airplay issue with OS X 10.10.4

I have a Mid 2011 iMac with 20GB of RAM, OS X 10.10.4 and iTunes 12.2.1.16 and an AppleTV 3 with 7.2 software.

Since the upgrade to OS X 10.10.4 just this week (July 28th 2015) AirPlay stops music playback to my AppleTV (connected to an amp with an optical cable) whenever the iMac display sleeps. As soon as I hit the space bar the display wakes and playback continues. I have the iMac set to never sleep but i dont really want the screen on when I'm just playing music. This issue also means that even though Apple Remote on my iPhone 5 can see my iTunes library on the iMac playback of music will not start unless the iMac display is awake.

Do we think this is an OS X 10.10.4 issue?