First Proto Builds -
Based on the success of cobaltmute's build and mine, cobaltmute decided to solicit some prototype builds. Beezar supplied many of the more critical chips. Several builders successfully built the r2 version of the "pup1 USB DAC" during March/April 2011. Cobaltmute started work on an updated version, to be referred to as "V2." It would incorporate RCA jacks, fix the regulator and resistor pads, improve the PCB layout around the opamp, and address some minor issues with grounding of the PCM chips.
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(forsakenrider's proto build)
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(Gotez's proto build)
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(forsakenrider's cased-up proto build)
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V2 of the PupDAC -
At right are the images of the new layout that resulted from cobaltmute's updated V2 pupDAC design and the experience from the prototypers' builds. Some of the changes:
- All parts SMD except for precision resistors and electrolytic capacitors
- Opamp moved to reverse side
- RCA jacks added
- Crystal replaced with clock oscillator
- LED changed to 3mm
- USB jack centered along PCB's width
- Ground plane on reverse side
- Electrolytics were upsized at key positions
- Charge pump changed to SMD SOT23-5
- PCB trace and pad errors in r2 version fixed
- Except for the output jacks, all parts were changed to Mouser part #'s
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(pupDAC V2 PCB design - top)
(pupDAC V2 PCB design - bottom)
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A new run of pupDAC V2 PCB prototypes was ordered in March of 2012. A half-dozen prototypers volunteered to construct the new PCB again. Beezar shipped the PCB's and as many IC chips as possible - plus the jacks. Construction on the new version started in earnest by cobaltmute in April, 2012. After finishing his prototype, cobaltmute noticed an offset in one channel that was higher than preferred. We waited until I could build mine before sharing the PCB's with a new round of prototypers. I finished the protos in early June, 2012. I tried building two at once, because I wanted to test out the new OPA2836 and directly compare it with the OPA2835. (My early pupDAC was built with the LMH6643.) However, I had a lot of issues with the OPA2836 version and switched back so that both PCB builds used the OPA2835. I had issues on that same PCB with the OPA2835, though. As it turned out, the newer chips use a type of plastic that is hygroscopic (absorbs moisture). I had used so much alcohol trying to get them clean that it never properly dried. The one pupDAC PCB kept getting knocked off line for some reason - it was very frustrating, because everything sounded great except for the intermittent operation. Once I realized it might be un-evaporated moisture, I put the PCBs out in the sun for awhile.
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(pupDAC V2 PCB - top)
(pupDAC V2 PCB - bottom)
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| That did the trick and operation has been uninterrupted ever since. My offsets ranged from 0 to 1mV, so we chalked up cobaltmute's build as a fluke and proceeded with the group prototyping. |
(Avro_Arrow's pupDAC V2 PCB) |
(Avro_Arrow's pupDAC V2 PCB)
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(Mullett's pupDAC V2 PCB - top) |
(Mullett's pupDAC V2 custom front plate)
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(Mullett's pupDAC V2 custom back plate) |
(MisterRogers' pupDAC V2 PCB - top)
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(TomB's pupDAC V2 PCB's - top) |
(TomB's pupDAC V2 PCB's - bottom)
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pupDAC V2R1 -
The prototype builds proceeded forward but as often happens, we discovered some other issues that really needed changing before we could finally go to production. Prototypers (including me) were still having issues with the PCM chips due to the short trace pads around the devices. The PCB-mounted RCA jacks had forward locating pins that did not line up with the edge of the PCB without some difficulty. Also, there was a chance that cobaltmute's output offset and my issues with the OPA2836 could've been the ground plane under the opamp. Cobaltmute admitted that this was not optimum and was simply forgotten in the PCB design for V2. These changes and a refined parts layout immediately surrounding the opamp were implemented. Cobaltmute developed a new PCB layout design, pupDAC V2R1, otherwise known as "pupDAC Version two, Release one." The layout design is shown at right. Another PCB prototype was ordered (just a couple) just to make sure (built version shown below).
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(pupDAC V2R1 Layout - top)
(pupDAC V2R1 Layout - bottom)
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(TomB's pupDAC V2R1 PCB - top) |
(TomB's pupDAC V2R1 PCB - bottom)
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The V2R1 PCB prototype was successful. The build was easy (lots of clearance around the PCMs' pins), the RCA jacks fit nicely in the PCB cutouts, and the opamp offset was minimal to non-existent. Production PCBs for the pupDAC were ordered!
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