USB DAC project

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ukram

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Joined 2003
hepso.dna.fi/misc/Ukram_Data.html
#1
I'm planning to build USB DAC, but there are some problems:

I ordered PCM2702 samples from TI (they ship them immediately by UPS), so i guess i'll go for it, although the SSOP28 package seems really small, which is a problem for PCB making and soldering.

I also tried to order Philips UDA 1321 samples, but so far no response to my email. Where could i get them? They are in SO28 or SDIP32 package, which are easier to handle.

One way would be just buying cheap USB DAC product (Xitel hifi link), but nobody sells them in Finland.

Is there any other possiblities?

I have Terratec 5.1 Sky sound card (with spdif output), so one possibility is to use SPDIF compatible DAC. Any ideas here?
 
#8
mwp:
Hmm... would be nice if they now came up with a 96Khz/24bit version using USB2.

I do not have any 96kHz source. Children play games, we watch mivies and listen to CDs. All are 44.1 and 48kHz sampling rate.

Rookie:
How do you like the sound of the PCM2702?

I love it. And is not tweeked in any way. It has 7805 regulator, which could be improved, chipest caps you can get. But you have to try it without output amplifier and filter. Probaj, nece ti bit zao.
 
#15
Re: Re: USB DAC project

planet10 said:
When i have considered this, i've always thot i'd start with a Griffin iMic which is a killer product in its own right. Add a power supply and a tube analog stage ...
Click to expand...


It is very good, I have had one for about a year. The software (Mac only) Riaa eq works really well too. But, you only get full functionality of the hardware under XP on the PC, it doesn't work too well with 98/Me or 2000.
 
#17
Well, i have a PCM2702 working now. It felt great when my win2k PC finally recognized the USB device, and automatically loaded drivers 🙂

I lifted every other pin of the SSOP chip up and soldered only half of the pins directly to PCB and connected the lifted ones with wire-wrap wire. Wire has to be thin to be flexible so that the pins don't bend.

I used my sound card and RMAA to measure, got about 88 db S/N ratio when using external +9V supply and +5V regulator (maybe it is better, and sound card is the limit).

I noticed that using USB +5V gave worse results, about 83 dB S/N ratio.

I'll try to use PCM2902, because it probably works better with USB power. Low dropout regulator (LM1117T-ADJ) can be used to get the regulated 3.6V needed for operation.

I'll try to do some kind of SSOP28 to DIP28 adapter to make things easier...

The PC ground noise disappeared when i "isolated" the analog ground from USB digital ground with 10 ohm resistor. This is what they should do in PCI sound cards...

About "duplex": PCM2902 has input and output (analog and SPDIF). The drivers support these as fas as i know.

http://hepso.dna.fi/misc/pcm2702/2702_top.jpg
http://hepso.dna.fi/misc/pcm2702/2702_bottom.jpg
 
#19
Got the PCM2902 chips from TI. Made a little adaptor PCB for SSOP28/DIP28 conversion. Soldering was like repairing a wristwatch. I used 5X magnifying lense and my nose was very close to the soldering iron...

http://hepso.dna.fi/misc/pcm2902/pcb_top.jpg
http://hepso.dna.fi/misc/pcm2902/pcb_bottom.jpg

2902 has internal 3.3 V regulator for DAC and ADC power, but an external regulator (3.6-3.85 V) can be used to get best performance. The TI example circuit uses REG103. It has low noise, so maybe the LM1117 is not the best possible choice.
 
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