Festivals in Late July?

Hi I'm visiting Amsterdam for the first time late July (20th ~31st) and wondering if there are any open air festivals, my reason for the visit is to 'relax' and listen to music in a cool atmosphere, I like electronic, soul, and deep house music. I was thinking if I needed to take a train to berlin I would.

So far I booked at the student hotel (cancelable) but any recommendations for a place where I could smoke on a balcony at city center would be appreciated!

Thanks!