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Old 7th February 2004, 10:32 PM   #1
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Default USB noise (noisy ground)

I just completed my USB to I2S converter and hooked it up to my dac.

With the USB pluged in to the computer there is noise, which follows the volume setting on my preamp, volume setting on the computer make no difference.

The noise is there with and without playing anything.

It isn't alot, but just knowing it is there annoys me.

Unpluging my USB thingy at the computer end, removes the noise completly.

I know there is a pretty big difference in ground from my system to the computer. I used to use the soundcard out to my preamp, and since my preamp only have one input I had to play it often and you can feel the difference with your fingers.

So I guess the noise is because of the huge ground difference, but what can I do about it?

I could use digital traffos, but with the I2S signal I have to use 3, which properly is the best option, but what about a quick fix of some sort?
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Old 8th February 2004, 03:47 AM   #2
MWP is offline MWP  Australia
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Annoying isnt it?

Just about the only thing you can do is isolate the I2S with transformers or opto

You could look at improving grounding schemes, but it wont remove all the noise
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Old 8th February 2004, 07:34 AM   #3
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I just tried measuring the ground difference and it is indeed huge, 102V AC, no wonder I could feel it.

I'am gonna buy some isloaltion traffos.
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Old 8th February 2004, 07:36 AM   #4
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I am interested to build usb to I2S converter too.
What chip did you use?
Would you like to post schematic?
Beside the noise what is your impression comparing with conventional CD player?


Thanks,
ticovski
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Old 8th February 2004, 08:15 AM   #5
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I've used a pcm2707.

Did what it said in the datasheet, USB bus powered and I2S out, (started with SPDIF) schematic on the last two pages.

Since I don't use the interal DAC or opamps, I have left pin 26-31 open, the same goes for pin 4,6,7,8,18.

Apart from the size, it is very easy to do.

I used to use the cdrom in my computer as transport (taking SPDIF directly from the drive).

I haven't done any comparings yet, because I know the noise is there and is bugs me big time (especaly now that I know it is the sound of 102V AC)

But I can only imagne that it is better than that hated SPDIF.
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Old 9th February 2004, 02:57 AM   #6
MWP is offline MWP  Australia
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I used a PCM2902.

All the details (schematics & pcbs) are here:
http://www.overclockers.com.au/~mwp/dac3
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