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Kind of a weird question..

Background:
My motherboard has started producing weird static(buzzing) noises on the audio output(3.5mm jack), it's not noticeable when using headphones, but on speakers it gets quite annoying, and it gets louder as I start 3d games. This is not a problem at all if I output the sound via HDMI, to my TV and then my speakers, so I figured it should be a problem with interference after the DAC on the motherboard. In an attempt to fix this without changing motherboard I thought I could try an external DAC through USB. I tried the simple USB-3.5mm jack adapter that came with my headphones and the noise is still there. This confused me slightly as I though USB was all digital, and the adapter should have it's own DAC.

Actual questions:
Can USB ports output analog audio to these small adapters?
Or do all USB to jack adapters have a DAC?
Is my DAC/Adapter outputting noise because there is noise on the USB power supply from the motherboard?
Would a new bigger DAC with it's own power supply fix this?

I tried looking at the USB 2.0 Specification but couldnt really find anything to help me there. This is my first question on StackExchange, and I don't know if this is the right section.

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  • \$\begingroup\$ Also, this question is a better fit for Stackoverflow as a general computer question. \$\endgroup\$ – Passerby Jul 18 '14 at 22:26
  • \$\begingroup\$ I would rather look at the caps on the mobo, and if these look OK just get a new PSU. Sounds a lot like yours produces an unhealthy amout of ripple already. \$\endgroup\$ – Turbo J Jul 18 '14 at 22:38
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    \$\begingroup\$ Try plugging the speakers into another PC to disprove it's the speakers. \$\endgroup\$ – Andy aka Jul 18 '14 at 22:47
  • \$\begingroup\$ @Andyaka From original post: "This is not a problem at all if I output the sound via HDMI, to my TV and then my speakers" - They work fine when sound goes through TV. (And I have also tried them with several laptops, and they work fine) :) \$\endgroup\$ – olehermanse Jul 18 '14 at 23:23
  • \$\begingroup\$ The noise is almost certainly power supply noise, since it correlates with other system activity. If it is worse now than it was previously, it means that the bulk bypass capacitors on your motherboard are failing. \$\endgroup\$ – Dave Tweed Jul 18 '14 at 23:25
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Can USB ports output analog audio to these small adapters?

No. Well, there are cell phones with USB ports that have multiple uses, when they can output analog audio in dock mode, but that's not the case with 99% of computers.

Or do all USB to jack adapters have a DAC?

Either external or internal to it's USB IC, yes.

Is my DAC/Adapter outputting noise because there is noise on the USB power supply from the motherboard?

Most likely. Same reason the HDMI does not. One, HDMI cables are designed to avoid ground loops. Two, the TV has it's own power supply, so only the data from the HDMI connection is used. Three, the TV probably has additional audio filtering, as size is not really a concern.

Would a new bigger DAC with it's own power supply fix this?

Maybe. Theoretically it should, but in practice, YMMV.

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  • \$\begingroup\$ Thank you! Fast response, very clear and informative. I'll mark this one as accepted answer. \$\endgroup\$ – olehermanse Jul 20 '14 at 1:41
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USB is a pure digital interconnect. It provides for delivery of data through logical pipes between applications (which might actually be device drivers or system services, but logically they are applications from USBs point of view) in the host, and endpoints in a device.

It handle audio to and from speakers, headphones, and microphones with a well defined protocol, which carries the digital audio stream to the device where it is ultimately converted to analog. In practice, the audio device is likely to be a single chip that implements the USB audio protocol and has analog outputs capable of directly driving headphones. In many cases, the audio on your motherboard is actually implemented by such a chip and connected to the rest of the PC via a USB connection that cannot be physically disconnected.

Your observed problems could have a number of sources, but power supply noise is fairly high on that list. To combat that, moving the DAC outside of the PC and powering it separately can only help. So an externally connected USB audio adapter is a good first thing to try.

Audio over HDMI to your TV is digital on the HDMI cable, and converted to analog somewhere in the TV (or soundbar or receiver...). The physical protocol is completely different from USB, but the principle is the same. Do the conversion to analog outside of the PC to avoid electrical noise from everything inside the PC.

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