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Old 29th April 2009, 08:44 AM   #1
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Default USB DAC isolation ?

Hi there.

I was wondering if somebody could share a great idea of how to galvanically isolate an USB 2.0 DAC, just by introducing a selfpowered DIY-adapter.

I am currently using an Opticics fibre optics cable, which does quite a good job.

The 2nd big advantage of the Opticis: A regulated 5V on the secondary allows to supply USB
powered DACs very easliy with clean power. (This should be also possible with the new solution)

The Opticis does USB 1.1 only and is pretty expensive.


THX
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Old 3rd May 2009, 08:01 AM   #2
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Ping.

Nobody ever tried it?
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Old 3rd May 2009, 08:17 AM   #3
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what about jitter introduced by optic transmitter? it may be added to the already very jiterry standard usb protocol doing more harm than good...
i think this should be done in another approach,using asynchronous usb transfer by putting low jitter clock near the DAC and slaving the PC to it just like wavelength company does and after that isolating the modern 24/96 delta-sigma DAC from the following equipment by using the benefit of galvanic isolation of the output audio-transformers attached after the DAC (with no OP-amps or any discrete amplification stages,because these delta-sigma Dacs already have a 2,5V output)

just my 2 cents
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Old 3rd May 2009, 08:32 AM   #4
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Hi there.

1. I am using asynchronous DACs. An M-Audio Transit (async at 44.1 and 48) and an EMU
0404 USB.
On my M-Audio Transit the Opticis is doing a great job.

2. I also used the Opticis on my PCM2707 based receiver. I wouldn't go without it.

Cheers
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Old 3rd May 2009, 08:50 AM   #5
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hmm i also used to have an M-audio transit but hated the sound coming out of it so i sold it
(maybe the AKM dac implementation followed by a unity gain buffer Op wasn't so good)
but i don't think it used asynchronous protocol over USB - are you sure about this info?
i mean, the guy from wavelength has spent 900+ hours writing it's own code for the chip for this implementation selling these new concept DACs at 7500$ (as i saw on their webpage) and m-audio selling the transit with 80$?
if it would be the case,then it's worth trying to tap the I2S signals from M-audio and drive some high-end DAC with them along with some good analog stages...
but again,i don't think it's the case for that because such an implementation has a good quality low jitter clock right beside the dac powered from a very clean PS and the PC is slaved back to it
(i have opened the transit and no such thing like a master clock beside the dac,it even uses that noisy 5v from PC's usb to power the Dac)
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Old 3rd May 2009, 09:15 AM   #6
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I did tweak the Tranist of course:

* 5V regulated supply
* Blackgate cap (for 3.3. regulator)
* Removed output caps
* Opticis USB cable
* AnalogOut opamp Maxim MAX4477

90% of the sound detoriation is usually caused by the peripharals and not the DAC chip itself.

What you pay for at Wavelength are the peripharals and the look.
I doubt that it takes 900hours for an experienced coder to get a bit of software in place.
The next project Gordan will probably do in 150hours.Of course there is a lot of marketing behind it. EMU and M-Audio won't even talk about it.


See below the stream parameters under Linux while playing back ( /proc/asound/card0/stream0). You can see that the interface plays in async mode up to 48khz.


Code:
M-Audio Transit USB at usb-0000:00:1d.2-1.3, full speed : USB Audio

Playback:
  Status: Running
    Interface = 1
    Altset = 3
    URBs = 2 [ 1 1 ]
    Packet Size = 196
    Momentary freq = 44100 Hz (0x2c.199a)
  Interface 1
    Altset 1
    Format: 0x20
    Channels: 2
    Endpoint: 3 OUT (ADAPTIVE)
    Rates: 48001 - 96000 (continuous)
  Interface 1
    Altset 2
    Format: 0x20
    Channels: 2
    Endpoint: 3 OUT (NONE)
    Rates: 8000 - 48000 (continuous)
  Interface 1
    Altset 3
    Format: 0x2
    Channels: 2
    Endpoint: 3 OUT (ASYNC)
    Rates: 8000 - 48000 (continuous)
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Old 3rd May 2009, 09:35 AM   #7
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those linux logs mean nothing...this is what i mean in terms of asynchronous usb transfer (and I'm sure M-audio doesn't use this) - please read Crowbar's explanation from following link

http://www.head-fi.org/forums/f46/as...b-dacs-271356/
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Old 3rd May 2009, 02:07 PM   #8
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Quote:
Originally posted by luxury54
those linux logs mean nothing
You seem to be the Linux expert here. Would be nice to hear your own explanation.
I am always happy if I can gain some expert knowledge.


Not to forget:

This thread is actually about isolating USB 2.0 up to 480MBit/s and not about USB modes.
Any expertise in this area?

Cheers
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Old 26th August 2009, 10:26 PM   #9
jkeny is offline jkeny  Ireland
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I know I'm reviving an old thread but I was looking into USB galvanic isolation & this is the only thread that mentions it.

The Opticics is too costly for me & I wondered if it (or something similar) could be done? What's inside the Opticics? How does it work? Any DIY versions?
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Old 27th August 2009, 01:30 AM   #10
wwenze is offline wwenze  Singapore
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Optocoupler, think of SPDIF through optical vs coaxial. Think of the associated jitter with optical too.

I did have a DIYed cable that takes power from an wall-wart instead of the computer, for use with a USB-powered DAC. If only signal is required from USB, it's as easy as taping two pins on the connector. Then get the power from somewhere else.
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