This question may have been answered but I’ve not seen it in my searches. I’m using Bose quietflight headphones (I know these are not ideal, but it’s what I have right now). When I plug these into the Headphone jack, the sounds is crackly tinny and really just terrible. But if I plug the headphones into out 3/L or 4/R the sound is perfect. Why is this?
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Any ideas, or suggestions for me to explore? Could I possibly have a faulty HP jack?
In the I/O settings; Have you tried lowering the volume on the headphone output jack? Are the output meters on the headphone output in the red?
I have, almost to the point where it is not audible. Nothing is in red - from my input reading, to the HP output. I’ve also tried adjusting the impedance which has not resolved the issue.
I haven’t experienced this with Audio-Technica M40 fs or Beyerdynamic DT 770 80 ohm cans. I would reach out to support.
Is there any harm in using the 3L output for HP?
I can’t imagine why there would be but I think you’re only hearing mono.
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Were you using the headphones while your speaker(s) was on? If so, it could be phase cancelation from the noise canceling headphones. Even the sound of your guitar, acoustically, could create some cancelation. Just a guess. 
Is there a chance the headphone socket is a little tighter and the headphones weren’t fully inserted?
same things happens to me… barely use headphones because of it and i dont know how or why it happens. im sending my qc to get fixed on another issue but had forgotten about this. i´ll mail support and hopefully we get a fix
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I’ve played with different HP adapters, and all plug fully in.
So when I’ve experienced it, it’s’ been JUST the HP plugged into the cortex. no additional speakers.
I’ve emailed support per suggested and I’m awaiting their feedback . I’ll follow up with what they suggest
If you can barrow a set of non-noise-canceling headphones or even some wired ear buds with a 1/4" to 1/8" adaptor, that would help rule out or confirm other possible problems.
it’s a known issue, multiple threads on it unfortunately… my feeling is that is just a not very powerful headphone amp put in to this units compared to others… (some low impedance headphone seem to have it quite bad compared to others that have better results such as bayer)
this problem was prominent on any type of HP solution with low impedance i was using.
i’ve went a great deal of back and forth with support and tried a few different quad’s and the problem persists in all of them…
workarounds for me were:
-cheap desktop stereo headphone amp (problem solved)
-seems to be less prominent when i have xlr out 1 & 2 connected to my studio monitors
-higher impedance headphones ( a lot of people don’t seem to have this problem with them)
-for my case having a high MULTI OUT level(0db) and a Low HP Level (-24db) seemed to be the magic ratio to have it without doing it the annoying clipping on the spikes
I will post below their email response to both the dealer and me about this issue.
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neural reply
"We tested to put just a gain block and start raising the level. Headphones start to clip before the red light on the lane output lights up. XLR outs have more headroom and start clipping much later. With 16 ohms headphones, this can be more noticeable.
The headphones’ output has an internal gain stage of +6dB. This could explain why you feel that the signal has more headroom while using the XLR outputs. Using the Headphones level between 0dB and -3dB should be enough to avoid internal clipping. Using Quad Cortex with the Volume wheel at 100 is also safe. Please, double-check if the clipping is gone after doing this.
Notice you can boost the output blocks by accessing the LANE OUTPUT CONTROL from the grid (tap and hold the output blocks to access this menu). After that, please boost the Headphones LEVEL from the I/O Settings as much as you need and check.
Let us know how this goes!"
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TO BE CLEAR this problem is only present when using stage iem’s (usually very low impedance)
I have had this issue with a set of Audeze heaphones (LCD 4z, 15 ohm). I had to purchase a headphone amplifier to remedy the issue. Interestingly, I also have a cheap set of Superlux HD 681 headphones (32 ohm) that don’t have the issue.
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Since the Bose are noise canceling phones, the input is not connected directly to the drivers. I assume the signal feeds circuitry that mixes in the inverted-phase signal from the ambient microphones and performs their “magic processing”. No telling what kind of load it is presenting to the QC’s headphone amp. My 36 ohm ATH50s work fine. I can get a good strong signal at below 12:00 on the volume knob. I’d try another set of phones. A set of good studio phones, with more flat response would be a better choice, anyway, especially if you’ll be using them to program the QC.
Update -
I’m still working with support who has me going down the same path that you went down @Tele .
I agree with this. I have ordered a set of Beyer HP, as they will serve me better in the long run.
This exactly - Great suggestion
Lastly - (rookie mistake)… I realized that the 1/8th" > 1/4" adapter I was using was Mono… I swapped it with a stereo adapter and it did clean up the sound quite a bit. Still not perfect - but much better. I will update once my new HP’s come in.
Thank you all!