If you google around, you will conclude the Raspberry Pi does have some noisy analog audio. Mainly due to a ground loop issue. Here's a good tutorial/ DIY solution for the electrically curious. For everyone else, keep reading. You can do a few things to alleviate the issue:
According to Porta Pi users out in the field, purchasing a ground loop isolator is the way the go. This eliminates the buzz completely and work solve in issue in other places, like your mp3 player plugged into your car stereo.
You should also Turn the alsamixer volume up higher (to about 80% or more). This boosts the pure audio over the noise.
- 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.
-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.