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The GrubDAC

post #1 of 1076
Thread Starter 
I made an off-handed comment in another thread and since people are asking for functionality in a board the haven't seen yet, I might as well make this public.

I decided to build my father one of joneeboi's USB headphone amp for his birthday. I ordered myself some of his prototype boards and then I went to order the BantamDAC to go along with it in the case.

And then I realized that I had a problem - the Bantam only has a full size USB jack. My father only ever carries a mini-USB cable with him, and I'm not about to make him carry another cable just for his birthday present.

So I decided to come up with this:
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What it is a PCM2707/WM8524 USB DAC that is designed to be a functional equivalent of the BantamDAC. It is the same size as the Bantam, just using a mini-USB instead of the full size USB. Hopefully anywhere you can use a Bantam you can use this.

The PCM2707 takes the USB signal and converts it to I2S. This signal is fed to the Wolfson Micro WM8524. The unique feature of the WM8524 compared to other Wolfson DACS is that it uses a charge pump to create a ground referenced signal. This means that there are no output caps. It also runs off of 3.3V so both chips can run off of one voltage regulator, in this case a TPS79333, making for a relatively simple board.

The schematic is attached. I know the PCM2707 section works as it is taken right from another working DAC I designed. The section around the WM8524 is pretty simple, so I don't think I mesed it up in anyway .

An example BOM for the board is here. The BOM does not include output jacks or a case. As for a case, it should fit in the same Box Enclosure that the Bantam fits in. The big thing right now is that PCM2707 seems to have gone into short supply.

I've got PCBs on order right now that I should be getting in a week or so. Once I've built one for myself to prove I didn't screw up, I will have extras if people are interested.

Any comments? I don't doubt that there will be something to be revised on this first version of the board.
LL
post #2 of 1076
Thread Starter 
9/21/09 - Pictures of the board at Post#48
9/21/09 - Excel BOM
9/22/09 - Finished PICS
9/22/09 - Boards available
9/30/09 - Added a important note about integration with other devices when using USB power only for both.
10/7/09 - Grub built as a CableDAC
2/21/10 - Pictures and Notes of the production board
post #3 of 1076
You basically took the half schematic and description I posted in another thread and "designed" a DAC around it?
One of these days I will learn to keep my mouth shut.

http://www.head-fi.org/forums/f6/lit...ma-1-a-438679/
post #4 of 1076
Thread Starter 
Hmm. While you may have a problem believing it, I did what I did based on the datasheet (pg21), so any resemblance there happens to be coincidental.

It is not like it is a hard DAC to implement or anything.
post #5 of 1076
oooh oooh im in line for one. Then i can finish shoe horning everything into my esd wand case so work will wonder why it has a headphone jack.
post #6 of 1076
I'll probably consider this to go with the Carries too, since it eliminates the DAC output caps. Here's to hoping pics will be up soon..
post #7 of 1076
Nice job with the USB breakout. That's reason enough to want one of these to pair with a Carrie!
post #8 of 1076
Very nice. It is interesting that the reason you designed tthis was due to the USB in. My only complaint about the setup was exactly that, I wanted a mini-USB style input.
If you need a prototyper, sign me up!
post #9 of 1076
One of the limitations I had on my DAC attempt was to keep the giant USB connector in the same place as the Bantam supposedly for swappability, but I realize now that such a constraint is unnecessary. It's good to see another Bantam-sized DAC to go with the Carrie. I'll offer a few comments:

- Why the pair of filter capacitors per channel on the output? Is that to allow the builder a choice between SMD and TH?

- I fear some of those vias surrounding the PCM2707 are at risk of bridging, particularly on pins 2, 15 and 20. The ones on pins 2 and 15 sit close to ground pins, and if a bridge forms, it could be a headache undoing it. Pin 15 is carrying a high signal to boot. You have room, so moving them a bit further away could increase build success. Pin 2's via can be moved left a little bit so that bridging is erroneous, and the other two can be moved slightly farther from the pins.

- I'm wondering about the LED placement. It will be hiding behind the rail caps on Carrie, so are you going to mount it on the panel?

Other than that, I like it a lot. Just an excellent choice with the Wolfson, and I like the breakout holes near the USB. C9 is large enough for those low impedance, low voltage electrolytic capacitors to keep Carrie's DCP turned on, and the routing is intuitive and simple. This DAC will certainly go well with the Carrie. Got any extra boards?

edit: Also, I prefer the I2S signal going to the Wolfson over getting SPDIF out, but that's up to you to give us access to it.
post #10 of 1076
Thread Starter 
Quote:
Originally Posted by joneeboi View Post
- Why the pair of filter capacitors per channel on the output? Is that to allow the builder a choice between SMD and TH?
The output has a RC filter on it (per the datasheet). Through hole resistor and SMD capacitor. The resistors are through hole due to comments over at DiyHiFi about the nickel barrier on SMT resistors being "the worst sounding material known to man"

Quote:
Originally Posted by joneeboi View Post
- I fear some of those vias surrounding the PCM2707 are at risk of bridging, particularly on pins 2, 15 and 20. The ones on pins 2 and 15 sit close to ground pins, and if a bridge forms, it could be a headache undoing it. Pin 15 is carrying a high signal to boot. You have room, so moving them a bit further away could increase build success. Pin 2's via can be moved left a little bit so that bridging is erroneous, and the other two can be moved slightly farther from the pins.

- I'm wondering about the LED placement. It will be hiding behind the rail caps on Carrie, so are you going to mount it on the panel?
We'll have to see about the vias. I looked at the soldermask layer and there looks to be good mask around the vias. They are all in front of the pins as well so fixing any problems should not overly hard. You however have now got me a bit worried about pin 18 as it is underneath the chip. A possible correction for r2 of the board.

As for the LED, I was never happy about the placement. In a standalone config, it is not an issue. In the Carrie, you can just remote it with wire to where ever you want. I may install it and just leave it on board for testing purposes as I don't think I need two indicator lights. I think I'll get it next to USB jack on the next revision

Quote:
Originally Posted by joneeboi View Post
Got any extra boards?
At this point, whenever they arrive, yes.
post #11 of 1076
This looks very intriguing! Likewise, since we are working on the newest Millett MAX board to use an onboard Bantam - this would work, too.
post #12 of 1076
Do you think you can add some pads/holes that makes it easy to tap to the I2S lines? Actually maybe I can just solder wires on the unpopulated WM8524 DAC, seems like space is an issue.
post #13 of 1076
Quote:
Originally Posted by tomb View Post
This looks very intriguing! Likewise, since we are working on the newest Millett MAX board to use an onboard Bantam - this would work, too.
That would be very nice!
Have you thought about another rev of the Bantam using the USB-Mini style connector?
post #14 of 1076
Thread Starter 
Quote:
Originally Posted by ShinyFalcon View Post
Do you think you can add some pads/holes that makes it easy to tap to the I2S lines? Actually maybe I can just solder wires on the unpopulated WM8524 DAC, seems like space is an issue.
I'm already playing with some possible layouts for r2 (gotta do something while waiting for the boards to arrive). I think it may be possible.
post #15 of 1076
Quote:
Originally Posted by Billyk View Post
That would be very nice!
Have you thought about another rev of the Bantam using the USB-Mini style connector?
I'll see if cetoole will add it.
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