How to stop digital (optical) out audio from going to sleep?

My iMac 2011 audio goes to via the optical cable to a receiver.

After 30 secsonds of no audio, I can hear a click/pop as the audio shuts down. Then, any sounds miss the first second or two when the audio is 'waking up', so that means in the case of system sounds I miss them completely. Very annoying.

I am using an app - written by a helpful user from another forum - that keeps the sound awake, but even that doesn't always work. (BTW, I've tried various apps that don't work, like Keep SoundAwake, Antipop, etc. byt none work.)

This behaviour only happens on the digital output as if I disconnect the optical cable, the problem doesn't occur through the internal speakers.

I should also add that the problem does not occur in Windows 7 Bootcamp, which suggests it's a OS X issue.