Petur, thanks for the reply
As I thought about it today some time, read some manuals and researched the net
I think its very likely a bad adapter battery.
So far I understand that the IRiver doesnt take any power from the adapter but in opposite
has some power ( 3,7 V ) to provide via the Line In...
So the only other thing is what you mentioned - that due to some strange circumstances the
adapter dont like the power ON the line in and so this combination drained out the adapter battery
I think the only way to find out is another field test under concert circumstances which is just bad if the recording fails like last night
Or maybe I just place my mics in front of my hifi speakers and record some hours with the IRiver and see if the battery lasts.