Hey I pressed the 'cds' button on my Alpine 7909 when playing a cd.
What exactly does it do ?
Supposedly it blocks out some tone and balance circuits on the player for a quieter sound...thats what the manual says...

 

It's a tone control bypass switch. There should be an amber led that comes on when engaged. Yes it bypasses the two sliders on the top of the unit....bypassing what is know as a noisier section of cd players. Cool feature.

 

Yes the amber led lights up after I press it..
When should I have it on ?
Is there suppose to be any noticeable difference/benefit ?

 

If you are going to use the tone controls...leave it off. When on, it should sound better....more direct signal path.