While the Mackie Onyx 1220i unit is plugged into my computer I can port the sound through to my headphones via the headphone jack on the console. When I listen to sound play for about 2 minutes I'll experience 'popping' sounds (kinda like clipping signal would sound) and then eventually the audio will stop playing through the headphones. This is experienced both when Windows 7 is configured to use the Onyx 1220i as default sound output, and when Sony Acid Pro 7 is configured for it to be the sound output device.

Are you sure the xlry cable is balanced? there are balanced and unbalanced xlrs. For phantom to work, you need a balanced xlr cable.

One problem is if somebody has put a plug into the INSERT jacks on the back. If signalls have been driven into these that are intended to be used for external effects, an op amp can be blown and it is a pain to replace. Try plugging a se of headphones into each and see if you can hear a signal. You will only hear it in the LEFT headphone and this is normal. Make sure ALL the channels have the L/R switches punched down as these send each cahnnel into the MAIN mixing bus.

You haven't told us where the "sound" is coming from... If you are ONLY plugged in one channel that is all that will be recorded. We can't guess at how you are setup. Please provide an adequate description. If you only have a mono signal to start with, then get a WYE connector and jumper to the second channel.

Look at the setups for voice and audio in the Windows control panel, "Sounds and Devices". Make sure the devices are set to the sources and destinations you want... This is not automatic and is a pain when you are going back and forth from recording and playing...

I would try re-installing the drivers:

http://www.mackie.com/products/onyxiseries/drivers/

And go for a firmware install as opposed to an upgrade, if it failed halfway through the upgrade you will need to load the firmware on from scratch.

You can do a major amount of troubleshooting using tests from the many jacks on this unit.
I can walk you through a lot of those if you keep getting back to me with the results. First test: When the failure occurs does one or both sides of the headphones plugged in the headphone jack ALSO quit? Also does the level meter LED's reflect loss of signal? If these are still OK I want you to plug the headphones into the MAIN MIX MONO OUT jack. The headphone signal won't be very strong, but is it OK and not distorted.

I hope you have remembered to use BALANCED (TRS) audio interconnect cables and not plain mono cables from your mixer to the amps. Use of mono cables shorts out one side of the balanced output driving amps in the mixer. Also the other send of these MUST connect to balanced amplifier inputs... NOTE some amps don't have this and you have to use a BALUN or a DI at that end.

Lastly, ALL inteconnected equipment MUST be powered from the same source or receptacle for system safety.

Feel free to contact me via email or whatever.

What is the setup you are using. What mics (make and model). What are the settings for the channels? With this info, I should be able to assist.

Please update this.

Thanks,
Dan

try hooking headphones to the output and see if the crackling is still there.

then, try it with the speakers, at a lower computer voume level and higher speaker volume level. perhaps the headphone output is too loud and distorting in the speaker inputs.

plug headphone in the effects send jack...If you get somme sound (won't be too loud) there, that means preamp sections are probably ok. Drive a signal into the effects return jack... If power amp responds, it is OK... If both these are OK, then suspect the switch contacts on these two jacks that forward the preamp to the power amp when effects unit is NOT plugged in.
These contacts are a chronic problem.

Contrary to what's posted above, the Marshall MG series does not pass the speaker power output through the headphone jack, and inserting the headphones into the jack does not disable the speaker. The headphone amp is a separate circuit and it's input is taken before the master volume. So Jan's post is exactly right. Turn off the master volume and turn up all the other gain/volumes as high as they will go. The other issue is that the headphone jack's output does not drive low efficiency headphones well, so you need some headphones similar to Sony's MDR-V700s to hear it well. Earbuds just don't cut it.