Thank you waedi for support.

I tried Garage Band, since I do not have reaper (yet)  - same behaviour.

Performed a total reset of totalmix and recorded on a stereo channel - same behaviour

I observe the meters in TotalMix while turning the pan knob in Ableton. Meters show what I would expect:

—signal on the left, when pan knob is turned all the way to the left - but in the headphones I hear sound on both sides, and on same full level

—and signal on the right, when pan knob is turned all the way to the right - but in the headphones I hear the sound again on both sides, but at about  -30dB

The strange thing is: This happens only when I connect wireless headphones. As I said, when I connect headphones directly to the UCXII, then everything is fine. Can’t get it. it seems to be very unlikely that two different wireless systems have exactly the same issue.

And by the way: you mentioned Reaper, and that is a marvellous invitation to go off topic. There are a number of reasons to stick to Ableton live In my case, which is live, looping, recording analog Instruments. It’s all about session view and about ClyphX Pro that lets me script actions to be triggered by a foot switch or by Push 2 to produce variety of songs structures with room to improvise. But maybe reaper provides similar functionalities meanwhile – I will have to check that out

I work with Aiaiai TM-2 and AKG SST4 inear monitoring system. No - no Bluetooth

I connected AKG iem to the line-out sockets of UCXII —> and NOW it works.

My humble logic tells me, that there must be something wrong with the interface’s headphone out. But why does it work correctly then with a pair of wired earphones connected? That leaves me clueless.

I used to connect the transmitters to the headphone out on the front of the interface. (Main and Click)
Now that I connect them to the jack outputs on the back - everything works fine

Obviously we want to know what cable and conectors you used...also obviously there is an adapter in use that is not c orrect stereo TRS. Otherwise it would work like any other headphone.

And what about if the wireless adapter is "smart" and if does not "detect" stereo signal than automatically switches to mono mode in which L goes evenly to L and R.  If panned to R no signal in L so no signal in headphones. If panned to L everything is in L and so L and R channel has the same signal....

What does it if the PAN is only partial? Or what it does do if you send signal only to L or R and PAN is on center?

I have one 3.5 to 6.35mm Earphone adapter too that does weird things, I don't remember exactly, but it was really like huh how does an adapter cause these strange things. I have many adapters from many headphones, so I just threw it away, btw it was an angled adapter, a nice and little bit expensive one.

Good afternoon all

And thank you all so far for the support.

I come back to this issue one more time just to get it done. I realized that using that y-cable - to go from 2 mono cables from ucxii to a small 4-channel headphone amplifier that only has a trs stereo  Input  -  messes up phasing or polarity or something. And meanwhile I picked up somewhere that those.y-cables are not designed to combine but to split anyway... So I assume that in the end it will be the best thing to get one of those headphone amplifiers with separate L–R Input jacks.

Thank you Kubrak for the hints.I am trying to wrap my head around it - I used this

https://www.dropbox.com/scl/fi/cyrd5so8 … m&dl=0

2 x female jack to 1 x male stereo to connect to a basic headphone amp and splitter, the Mackie HM4.

One thing is, that I do not understand why I myself was having heavy issues with sound quality while my bandmate wasn't. We both had wireless devices attached to HM4, my bandmate came with a nux B-7PSM, and I had an AKG SST4 - yet another y-cable .... I am absolutely aware now that everyone with reasonable knowledge will frown at this ....

And the other thing: would you mind to point out the spot where exactly the short circuit occurs in the y-cable, so that it might affect the ucxii in a destructive way? Or if it was too much of a hassle to explain, would you then mind to point at a resource where I could try to find out myself?

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Edith said I wasn't even able to tag a pic with  img html .....