As Tom has said, this appears to be normal. I have some suspicions as to what causes it, but haven't had time to track it down.
It does appear to be related to spikes on the mains power that get fed into the dac (either via to output or the PC side). Similar kinds of things happen when I run turn on/off a desktop lamp near my rig.
As a test, if you are comfortable with removing parts, I might try to remove C5.
So I finally got some time to get back to debugging this. Thanks for the info.
Removing C5 didn't help.
I'm currently using the setup with my laptop and it's made the problem a bit easier to track down, it also reconnects the DAC automatically which is nice. I'm fairly sure that it's a spike being fed back from the mains via the output to the amp, I can trigger the problem by turning my soldering iron off (plugged into the same strip as the amp) while the DAC is powered from my laptop (on battery power). dmesg log below.
I'm also finding that the DAC is unusable when my laptop is plugged into the mains. I am using a cheap replacement charger though so that's probably not helping.
Any ideas for workarounds?
I'll try to get some 'scope shots of what's happening on the grubdac outputs after the holidays.
[20837.640106] usb 3-1: new full-speed USB device number 19 using uhci_hcd
[20837.834359] usb 3-1: New USB device found, idVendor=08bb, idProduct=2706
[20837.834372] usb 3-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[20837.834379] usb 3-1: Product: USB Audio DAC
[20837.834386] usb 3-1: Manufacturer: Burr-Brown from TI
[20837.866346] input: Burr-Brown from TI USB Audio DAC as /devices/pci0000:00/0000:00:1a.0/usb3/3-1/3-1:1.2/input/input215
[20837.866497] hid-generic 0003:08BB:2706.00CC: input,hidraw0: USB HID v1.00 Device [Burr-Brown from TI USB Audio DAC ] on usb-0000:00:1a.0-1/input2
[20840.088265] hub 3-0:1.0: port 1 disabled by hub (EMI?), re-enabling...
[20840.088275] usb 3-1: USB disconnect, device number 19
[20840.380109] usb 3-1: new full-speed USB device number 20 using uhci_hcd
[20840.575289] usb 3-1: New USB device found, idVendor=08bb, idProduct=2706
[20840.575297] usb 3-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[20840.575301] usb 3-1: Product: USB Audio DAC
[20840.575304] usb 3-1: Manufacturer: Burr-Brown from TI
[20840.606463] input: Burr-Brown from TI USB Audio DAC as /devices/pci0000:00/0000:00:1a.0/usb3/3-1/3-1:1.2/input/input216
[20840.606776] hid-generic 0003:08BB:2706.00CD: input,hidraw0: USB HID v1.00 Device [Burr-Brown from TI USB Audio DAC ] on usb-0000:00:1a.0-1/input2