Are these settings you use than?
No. I gave MQA support in the top section set to none, not renderer only.
Okay tnx, stuttering is gone 
Led on the Zen Dac remains green, not purple anymore, but is the MQA output still there?
Roland if it’s Green then there is no MQA, but it’s better to have good sounding music rather than stuttering music.
I listened to a MQA album from a Pi4 on VitOS to my Zen DAC earlier and it was perfect. It might be just something in your system. You’re not really losing anything by not having the MQA working well (IMO), unless you really want MQA that is.
Mike
Enable the MQA core decoder. You’ll then get the coveted purple LED. 
Roland I am sure I see more in my path (and the purple light). Maybe Roon is unfolding the first part but not getting the second as you have told Roon that it’s not MQA compliant.
I will send you a screenshot shot a bit later of my study setup.
Again I would rather have decent high resolution music that works well than stuttering Purple badge.
Mike
For IFi DACs MQA capabilities should be set to Renderer as that is what the Ifi is. Ensure Roons MQA decoder is engaged as this is essential as Roon must perform the first decode stage.
Plug the Ifi DAC into your Mac using the same usb cable , set it up the same and see if this plays back with out any problems and the DAC shows its playing MQA. Make a note of Roon processing speed in the signal path to.
Then connect to the pi setup as described play same file, does it have issues? If so check processing speed in signal path again and see what it says.
Are you using WiFi in your playback chain? If so where is it being used?
How is that DAC powered, seperate PSU or is it using USB power. What PSU is on the pi official USBC power supply or something else and Is it specced to produce 5v 3a?
Have you tired a different usb cable as they can cause issues sometime.
Hi Roland
Here is my signal path for my iFi Zen DAC just picking the last MQA album I listened to.
Running into My Hifiman Sundara’s from 4.4mm Pentacon balanced connector, though I have the unbalanced without issue as well without any issue.
And here is my Device setup
If you want me to test any settings let me know and I will give it a go.
Mike
Problem found 
With the settings described by Mike and Simon the stuttering remained. But together with the Zen Dac I bought an IFI Purifier3 to clean the noise of the Rpi.
Removing the Purifier was the key: MQA-purple led-no stuttering anymore.
The Purifier however did contribute to better sound quality significantly so I don’t know what I’m going to do about that.
Any one a suggestion for this? Using my Mac as source without the purifier , another Noise canceler or …?
Anyway thanks a lot for all your effort and help, most appreciated.
Regards Roland
Hmm I have one on my pi it never caused issues with MQA, tbh it didnt make much difference to SQ with my SMSL SU9 at all either.
Roland good that you have tracked down the problem. I have the Audioquest Jutterbug on all my devices doing the same sort ok Job and no issues with MQA on any so might be worth a try.
Are you using the provided iFi usb cable or a third party cable? The iFi is the only device I use the supplied cable with as it feels pretty good.
Otherwise I generally use audioquest cinnamon cables. Expensive but very good.
I would be interested in the direction you go with this, so let us know.
Mike
Yes, I used the provided cable by IFI. When I stream (from the Mac this time) with Tidal there is no problem with the Purifier 3, so there must be something in Roon…
I think I’ll give the Audioquest Jutterbug a go, keep you informed.
Roland
Roland that is very strange indeed
It will be interesting if a different USB cleaner makes a difference. It doesn’t make sense that it would, but then your problem doesn’t make sense either, just one of those weird issues where you have to remove every part one by one and see what makes a difference.
Good luck with the jitterbug I hope this resolves the issues and look forward to hearing the result.
Mike
Well, the jitterbug does its work very well, without any stuttering.
Again thanks all of you.
Roland
Roland Surprising but good news
Makes me feel that I made a good decision going the way of the Jitterbug. I have them on all my USB based Pi’s and while I did not see a huge difference it was by using a good iFi power supply, good USB cables and the Jitterbug together made a difference in my ears and my family’s ears when they had no idea what I was doing.
Glad to hear you have both a purple light and music you can enjoy without issue.
Mike
The iFi iPurifier3 is an active device that contributes to power draw. I wonder if the Raspberry Pi’s USB port (or upstream power supply) is simply not able to deliver sufficient power to run both the iPurifier3 and the ZEN DAC.
Whenever a RPi is used with a bus-powered DAC, I recommend inserting either the iDefender+ with iPower/iPower X or the micro iUSB3.0. These relieve the RPi of power delivery duties.
It looks like the ZEN DAC can optionally be self-powered. Have you tried adding an iPower 5V?
David
Roland has fixed his problem by getting a Jitterbug now, but you may have hit the nail on the head with your comments as too the reason why it happened.
Mike
Yeah…but if I’m right, getting better power for the ZEN is still probably a good idea.
David that makes sense, I forgot that it doesn’t come with the iFi power supply as I bought mine with the power supply and the Jitterbug.
I have been extremely happy with my Zen DAC with the set-up with VitOS (thanks David) and my newly purchased (well newly delivered) Oidio 4.4mm balanced cable. With the Sundara’s it really does sound like a higher end system.
For my study listening while working I couldn’t ask for much more (ok maybe a pair of closed back headphones as well)
Mike
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