this DAC.  That removes the excess flux and lets you see the joints better - try not to proceed unless you are confident the pins are clean and soldered well!  Even so, you can still re-flow the chips on a completed pupDAC if you remove one or more electrolytics.  I just did that on one last night, re-flowed one side of the DAC chip and re-installed the electrolytic cap that was in the way - not too difficult if you find you have to do it.

 

2. DMM trace continuity checks.

With this method, you place one probe of a DMM at pin right at the exit out of the chip's plastic - above the solder joint, IOW.  Follow the trace that the pin is connected to until you're well away from the solder joint and place the other probe at that spot.  You should read zero resistance.  A good thing to do at this point is move one probe to the next pin/trace to also check to see if it's bridged.  This is obviously much more tedious than #1, but is a good fall-back if you're really having trouble pin-pointing a problem.

post #22 of 296
Quote:
Originally Posted by Angelbelow View Post

Quick question, would swapping the op-amps from this list to the AD797BRZ be possible or would the entire system have to be redesigned?

 http://www.diyforums.org/PupDAC/bom/pupDAC-BOM.htm
The AD797 isn't spec'ed to run at +-2.5V, which is what the output opamp is running at in the pupDAC.

With some careful looking at the schematic and board, you may figure out a way to get +-5V, but I'm not going to say anything about how it would actually work.
post #23 of 296

my opa2835 bag only contains a card and dehydrate packet lulz

post #24 of 296

woops nvm, it was somewhere unexpected

post #25 of 296
Thread Starter 
Quote:
Originally Posted by kyoshiro View Post

woops nvm, it was somewhere unexpected

Yep - it's hard to find that tiny little opamp amongst all of that anti-moisture business. tongue.gif

 

If Mouser hadn't packaged this stuff, there's no way I'd package one of those opamps like that.wink.gif

post #26 of 296
Quote:
Originally Posted by tomb View Post

Yep - it's hard to find that tiny little opamp amongst all of that anti-moisture business. tongue.gif

 

If Mouser hadn't packaged this stuff, there's no way I'd package one of those opamps like that.wink.gif

yea no kidding. I also figured out something during the soldering of the many leg SMDs. Good flux is a must to even solder it properly with swipe method. The cheap pen flux i was using didnt do the job so I ordered some BGA gel flux and it all came together very quickly lol

post #27 of 296

Tomb,

I've been super busy, but I'm finally getting around to working on this now. Is there any reason to swap the PCM2706 for PCM2707, or is this just to add some variety?

Similar performance out of both?

post #28 of 296
For our purposes they are exactly the same. The choice of part for the kit is based upon availability.

IIRC, the difference is based upon the PCM2706 being programmable by an EEPROM.
Edited by cobaltmute - 1/16/13 at 3:30am
post #29 of 296
Quote:
Originally Posted by cobaltmute View Post

For our purposes they are exactly the same. The choice of part for the kit is based upon availability.

IIRC, the difference is based upon the PCM2706 being programmable by an EEPROM.


Got it, thanks. I just updated my BOM last night, changed a few parts. You used some nicer caps than I speced, so I switched to yours.

 

Anyway, I need to figure out another project (a pedal for a friend) before I place the order. But, soon! I'm very excited still.

post #30 of 296

argghh LED not lighting up : (

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