USB DAC isolation ?

[soundcheck]

Originally Posted by eclectic2k

the answer:
http://www.circuitsathome.com/mcu/usb/usb-isolator

from a posting by cuibono, here:
http://www.diyaudio.com/forums/pc-based/151844-usb-galvanic-isolation-post1927483.html

Unfortunately you won't get around the 12Mbit limitation!
I've been in contact with AD earlier about it.

If you get along with the limitation, it can be more attractive than the Opticis or Rover.
Price wise it is obviously the winner.

[soundcheck]

Did I mention this link before. You'll get some background info.

[eclectic2k]

Originally Posted by soundcheck

Unfortunately you won't get around the 12Mbit limitation!
I've been in contact with AD earlier about it.

If you get along with the limitation, it can be more attractive than the Opticis or Rover.
Price wise it is obviously the winner.

Gotcha. I was mistaken, or a bit off...

I thought there were some higher speed options, but it does look like they are not fast enough for USB high speed, and apparently they are not available in the bidirectional versions as I had thought. I bet it won't be too much longer before they can make it work.

for my review, 2 questions:
- I don't know of much 24/192kHz material. You really can't live with 24/96 and 12MBps?

- isolating the I2S signals *after* the USB controller is not acceptable? This way the analog output is still effectively isolated. (and can be done with current ADuM parts) This seems like a good solution to me? And is much like the SPI/I2C examples mentioned in some of the docs? No?

[soundcheck]

Originally Posted by eclectic2k

Gotcha. I was mistaken, or a bit off...
- I don't know of much 24/192kHz material. You really can't live with 24/96 and 12MBps?

For sure you can.

98% of the tracks out there are 44.1/16 anyhow,

Originally Posted by eclectic2k

- isolating the I2S signals *after* the USB controller is not acceptable? This way the analog output is still effectively isolated. (and can be done with current ADuM parts) This seems like a good solution to me? And is much like the SPI/I2C examples mentioned in some of the docs? No?

I agree. I don't think it really matters at what point you decouple. You just need to do it.

I am wondering what Ayre is doing. They do optical decoupling.

[jkeny]

Ok, I'm back to this topic again as I have hiss on my set-up. I put a scope on the output from my USB DAC, which has a PCM1793 DAC with differential Vout through a transformer. I attached the scope leads to the secondary side of the transformer & I see about 20 to 25 uS peak-to-peak waveform at about 8mV height coming from it but ONLY when I connect the USB cable to my PC (no music playing). I can power the DAC separately from an external supply & the scope is relatively clean when it is just powered on but as soon as I connect the ground through the shield of the USB cable, I get the waveform.

I have powered the Laptop (Dell Inspiron 6000) through an ungrounded SMPS & it makes some difference but I didn't note it as it's late here & these were quick test before bed.

So it looks like a different laptop is needed or better a USB isolator like the ADUM (I know it won't do 24/192 but clean 16/44 is better than dirty 24/192)

I'll do some more tests tomorrow but first a question:
- is there not a difference between doing the de-coupling at the USB end rather than the I2S end, in that jitter could be introduced into I2S but it's immune to it on the USB side?

[maxlorenz]

Hi Jkeny,
Did you try battery powered laptop to see if the pick is there?

Adum4160 evaluation board has a delay or is in backorder at Digikey...maybe a couple of months delay...

[jkeny]

Originally Posted by maxlorenz

Hi Jkeny,
Did you try battery powered laptop to see if the pick is there?

Adum4160 evaluation board has a delay or is in backorder at Digikey...maybe a couple of months delay...

Unfortunately, my laptop battery is gone & I'm reluctant to buy another one - I just might buy a new nettop instead! I'm going to find somebody with a laptop with good battery & I'll try that. The SMPS could well be the issue!

I order a $14 board + ADUM4160 from here: http://www.circuitsathome.com/store

[maxlorenz]

Thanks for the link. Please report about the performance of that module.

M

[2A3SET]

By following the recommendation, now I can get HDMI connected to HTPC(using TV as monitor) at same time I am listening to the USB 1543 DAC. I used to have pretty strong hum and hiss because of the HDMI connection. After the modification, with HDMI connected, it even sounds better than before without HDMI.

Thank you!

[jkeny]

Sorry guys, I've nothing to report on the ADUM board as I haven't built it yet - it won't work with high-speed USB so I had no need!

2A3SET, what modification did you apply?