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USB noise or ground loop?

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drummerdimitri:
I have a USB DAC/Headhpone AMP that also supports TOSLINK connection and with very sensitive IEMs on USB I can hear a lot of nasty noise when no music is playing at any volume level.

When I switch over to the TOSLINK connection, all that noise goes away. I have found two devices that eliminates USB noise by filtering or by disconnecting the 5V rail since my DAC is mains powered.

The only issue is I'm not sure how to differentiate a noisy USB connection vs a ground loop. If I can identify the problem then there are products that do both.

jonroger:
I would just go on ebay and pay $17 for a USB isolator that isolates everything.

Nominal Animal:

--- Quote from: drummerdimitri on March 23, 2019, 10:57:48 am ---a noisy USB connection
--- End quote ---
USB is a digital protocol, with data encapsulated in packets with error checking.  If the data is garbled, the audio will cut off and click; it won't just have some background noise or hiss.  (Jitter might occur with USB audio, but not as background noise/hiss.)


--- Quote from: drummerdimitri on March 23, 2019, 10:57:48 am ---a ground loop
--- End quote ---
That is what you have. The cheap USB isolators operate only on USB 1.1 Full Speed, or 12 Mbit/s, so you need to use an isolator on the audio USB link only.  (Some isolators have a switch to choose between 1.2 Mbit/s and 12 Mbit/s data rate.)

They are based on Analog Devices' dedicated ADuM4160 or ADuM3160 chips, and their reference design.  The only thing that varies is the DC-DC converter used; it must be isolated for the device to work correctly.  When you get one, just check the largest component on the board; that will be the DC-DC converter.  Mornsun B0505S series is commonly used (they vary by the amount of power they pass), and work fine for ground loop isolation.  Find its data sheet, and verify that it is an isolated one (minimum 1 kV isolation), and passes sufficient current for your device. (For powered USB devices, 100mA is usually enough.)

1 kV isolation means that if the two sides are powered by different 240 VAC mains circuits, there can be about 300 volts potential difference between the zero potentials and the isolator will still work, even if the two circuits are not synchronized in any way.  So, I personally use one when I ground loops are a possibility between two devices powered from the same building or "fuse box", but possibly from different circuits.  If the two sides are powered by e.g. different buildings, I use Ethernet (which is magnetically isolated) or an optical connection.

Any proper elechicken can explain this better, I'm sure.

GeoffreyF:
Did you probe the differential between the devices and other grounds with a DVM or an oscilloscope?   That would answer your question completely. Getting a choke (ferite core) around the USB cable would possibly be a remedy as well.   If all that reveals nothing, then it's not what you speculate.

drummerdimitri:

--- Quote from: GeoffreyF on March 23, 2019, 04:05:19 pm ---Did you probe the differential between the devices and other grounds with a DVM or an oscilloscope?   That would answer your question completely. Getting a choke (ferite core) around the USB cable would possibly be a remedy as well.   If all that reveals nothing, then it's not what you speculate.

--- End quote ---


What do you mean by differential? What exacly should I be probing with my scope?

Will try your suggestion of a choke even though I'm almost sure it won't solve my problem.

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