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Jambo DAC

post #1 of 219
1/21/08 at 8:52pm
Thread Starter 
Hi Folks,

I thought I'd share my DAC PCB design with you. Got the PCBs today and have it working in hardware mode.

The design is as follows:

Inputs:
Burr Brown PCM2707 USB to SPDIF converter
2 x Optical SPDIF
1 x Coaxial SPDIF (although I made a mess of the PCB footprint for the RCA socket)

Outputs:
RCA
De-jittered Optical SPDIF

SPDIF transciever: Wolfson WM8805 (multiplexed version of WM8804)

DACs: Wolfson 8740/41 in dual differential mode

Output filters: Analog Devices AD797s

Control: Xilinx FPGA over I2C bus to WM8805.
I have the Verilog written for this but underestimated the size of FPGA required so need to get the board re-spun. At the moment the WM8805 is configured in hardware mode (I thought ahead here and provided that option), and receives data nicely from one of the optical inputs.

NO caps in signal path, plenty of sites for decoupling caps. Pictured below...

Let me know your thoughts, I'll do a run of these if anyone's interested?

Cheers...
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post #2 of 219
1/22/08 at 12:11am
I'm interested
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post #3 of 219
1/22/08 at 6:57am
yeah looks very interesting. how's it sound?

expected cost?
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post #4 of 219
1/22/08 at 7:36am
Sounds interesting, any reason you are using SPDIF for transmission and not I2S?

A schematic would be really nice to see how everything works together
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post #5 of 219
1/22/08 at 7:42am
Great work Jambo. Maybe you can get same size xilinx with more logical cells?
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post #6 of 219
1/22/08 at 8:00am
Thread Starter 
Quote:
Originally Posted by FallenAngel View Post
Sounds interesting, any reason you are using SPDIF for transmission and not I2S?
Simply because the WM8805 is an SPDIF multiplexer and I wanted Toslink and Coaxial SPDIF it seemed logical to also connect the PCM2707 in this fashion. I2S still runs between the WM8805 and the DACs though.

Quote:
Originally Posted by FallenAngel View Post
A schematic would be really nice to see how everything works together
Sure, I'll get the schems into a nice format and post them....
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post #7 of 219
1/22/08 at 8:04am
Thread Starter 
Quote:
Originally Posted by borisov57 View Post
Great work Jambo. Maybe you can get same size xilinx with more logical cells?
Thanks. I have looked, there is one with a few more but not quite enough, I think respinning the board is the only option! I guess a micro would be a much more cost/space effective way to do this, but I have FPGA/verilog experience so this was the obvious choice.

Something I didn't explain earlier is that there are headers on the PCB for LEDs and switches to connect to the FPGA for controlling source selection
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post #8 of 219
1/22/08 at 8:13am
Thread Starter 
Quote:
Originally Posted by d-cee View Post
yeah looks very interesting. how's it sound?

expected cost?
It sounds great so far!

Cost will depend greatly on volume - it has a lot of quite expensive components on it, but if ordered in quantity this comes down a lot.

I have some ideas for a costed down version too:

1. Use in hardware mode only - don't need FPGA/PROM and can also get rid of extra toslink/USB circuitry.
2. Use something other than AD797.
3. I expect half of the OSCONS could be ditched without a noticeable degredaton in sound quality.
4. Use WM8740 instead of WM8741 (the WM8741 is hard to get right now anyway, but I have designed this so that both will work and so that the WM8741's digital filters can be used, though WM8741's DSD mode will not work).

I have acquired an FPGA development board so I am going to test the I2C functionality using flying wires before respinning the board, hope to get that working within the next couple of days.

Keep the questions coming!
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post #9 of 219
1/22/08 at 9:31am
What about volume control of the DACs?
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post #10 of 219
1/22/08 at 9:34am
Thread Starter 
Quote:
Originally Posted by naamanf View Post
What about volume control of the DACs?
The DACs are in hardware mode so this feature isn't available yet. But I think I might make it available on the new board revision - would this be useful? I personally prefer to use my amp as my only volume control.
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post #11 of 219
1/22/08 at 9:36am
I think it would be. Then it could also be used with speaker amp that don't have volume control and also ditch volume controls in headphone amps.
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post #12 of 219
1/22/08 at 12:17pm
You would lose bit-resolution with such a volume control, no? That is, if it's done by scaling the digital audio data...
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post #13 of 219
1/22/08 at 1:39pm
Thread Starter 
I have had a quick look at the datasheet and it is indeed done in the digital domain. Although I think I can see what you're getting at, I'm pretty sure the bits aren't truncated at all. Volume control looks easy enough to implement, only the WM8741 supports 2-wire control though, the WM8740 doesn't.
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post #14 of 219
1/22/08 at 3:45pm
Jambo, If you are or plan to start keeping a list of interested folks for a group buy once the design is finalized, I am interested, at least one, possibly 2 or 3 boards, and same for any component group buys. Thank you!

aamefford
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post #15 of 219
1/22/08 at 5:32pm
I'd be in for a board or two definitely.
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