For a better understanding and experimenting with the LFO´s you can control a parameter on the DN with a LFO from the MIDI-Track. In this case the changes from the LFO are visible on the LFO destination.
I wish this would be also possible with the LFO´s from the audio tracks.
AMP envelope attack 
That’s a fantastic idea, mate! I always thought this is the most underrated and forgotten left out feature on all Elektron boxes. Better than blindly walking in the dark.
setting LFO destination to Pitch ALL is a great way to understand what happens as well 
I use LFO Destination Pitch to get a feel for what my LFOs are doing 
Very handy!
Same here. Going for a second cup.
That’s indeed the way I should have approached this with TRIG 1st, AMP ATK Env to a high value and then RELEASE to INF.
Or trig length to a length you want or INF. I tend to never use length or release set to INF as it can cause other annoyances. Most of the time better to set the length high and add a trig at the end of that length with a condition and AMP enc attack short. This way it will take over from the initial slow attack trig at the end of it. So the trick here is to use conditional locks in such a way that trigs take over from one another, slightly overlapping with release time for instance. Can become a bit of a puzzle, but it offers more flexibility and consistency I’ve found
If that makes sense 
That makes sense. Does this mean: Overlapping trigs take the AMP values over without retriggering but when they don’t overlap they trigger the AMP env again?
Yeah or overlap and lock amp attack to a short value so it doesn’t “restart”.
There’'s also a AMP ENV reset on/off button on amp page 2. But I don’t use it as much. It seems a bit bugged out, but I have to dive more into that to make sure. It looked like odd behaviour but maybe I used it wrong sometime.
It’s like reading a UFO, sorry, LFO novel.
In similar cases i just hit func - yes. Bring bpm down, live record the automation at zen buddha speed and bring bpm up again after your automation recording. Not satisfied hit func no and start over. Be the lfo.
Maybe that’s why most of the time I produce at very low BPM (30 to 45). Ah, that subconsciousness!
@PekeDorty try this:
On trig page set LFO.T to OFF
On LFO page set LFO SPH to 90, LFO trig mode to TRG or ONE or HALF depending on desired fade result. Set wave to TRI. Set speed/mult according to your desired length, and have AMP volume at 110 (default) set LFO destination to amp vol, depth to 127.
On pattern p-lock LFO.T to on for just the first step, you should hear your pattern fading in, then just afjust speed and mult to get desired result.
Let me know if it works.
Edit: if you don’t want the start to be silent try lowering LFO depth to around 80.
Not at my DN to check now, but I was wondering whether the arp triggers the LFO on every “played” note even if there’s only one initial trig in the sequencer. Because that would reset the LFO back down to the minimum value on every note and cause nothing to be heard.
For a simple fade in I don’t see why not using Attackk parameter.
I opened a thread about LFOs some time ago as i find the parameters very conter-intuitive. The drawings of the shapes on the OT don’t make sense at all.
In general the time i found correct parameters is enough to forget what was the idea… but i have a really bad short term memory as i was about to react saying something totally different and forgot it as well 
Saving LFO presets would earn a lot of time, especially whenever you want a Sine wave.
Because OP is trying to fade in an arp.
I forgot the OP as well it seems…
Fwiw - This is how I do slow fades
Ah, possibly right, I’m not in front of mine either.