Hello

I am having delay issues in a mixing project, when using outboard gear as an insert, on a send or as an I/O on my Fireface. I am just making my first experiences with outboard gear (so far, just mixed "in the box"), so please help me understand and solve my problem.

Setup:
RME UFX II as the master (incl. wordclock-master), a Fireface 800 connected to it via 2 ADAT-cables (just as an I/O expansion), ProTools 2018.7 running at 88.2kHz/24Bit, buffer 1024, automatic delay compensation ADC switched on.
Analog compressors, EQ and Summing Amp are connected via a patchbay directly to the analog I/Os of the interfaces (in groups: 3/4 in -> 3/4 out etc., to allow them being used as "inserts"). The Summing Amp has 8 stereo input channels, 7 of them can be patched to the interfaces' outputs (outputs 1&2 of the UFX II are used for the monitors), the Summing Amp's stereo output is patched to input 1&2 of the UFX II. RME matrix reflects this pretty straightforward setup.

I know, that in many cases, in Pro Tools, you need to enter a value for delay compensation in the I/O setup. But, as a also read, some interfaces' drivers are telling Pro Tools about the required delay compensation. And here, my problem starts.

Whenever I use analog outboard gear as an insert, on a send or as I/O on the UFX II, I do get a delay on the return path. So I am trying to figure out, whether Pro Tools "knows" about the delay compensation time of the UFX II or I need to tell Pro Tools about it in I/O setup. But: I am always experiencing delay issues!

My test to hear the delay issues:
New session with 2 tracks, both holding the SAME stereo music file.
When I play back both tracks simultaneously, both stereo tracks just "add up in volume", but sound clean, just as the original.
But when I insert any analog outboard gear via the UFX II on the second track, and play back both tracks simultaneously, the second track is delayed and produces phasing / comb filter effects in combination with the first track.
So, there must be some delay introduced, that needs to be corrected.

At first, the delay compensation value in I/O setup was still "0". I found out (more on that later), that the "roundtrip delay" of the UFX II must be 25ms. If I enter this value in I/O setup and repeat the above mentioned test, the phasing effects only get worse. So, to me this looks as if Pro Tools actually already "knows" about the delay of the UFX II (through the driver??) and by entering a value of 25ms in Pro Tools' I/O setup, I only add more delay than to correct the situation.

Here's, how I found the value of 25ms for the "roundtrip" delay:
A snippet of music is on track 1. I choose analog outboard gear as an insert on track 1, the output of the track is recorded (printed) to a new track. I measure the delay between track 1 and 2 and the result is 25ms.

Up to here, I described, what I experience, when I use my analog outboard gear as an insert or on a send.

Next problem: 25ms delay, when using the Summing Amplifier.

As described above, I can "hardwire" 14 analog outputs of my Firefaces to the inputs of the Summing Amplifier, the stereo output of the Summing Amplifier is connected to analog inputs 1&2 of my UFX II.
In my session, I want to create stems from certain groups of musicians and leave some other tracks untouched.
So, first, I send the first group of musicians to the 14 channels of my summing amp by choosing the respective tracks' outputs (musician 1 to input 1, musician 2 to input 2 etc.). The stereo output of the Summing Amplifier then is recorded / printed to a new stereo track in this session.
I repeat that with the next group of musicians and end up with a bunch of new stereo tracks (my stems) and some remaining tracks, that I left "as is".

But: these stems are delayed by 25ms to the remaining tracks.
So, when I am finalizing the mix, I of course mute the original tracks used for the stems, but still have this 25ms delay between a few remaining, "unstemmed" tracks and the newly created stems. Of course, I cannot continue my work with tracks, that are not perfectly in sync.

Thanks for reading this - I hope, that I provided all information you need to help me in the matter: how do I get rid of any delay issue, when using analog outboard gear as inserts, sends or as I/O with my UFX II?