Modding USB cable to power DAC with external PSU - diyAudio
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Old 8th April 2013, 11:05 AM   #1
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Default Modding USB cable to power DAC with external PSU

Hello all.
My DAC sounds fantastic through its SPDIF input but when driven through USB the sound quality strongly depends on the associated computer.
I know it should not happen with async USB but the difference is clearly audible.
With my laptop (battery powered) it sounds great, with one of my Mac Minis it sounds acceptable and with my newer Mac Mini with integrated SMPS it sounds awful.

As my DAC's USB receiver is powered by the computer and my HiFace EVO SQ clearly improved when I replaced the cheap PSU for a nice regulated supply I began to investigate the ways to power the USB receiver chip externally. I tested the DAC with a Placid supply and usb power injector cable but it did not work with any of the Macs. The DAC USB led always lights up but the computer does not recognize it.
I then modified an USB Female B to Male B adapter. Isolated the +5v coming from the computer and soldered the Placid cables to pin 1 and 4 at the USB male end leaving pin 4 connected to both ends and it did not work either. Would it be possible that the computer +5v needs to be terminated in a certain way or cannot be simply cut because it is used to negotiate with the USB device? Or should the external PSU be connected to pin 1 and ground instead?
I searched all over the forums and it seems that there is no consensus on the right way to do this.

Before going for a commercial solution e.g. the AQVOX USB Supply or purchasing a High End USB Hub I would like to have a clue of what is really needed to power an USB DAC externally in a manner that the DAC is recognized by any Mac.

Any help will be greatly appreciated.

Rod
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Old 8th April 2013, 07:15 PM   #2
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Quote:
Originally Posted by rodelius View Post
I searched all over the forums and it seems that there is no consensus on the right way to do this.
USB supports both self and bus powered devices and provides specific timings for device enumeration in response to cable attach or bus power up/down and reset events---refer to the USB specifications for details. From your description I would guess the power splicing is breaking device enumeration. You might be able to work around this by slaving the external supply to the bus supply but there are good reasons the USB Implementor's Forum does not certify extension cables, power breakouts, and so on as USB compliant.

Assuming the DAC is operating as an asynchronous USB Audio 2.0 class device as expected you're probably better off leaving the USB interface bus powered and focusing on noise management around the A/D supplies and ground bounce. Hard to say much meaningful without a schematic of the DAC and measurements showing what the problem is---"sounds awful" is many things but precise is not one of them.
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Old 8th April 2013, 09:10 PM   #3
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It's possible that the timing of the power connection might be making some difference and if you plug it into the computer and then turn on the power, or vice versa if this is what you're doing already, it may work.
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Old 9th April 2013, 07:59 AM   #4
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Quote:
Originally Posted by twest820 View Post
Assuming the DAC is operating as an asynchronous USB Audio 2.0 class device as expected you're probably better off leaving the USB interface bus powered and focusing on noise management around the A/D supplies and ground bounce.
DAC is a commercial unit for which I don't have any shematics. I assembled several DACs but I don't feel comfortable fiddling with this DAC or with the computer.

For some unknown reason the Mac cannot be fooled or I am doing something wrong.

As there are several commercial units that do what I am trying to do (AQVOX ifi, I will keep investigating this route.

Thanks!
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Old 9th April 2013, 08:01 AM   #5
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Quote:
Originally Posted by counter culture View Post
It's possible that the timing of the power connection might be making some difference and if you plug it into the computer and then turn on the power, or vice versa if this is what you're doing already, it may work.
I tried that. It did not work
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Old 19th April 2013, 11:43 PM   #6
phase is online now phase  United States
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I believe that there needs to be a 220 ohm 1/2 watt resistor on the host(mac) side, between the usb +5v supply and -5v. Pretty sure that the chassis ground/connector shell is not included in this operation. This tells the mac that there is a load/device, and to look further at that port for data.

I've done this with automotive computers to allow modern engines to be used in older cars that don't have the features, sensors that the new cars have(solenoids in the transmissions, roll-over fuel shutoff, etc). Same action, just canbus instead of USB.

I'm about to jump into this myself, but with an old PC. I want to expirement with different wire for the signal, since it's pretty apparent that the wire itself acts as a filter. Even in the recording studio forums there are rampant "discussions" on that topic.
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Old 20th April 2013, 03:21 AM   #7
jerry g is offline jerry g  Uzbekistan
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here is a great link...

2 Channel Audio : DIY - Solid core silver audiophile USB - Yes your cable makes a difference

What I did was modify a generic 2 foot long USB extention cable with a female and male end for multi purpose playing.
I broke the + line (red) and connected a 5volt switching supply + to the far side of the cut red lead that goes to the output plug at pin 1. I connected the - of the 5 volt switching supply to the - output (black) at pin 4 without breaking the line (scraped off the dielectric)
I didn't connect a diode and resistor. It works into my $42 hifimediy usb/dac from my laptop, but it doesn't work from my PC to my U link.

I might go buy a diode and resistor and try to fool the output from the computer, but the Bel Canto ulink is so great sounding by itself, why bother?
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Old 7th February 2015, 04:44 PM   #8
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Quote:
Originally Posted by jerry g View Post
... I might go buy a diode and resistor and try to fool the output from the computer, but the Bel Canto ulink is so great sounding by itself, why bother?
Hi do you mean that you are using the Bel Canto connected only to the PC usb port ? if so it means that it is very well designed and built because power on usb is bad quality.
But it could sound even better with a solution like this one ... i am not sponsoring it just describe what i have in mind now
Click the image to open in full size.
it is clear that the better the quality of the +5VDC the better the performance.
Problem is that i would just need the piece in the attachment
because i have already a high quality +5VDC power supply ...
Any idea ? thanks a lot and kind regards, gino
Attached Images
File Type: jpg Capture.JPG (9.8 KB, 163 views)

Last edited by ginetto61; 7th February 2015 at 04:52 PM.
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Old 7th February 2015, 04:55 PM   #9
Julf is offline Julf  Europe
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Quote:
Originally Posted by ginetto61 View Post
it is clear that the better the quality of the +5VDC the better the performance.
Is it?

Quote:
Problem is that i would just need the piece in the attachment
because i have already a high quality +5VDC power supply ...
Any idea ? thanks a lot and kind regards, gino
Get a cheap USB cable, cut it at one end, and solder it to your PS.
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Old 7th February 2015, 05:12 PM   #10
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Quote:
Originally Posted by Julf View Post
Is it?
Hi and thanks for the reply
Well i think because the quartz inside the dac/converter usually benefits from a clean and stable supply (i am guessing here)
Moreover i read many reviews of these devices and usually the conclusions are pretty positive.
So yes i think they are beneficial

Quote:
Get a cheap USB cable, cut it at one end, and solder it to your PS.
Looking at the cable structure in the attachment do you mean that i can connect the red to the +5V and the black to 0 from the power supply ?
Just that ? i will try for sure
Thanks a lot. Kind regards, gino
Attached Images
File Type: jpg usb cable structure.JPG (31.8 KB, 153 views)
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