See Ayre QB-9 Twenty Upgrade Announcement

I have since sold my Ayre QB-9 DSD DAC but it “stop working” and had to be retuned to the factory for a transformer repair. As you noted, I wonder if your humming issue is due to a transformer issue and you might need a new transformer.

I was told the Ayre Codex DAC employs a very high quality linear power supply. This is why it sounds so good in balanced mode. This also means that the voltage is fixed

Based on my experience, please ask Ayre if they will repair the transformer only under their warranty program.

Thank you for your answers.

I did try to plug it in a few outlets but the hum persisted. I bought an Emotiva CMX-2 to eliminate DC offset, and the hum persists.

The answer I had from Ayre does not encourage me to insist for a repair:

Yes, a QB-9 with your serial number has an internal transformer for Buss Power. And they do tend to hum a little. There’s not much you can do about it unfortunately.

All they suggest is the Twenty upgrade which, with the improved audio board does not have buss power.

For the moment, I installed the unit in behind a door in a closed furniture. It dampens the noise. I now want to try to cover it with a pillow... Most of the time, however, I don't hear it from my listening position... but when I do, it's annoying... and it is frustrating to hear it everytime I approach my installation, for example when putting vinyls for audition. This particular model does not have on/off switch, and unplugging it proves fastidious in a closed furniture.

I hope the hum is not precursor of a failure soon!
Thanks again.

@vepitesk, I understand that spending $1,500 for the Ayre upgrade is expensive based on your purchase price. 

Please call Ayre, again, and ask them is them if they just repair the humming transformer. Of course, they want to sell you the upgrade but you only need them to repair your humming Ayre QB-9 DSD DAC. They should be able to make this repair. Please keep us posted.

I have an a QB-9 that was the "second generation" (first generation only went to 24/96 files, mine could handle 24/192) and had it upgraded to "DSD". I don't hear any hum regardless of how close I get to the unit. I'm also running a stock NUC as a server. When I bought it, I intended to get a fanless case. But I can only hear the fan when I get close, not from the listening position even without any music playing. Could well be that I'm not as sensitive to extraneous noises compared to you?

I am in the queue for the Twenty upgrade. I've been really happy with the SQ improvements when Ayre releases upgrades.

Thanks all for your comments. I am now quite sure that my humming transfo is defective. My issue is related to https://forum.audiogon.com/discussions/ayre-qb9-dsd-buzzThe guy from which I bought my unit had it upgraded to DSD capabilities, as in the above link.

Ayre themselves deny the issue ("...a QB-9 with your serial number has an internal transformer for Buss Power. And they do tend to hum a little...")
I will contact my local Authorized Dealer to have a check. My belief is that the hum is not "normal little hum" but rather a buzz as described in the above forum link.
In any case, thank you all for your comments that encourage me to have my unit serviced.