Anyone interested in a general purpose DIY SPDIF/USB/ADC/DAC interface board using th - diyAudio
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Old 24th July 2010, 01:56 PM   #1
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Default Anyone interested in a general purpose DIY SPDIF/USB/ADC/DAC interface board using th

(subject was cut off: General purpose DIY SPDIF/USB/ADC/DAC interface board using the PCM2906b)

I've been lurking these hi-fi audio forums for a while...

Here's my problem: I have a GT-8 guitar effects pedal that has an SPDIF output but I'm using a laptop with no SPDIF input. I don't want to pay $100 for a USB/SPDIF audio interface so I've decided to build my own. Looking around, this has been done already, but in the other direction: ecp.cc

But this chip has so many features, like a DAC and ADC. Why not make a general purpose board that can make use of them? So I plan on building a board based off these components, but add in space on the PCB for:

1) SPDIF output
2) SPDIF input
3) make use of the on-chip DAC and put a headphone output from a headphone amp IC. With space for a variable gain pot.
4) make use of the on-chip ADC and put in an input. For my purposes I'd design it for a 100mV to 1V rms signal (guitar pickup) or from a mic that doesn't need voltage biasing (1mV signal). Variable gain pot needed here.

Of course, this board won't be the smallest (not important for me) or perform any of these functions the best (but well enough). However, it will have all these options available and customizable and be good enough for entry-level audiophiles. If you don't need any feature, just leave those items unpopulated.

I envision being able to use it as SPDIF -> USB when recording from my effects pedal, as a high quality mic -> USB, or as a high quality USB -> headphones, or as USB -> SPDIF for those who need it. As long as the USB driver supports these features (the only thing I'm not too sure about)

So would anyone else be interested in these boards? They will be very cheap. And the IC's can be obtained as free samples from TI, saving on a lot of the cost.

I believe there's enough like-minded people that would have a use for one or more of these features. If there's enough interest, I can probably sell the boards at $10 - $20 depending on how many I buy at once. Shipping costs will be $5. Also, if people think it's a good idea, I can offer customizable kits that come with only the parts/jacks that are needed for the features you need.

(cross-posted to head-fi)

Last edited by allanw; 24th July 2010 at 02:23 PM.
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Old 24th July 2010, 02:15 PM   #2
Pano is offline Pano  United States
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Welcome to the forums.
For something like this to fly on this forum, you'll need to provide the schematic and an image of the PCB, at the very least. If this is not a project you wish to discuss in much detail, then we can move you to the Vendor's Bazaar where you can offer boards for sale.

Have a look around at the other projects on this forum. There is a healthy discussion of the circuits used, choice of parts, etc. You are off to a good start above, but please be prepared to provide a LOT of detail about the project. A schematic is the best place to start.

Best of luck!
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Old 26th July 2010, 09:07 PM   #3
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This sounds like the sort of thing I'm after myself. Although I'd be more interested if it had more input channels. Would this be a possibility?
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Old 26th July 2010, 09:12 PM   #4
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That's probably not easily done with this chip. While you can multiplex the ADC, the USB driver wouldn't know how to separate the input channels and present them as multiple recording sources.

Anyway, I was seeing if anyone else would be interested in it, so I could know if I should just get a few boards or 50. Once I make a schematic I'll share it and see what people think and then share my board layout.
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Old 27th July 2010, 08:47 AM   #5
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The ADSTech "Instant Music" comes fairly close to that. It uses the PCM2902E, does line level analog and Toslink S/PDIF in and out. It's got low-pass filters on the analog inputs and outputs, just like the evaluation board that TI sells for the PCM2902. There's an unpopulated connector location on the board that appears to go to the input pins on the PCM2902 for HID key inputs for vol up/down and mute. The external look of those units has changed since I bought mine in 2006, so it's possible that ADSTech has also changed what's inside.


The Behringer UCA202 uses the same chip, and has a headphone jack, but IIRC no digital in. And according to a RMAA comparison someone did, it performs a bit worse than the ADSTech, possibly due to poor power supply bypassing or regulation. Behringer has produced several variations on this, like one that includes a phono preamp.

Anyway, what's my point? If you just need one versatile USB thing, the ADSTech is pretty good. Lack of a gain control on the analog in is a drawback, and headphone and RCA digital I/O would be nice as well.

So, I would be interested in a board that let me build something like the ADSTech for a lot less money. And, if you're adding a preamp, how about using a layout that could allow using a phono (RIAA) preamp instead if desired. Maybe one that happens to look a lot like the Hagtech Bugle, or the venerable UREI 1620 phono stage (but using two dual opamps instead of a quad).
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