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?
Thanks
Salsero
Thanks
Salsero
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.
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.
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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
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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
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.
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
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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
Thanks
Salsero
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.
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.
I'm having similar trouble with a USB bus powered sound card. Switching noise from the USB power gets into the recorded signal, mostly at around 25 kHz, but also at 400 kHz, 1 kHz, and harmonics thereof.
For the record, I don't have the problem when the Onkyo SE-u33 is plugged into the USB ports of my desktop computer, but all the various notebooks and netbooks I've tried have more-or-less the same issue.
Also, if I used 16/44.1 bit recording the problem would not really show up, since all the stuff >20kHz is caught in the brick wall filter and the rest of the noise baseline is much higher from the 16 bit LSB dithering.
However I want to record in 24/96 and I want to use my notebook, so...
So far I've attacked the problem by connecting the card to the notebook via an external USB hub that has a 1.35/3.5mm DC power jack. Right now its powered by a little switching wall wart, so the switching noise is still present (but less noise overall than with the notebook USB bus power). Next I plan to replace the wall wart with a linear or battery supply.
The chart below shows the FFT spectrum of the baseline noise, with the input volume at zero (purple) and in the normal position (green), with the phono stage attached. The phono stage output noise covers up the switching noise in the audio band, but the strong signal at 25 kHz gets through. It's probably so low as to be nothing to worry about, but it bugs me all the same.
For the record, I don't have the problem when the Onkyo SE-u33 is plugged into the USB ports of my desktop computer, but all the various notebooks and netbooks I've tried have more-or-less the same issue.
Also, if I used 16/44.1 bit recording the problem would not really show up, since all the stuff >20kHz is caught in the brick wall filter and the rest of the noise baseline is much higher from the 16 bit LSB dithering.
However I want to record in 24/96 and I want to use my notebook, so...
So far I've attacked the problem by connecting the card to the notebook via an external USB hub that has a 1.35/3.5mm DC power jack. Right now its powered by a little switching wall wart, so the switching noise is still present (but less noise overall than with the notebook USB bus power). Next I plan to replace the wall wart with a linear or battery supply.
The chart below shows the FFT spectrum of the baseline noise, with the input volume at zero (purple) and in the normal position (green), with the phono stage attached. The phono stage output noise covers up the switching noise in the audio band, but the strong signal at 25 kHz gets through. It's probably so low as to be nothing to worry about, but it bugs me all the same.
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