[motoman4540]

My setup:  a 2014 iMac with i7 intel processor, 24 gig RAM, 1 Tb fusion drive, an O2+ODAC rev. B and a set of Sennheiser HD650 headphones.  All music resides on a Netgear 516 NAS in FLAC, probably 50% 16/44kz, and the rest 24/92 and some 24/192.  Music is played through the iMac using VLC or sometimes Plex
 
The problem:  I have been happily using my ODAC and HD650's since November 2015.  Everything sounded great . . . until yesterday.  Then, I sat down to listen to some old rock (Yes, Fragile, Roundabout) and the music sounded very distorted, like a very bad quality recording.  I listened to a couple of different versions -- all sounded distorted. Then I played it on my audiophool stereo system -- sounded great.  Then I put on my "test" FLAC -- an excellent quality recording that I use to compare stereo components --  through the ODAC, and got the same distortion.
 
Now in troubleshooting mode, I did the following:
 
- checked to see if it was the headphones by plugging them directly into the iMac.  Played fine, so it wasn't the headphones, the server or the connection between the server and the iMac.
- Swapped USB cables between the iMac and the ODAC.  No change in the distortion, so the cable was not the problem.
- Verified that the sound settings on the iMac for the ODAC were 24/96khz (also tried 16/48K).
- Hooked up the ODAC to my Macbook Air -- to my surprise, no distortion, albeit the volume was softer and needed higher gain.  Seemingly, there was no problem with the ODAC, therefore, and the problem was somewhere on the iMac.
- Switched to another USB port on the iMac.  Still the same distortion.
 
The USB ports on the iMac seem to work fine for things like data transfer to a USB drive, my iPhone, even a desk phone with a headset that connects to the computer via USB for things like Skype, FaceTime or Google Voice.
 
Other than to eliminate the ODAC as the problem,Tech Support at Mayflower Electronics was useless at this point, now that I had figured out that the ODAC was apparently not the problem.  The tech support guy said he didn't have a Mac so he couldn't help me.
 
I thought that there might be some driver that got corrupted for some reason yesterday, which would explain why swapping USB ports didn't fix the problem, but it doesn't seem like the ODAC needs any drivers, and as I said, the ports seemingly work fine.
 
So . . . what do I do? 

 

[NA Blur]

Hopefully this is not a driver issue as there is not much that can be done about that, but here do try lowering the volume on your Mac to say 80%. My guess is that it is currently at 100% and thus causing some clipping. If this is the case then the software controlling the voltage on the USB ports has changed the output voltage.
 
If the problem persists you and your DAC has optical input you can try using an optical cable and eliminate the USB ports are the source of the problem. I know for Windows it is helpful to remove the "Power Management" for all USB-hub devices to help prevent these types of issues.

 

[NA Blur]

When in doubt try a USB isolator which runs the USB through an opto-isolator.
 
There are some inexpensive ones out there like the Behringer makes the HD400 that goes between the output of the DAC and amp.
 
In the end it is about lifting the ground from the USB port usually done by removing the ground pin on one side of the USB cable.

 

[motoman4540]

Thanks for the response.  Unfortunately, adjusting the output volume to make sure that it wasn't clipping was the very first thing that I did, even before determining that there was an actual problem.  I never had the iMac's output volume at over 80% anyway, but even reducing it to much less than 80% didn't fix the problem.
 
As for optical inputs, the ODAC only has a mini-usb input and the iMac doesn't have an optical output.  However, are you saying that ALL of the USB ports are defective and need to be bypassed?  That doesn't make sense to me, as the ODAC has not been moved since I hooked it up in November.  In other words, I didn't plug it in to the iMac and unplug it several times.  It has remained plugged in.  I just turned the ODAC on yesterday morning and had this problem.
 
I have read some comments about power management.  I looked in the System Preferences on the iMac and didn't see any settings for the USB devices.  Do you know that there is setting a setting for the iMac?

 

[NA Blur]

Keep the Mac and ODAC plugged into the same outlet?

 

[motoman4540]

Keep the Mac and ODAC plugged into the same outlet?

If you are asking whether the iMac and the ODAC are plugged into the same wall outlet, the answer is "no," although they are both plugged into the same UPS.  Do you think that that could make any difference?  And if so, why has it worked perfectly since November up until yesterday?

 

[RRod]

Have you check the VLC volume? I know I have a horrible habit of setting it over 100% accidentally.

 

[motoman4540]

  Have you check the VLC volume? I know I have a horrible habit of setting it over 100% accidentally.

the volume on VLC was 100%.  However, it was set at the same volume when I listened through the headphone jack on the iMac and the distortion wasn't there.  Plus, wouldn't the volume on VLC not matter because the ODAC has its own volume control to raise and lower the output?

 

[RRod]

 
the volume on VLC was 100%.  However, it was set at the same volume when I listened through the headphone jack on the iMac and the distortion wasn't there.  Plus, wouldn't the volume on VLC not matter because the ODAC has its own volume control to raise and lower the output?

 
VLC is the first thing to see the file (it's prepping it for the audio server / DAC), and thus it can manipulate the digital audio stream, so unless there's a setting to disable all touching of the stream, then its volume would indeed affect things. But unless you had it over 100% that's probably not the problem.

 

[motoman4540]

Yesterday, I installed an update to El Capitan pushed by Apple.  Whether it was something in the update or the fact that the iMac rebooted, the problem is no longer there and the distortion is gone!  In looking back on the issue, I am pretty sure that one of the things that I did when the problem started was to reboot the computer, but I guess its possible that I forgot to do one of the obvious things (probably because, after switching from Windows, I don't have to constantly reboot the computer to fix things).  Anyway, whether it was something to do with the update or simply rebooting, it's fixed!  Thanks for weighing in to help out!