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Old 27th December 2009, 12:40 AM   #1
Salsero is offline Salsero  United States
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Smile USB cable BUS modification

I just bought a USB to Coax converter to connect my lap top to my DAC. Based on some feedback, I would like to use a regulated power supply to feed the converter instead of the USB BUS. How do I modify the USB cable to achieve this? IS there a color code for the +5V line? Is 500mA a sufficient supply current max?
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Salsero
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Old 27th December 2009, 06:49 AM   #2
wwenze is offline wwenze  Singapore
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Here you go:

USB pinout and wiring @ pinouts.ru
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Old 9th January 2010, 01:07 PM   #3
qusp is offline qusp  Australia
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the pinout is not enough for this, as the host will try to 'ping' the device on the 5V line to check it is there. without the 5V line connected no audio will be sent to the device.
there are ways around this though Analogue devices makes a chip for isolating the USB power supply the ADuM4160 which can be used in circuits like this and only this will allow you to add your own higher quality power supply. before this device I made a couple of cables simply splicing power into the +/-5V supply lines with a DC plug on them, but it doesnt work, the device isnt recognized (much to my dismay) but the PCb linked is quite cheap as is the fully built unit if you wish an out of the box solution.

Last edited by qusp; 9th January 2010 at 01:13 PM.
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Old 9th January 2010, 02:26 PM   #4
Salsero is offline Salsero  United States
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Thanks, very useful feedback. Definitely more complicated than splicing cables.
Cheers
Salsero
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Old 9th January 2010, 08:12 PM   #5
qusp is offline qusp  Australia
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No problem, yeah it is unfortunately. the 'ping' is made when the host wants to negotiate the speed, full speed, high speed, USB 2.0 etc. devices that have their own power supply for the usb receiver/dac dont always need it and the power wires can be omitted in this case. I have never struck one though, because of the reasons I mention above. even in devices that have their own power often still pull the power for the usb codec/receiver/i2s from the 5v line

Last edited by qusp; 9th January 2010 at 08:18 PM.
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Old 9th January 2010, 08:23 PM   #6
Salsero is offline Salsero  United States
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If I can't use an external supply the easy way (i.e. splicing cables, etc...), would a cap or other circuitry help clean the PC power source?
Cheers,
Salsero
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Old 9th January 2010, 11:03 PM   #7
wwenze is offline wwenze  Singapore
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Quote:
Originally Posted by Salsero View Post
If I can't use an external supply the easy way (i.e. splicing cables, etc...), would a cap or other circuitry help clean the PC power source?
Cheers,
Salsero
Many use an RC or LC filter.

But try the splicing cable method first if you can, it worked for my PCM2702 DAC.
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Old 10th January 2010, 01:35 AM   #8
qusp is offline qusp  Australia
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thats because that chip is really only a USB receiver that has a rudimentary dac function, but sure give it a try its a cheap solution

Last edited by qusp; 10th January 2010 at 01:40 AM.
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Old 10th January 2010, 12:21 PM   #9
Salsero is offline Salsero  United States
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The same must then hold for the PCM2902 - the chip used in the USB to SPDIF converter I am using. Please confirm.
Thanks
Salsero
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Old 10th January 2010, 01:05 PM   #10
star882 is offline star882  United States
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I have made a simple adapter to supply more current to USB devices by simply rewiring the output power connection to a regulated 5v wall wart. Everything I have used it on work just fine.

However, you're not going to get any isolation since the grounds will still need to be connected for data. Use an optical connection for that. On the digital side, reducing noise is not going to do anything if it is operating properly in the first place. USB to S/PDIF adapters have built in voltage regulators anyways.
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