Question RE: Mac does not detect Ayre QB-9 DAC

I returned from vacation and powered on my audio system. It seems my MAC computer is not detecting my Ayre QB-9 DAC. I go into system preferences, select sound and the Ayre QB-9 is not listed as an output. I called my dealer and they are checking but no answers yet.

I removed and reconnected the USB cable and still no QB-9. Everything worked pefectly before I powered down. A few software upgrades were made but nothing looked strange. I re-booted the MAC several times but the QB-9 is not in the list. It seems the MAC is not seeing the QB-9.

Someone at the store suggested maybe my USB cable has failed. Any suggestions?
Same thing happened to my Macintosh C48 but everything was back to normal after I unplugged the C48 and waited 60 seconds to power it back on.
Try a different usb port. You can also try plugging it into a different computer just to see if it tries to connect.
My dealer connected my Ayre QB-9 DSD DAC to their MAC computer and it also failed to detect the QB-9 DAC (using a different USB cable). This is an exact duplication of the problem I had using my MAC Computer. We checked the fuses and they were okay. The unit is being returned to Ayre for repair. They loaned me a Musical Fidelity M1 DAC. Estimated down time is TBD.

As suggested, we tried a different usb port and also plugged it into a different computer. And, we also checked the Audio MIDI setup. The QB-9 failed to operate in all cases. Thanks again for the suggestions.
My Ayre QB-9 DSD DAC has been repaired by Ayre and I picked it up today.
It was shipped to the factory on May 6 and returned today, June 5. We tested it at the dealer to ensure it works. It does. I am very happy to get it back.
As I noted above, I picked my repaired Ayre QB-9 DSD DAC last Friday (6/5/15) and it sounds wonderful. Ayre confirmed the problem with the QB-9 was a board-to-board connection.

I quickly discovered my QB-9 would NOT go into sleep mode when I turned my MAC computer off. I "goggled the problem" and found a post by Mr. Charles Hansen (Ayre Chief Designer). He said:

“First, I would simply just change the QB-9 power switch to "B".

Then the - - - lines will only show up when your DAC is receiving an active audio signal from the computer. This will be only briefly and/or between songs, as it monitors the "Active" line instead of the "Link" line. Then it will stay in - - - mode for 30 after the music ends and then returns to the single green LED by a TIMER instead of looking at any signals that are present (or missing! as the case may be).

I immediately changed the switch setting (on the back of the QB-9) from A to B and it worked perfectly. My QB-9 now goes into sleep mode when my MAC Book Pro computer is turned off (and no longer stays in the always on mode).

I doubled checked the AX-7e Manual and found it confusing. I suggested to Ayre that their A-B switch operation be better explained to what was said above. I found several other on-line posts with people having the same problem.

I am very happy to get my Ayre QB-9 returned. It is wonderful sounding DAC.