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usb cable , external powered 5v adapter , workable?

post #1 of 12
4/6/10 at 7:48am
Thread Starter 
usb cable , external powered 5v adapter , workable?

I saw one of the thread, someone is able to cut the 5v supply of the usb cable, and attached an external 5v, so this power supply will be a clean source to the DAC

I have tried it and it doesnt work, what am I missing?
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post #2 of 12
4/6/10 at 8:13am
You likely have a sequencing issue. USB power may be used to indcate connection to the host.

Does it work if you:
Attached the cable to your PC
Attach the cable power to the wall
Attach the cable to your device
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post #3 of 12
4/6/10 at 9:41am
Thread Starter 
the pin connected from pc to dac is only the USB Data - and USB Data +

the 5v and gnd cable is disconnected, and connected to a 5v external source. this is connected to the dac
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post #4 of 12
4/6/10 at 9:59am
I understood your cable.

What happens if you connect it to your devices in the order I specified.

That is, attach cable to PC, attach cable power to wall, attach cable to DAC.

If the DAC uses something like a PCM2707 as the USB receiver, it uses the power down to USB cable to know when to start communicating on the USB. If you have power on the cable before it is attached to the PC you may have a problem.
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post #5 of 12
4/6/10 at 10:38am
Thread Starter 
good point, I will try it out
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post #6 of 12
4/6/10 at 12:17pm
Quote:
Originally Posted by alamakazam View Post
usb cable , external powered 5v adapter , workable?

I saw one of the thread, someone is able to cut the 5v supply of the usb cable, and attached an external 5v, so this power supply will be a clean source to the DAC

I have tried it and it doesnt work, what am I missing?
I hope cobaltmute's suggestions work, but you've entered into the law of unintended consequences, I'm afraid.

If it's the thread I'm thinking about, the general consensus (of those in the know) was that more than likely even a tiny DAC would have onboard regulation that would make switching out the USB power moot. Note that the most vociferous poster in that thread against USB-power was not a DIY-er and was throwing out much useless information and bad references. My concern was always that some fledgling DIY-er would attempt an experiment such as this with perhaps not-so-good results.

A DAC is often designed specifically for the power requirements intended. If USB power, then more often than not, specific regulation is included to clean up the USB power (which is often not as bad as many are led to believe). Further, if you apply separate power supplies into an existing design, you need to be well aware of the variables in play. For instance, what kind of walwart are you using? If unregulated, you may be feeding 6-9VDC into the very light load of a tiny DAC. That could burn up the regulators or worse - perhaps fry the DAC if the regulation is internal. If a switching supply, it might throw out more trash than the USB power.

Just some things to think about. I hope cobaltmute's suggestions work out for you.
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post #7 of 12
4/6/10 at 12:52pm
Quote:
Originally Posted by alamakazam View Post
the pin connected from pc to dac is only the USB Data - and USB Data +

the 5v and gnd cable is disconnected, and connected to a 5v external source. this is connected to the dac
The gnd needs to stay connected between your DAC and your PC. The DAC needs a reference point or it wont work...

So you need to leave the GND connected and splice in the PSU to that line.
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post #8 of 12
4/8/10 at 12:42pm
Thread Starter 
Quote:
Originally Posted by bou View Post
The gnd needs to stay connected between your DAC and your PC. The DAC needs a reference point or it wont work...

So you need to leave the GND connected and splice in the PSU to that line.
so thats what I am missing
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post #9 of 12
4/8/10 at 12:44pm
Thread Starter 
Quote:
Originally Posted by tomb View Post
I hope cobaltmute's suggestions work, but you've entered into the law of unintended consequences, I'm afraid.

If it's the thread I'm thinking about, the general consensus (of those in the know) was that more than likely even a tiny DAC would have onboard regulation that would make switching out the USB power moot. Note that the most vociferous poster in that thread against USB-power was not a DIY-er and was throwing out much useless information and bad references. My concern was always that some fledgling DIY-er would attempt an experiment such as this with perhaps not-so-good results.

A DAC is often designed specifically for the power requirements intended. If USB power, then more often than not, specific regulation is included to clean up the USB power (which is often not as bad as many are led to believe). Further, if you apply separate power supplies into an existing design, you need to be well aware of the variables in play. For instance, what kind of walwart are you using? If unregulated, you may be feeding 6-9VDC into the very light load of a tiny DAC. That could burn up the regulators or worse - perhaps fry the DAC if the regulation is internal. If a switching supply, it might throw out more trash than the USB power.

Just some things to think about. I hope cobaltmute's suggestions work out for you.
battery is my power supply, I do understand my dac chip is working 3.3 to 6v
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post #10 of 12
4/8/10 at 2:56pm
Quote:
Originally Posted by alamakazam View Post
battery is my power supply, I do understand my dac chip is working 3.3 to 6v
My apologies, but there was a key word in your first post: "adapter." Good luck on finding your issue.
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post #11 of 12
4/8/10 at 3:27pm
I have built a cable like that. I used it for an external HDD, and it worked really good, but you have to connect the ground to the USB ground and the device's ground, and you should connect the +5V only to the device's 5V supply. I don't know if it will catch noise or not, but you should build a well shielded cable for that. I hope I could help.
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post #12 of 12
4/8/10 at 6:16pm
Quote:
Originally Posted by cobaltmute View Post
I understood your cable.

What happens if you connect it to your devices in the order I specified.

That is, attach cable to PC, attach cable power to wall, attach cable to DAC.

If the DAC uses something like a PCM2707 as the USB receiver, it uses the power down to USB cable to know when to start communicating on the USB. If you have power on the cable before it is attached to the PC you may have a problem.
While it would take a little more work and would require a PCB or piece of protoboard, you could use the USB's power to trigger a relay that connects the external power supply which would solve this issue.
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