If you google around, you will conclude the Raspberry Pi does have some noisy analog audio. It's just part of the package. Better than it B+ counterpart, but still not perfect.
You can do a few things to alleviate the issue.
- Connect the RPi Ground to the Amp's Ground. (Eliminate ground potential). I thought sharing the same power supply was enough, but what I failed to remember is the amp has a boost converter and the Pi has a buck converter; after the step up and step down there might be a difference in ground potential between the two boards. Sharing the ground helps significantly with buzz. If you bought a kit from me, chances are your amp might already have a ground lead going to side buttons for volume up+down. Splice the ground and make this connection, or there should be another gnd on the amp you can plug into.
-Don't use an audio cable, Get an Audio/Video cable with 4 poles. The B+ shares video in the same plug- so a regular audio cable connects both Gnd and Video. This seems to couple noise from the composite video signal even when disabled with HDMI is present.
-Turn the alsamixer volume up higher.
-Add a ferrite bead clip to your power supply cable and audio cable.
Didn't get a kit from me? These are some alternate solutions.
-Use the same power supply for the Pi and amplifier (aka removing the ground potential).
-Use a shorter audio cable.
-Use a quality (UL listed) power supply.