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Old 9th October 2006, 04:05 AM   #1
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Default How does a USB DAC work and..........

How does a USB DAC work and are they any under $300? I have a tone of FLAC files on my laptop and I want to stream them to a NAD 320BEE.

Thanks
RH
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Old 11th October 2006, 03:20 AM   #2
dangus is offline dangus  Canada
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As far as I can tell, they emulate a sound card to turn USB into something like S/PDIF then drive a DAC from that. I got one for $14.99. It sounds much better than I expected for the price, and the chip has an S/PDIF output pin, so it has the potential to drive a real DAC or processor. This used a chip brand I'd never heard of.

The Burr-Brown/TI chips have much better specs:
http://focus.ti.com/docs/prod/folder...t/pcm2706.html

Search on those part numbers and you'll find all kinds of stuff (products, DIY projects).

You could pick up the ADStech "Instant Music" to experiment with; it allegedly uses one of the DAC/ADC versions of the TI/BB USB chips, and has both analog and Toslink inputs and outputs:
http://www.adstech.com/products/RDX-...sp?pid=RDX-150
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Old 11th October 2006, 11:18 AM   #3
dangus is offline dangus  Canada
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Some notes I made earlier on USB ADC/DAC stuff, possibly too much information, but anyway:

PCM2702: USB1.1 DAC, no S/PDIF, but better S/N ratio. Under $12 at Digikey and in stock.
PCM2704: USB1.1 DAC with S/PDIF and analog out, used by http://www.hagtech.com/hagusb.html (schematic is there)
PCM2902: USB 1.1 with S/PDIF, 89 dB as ADC, 98 dB as DAC (and has some HID key inputs for vol up/down and mute). (possibly used in the ADSTECH box or maybe the PCM2906)
PCM2906 - USB ADC/DAC from TI, in stock at Digikey, similar to '2902 and is used in http://www.hagtech.com/ripper.html
also in http://us.hifidiy.net/Article.asp?ArticleId=181

"The Source" (formerly Radio Shack Canada) had a "Nexxtech" thumbsized USB headphone/speaker adapter for $15, part number 2616365.
Comes with disk, although it appeared to be mostly for 3D effects, since Windoze recognizes it as a standard USB audio thang. It has limited volume control ability; at 50% it was almost completely muted. It sounds pretty good through the MDR-V900 headphones.

Inside, board says UD-302 on underside, ver 1.0.
Output is through 6.3V 220 uF electrolytics.
Chip is CM102S, 18 pins with a 12 Mhz crystal.

Google reveals this chip is from Cmedia, who designed it for powered speaker applications. It has an LED output pin during play, S/PDIF output on pin 17. S/N is very good, but THD + N isn't, and gets worse with lower impedance loads.
Data sheet: http://www.cmedia.com.tw/files/doc/U...et%20v1.01.pdf
Searching on UD-302 revels that Juster spawned it: http://www.juster.com.tw/product/usb/ud302.htm
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Old 11th October 2006, 04:52 PM   #4
dangus is offline dangus  Canada
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I just looked inside the ADSTECH "Instant Music" box: it IS the PCM2902E. Build quality looks pretty good (Taiwan, not China); the chips are all major brands.

Probably the best sound for your $300 is buy something like the "Instant Music" for $50 to get S/PDIF out and feed that to whatever second-hand external DAC you can get for $250.
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