I have built a tube amplifier and modded it with a line out for headphones based on this article.
I measured the output voltage on the artificial "line out" is peaking around 4-5V. As a reference I also tested what my mp3 player delivers and that is around 500 mV
is it safe to introduce 5 V to my mixer (Behringer Xenyx 502) line in?
the manual specifies that the line ins are "designed to handle typical line level signals". based on this post typical is around 300mV, however it may peak to 3 V
I might have answered my own qustion, but a confirmation would be nice