Static sound when running amplifier and usb charger off same battery
This is not really an electronic design question.  If the noise does not come from the amplifier when the iPod isn't playing (but continues charging) that tends to point toward a ground loop issue affecting the iPod itself. When the iPod's headphone output drives just headphones, those headphones are not grounded to anything. Current just flows to them and returns. But when you connect to the amplifier, the output couples with a ground in the amplifier, and that ground couples to the charger's ground via the battery connection. Voltages on the ground line caused by the charger can then find their way into the audio circuitry in the iPod. This could happen even if the power amplifier itself is immune (has good rejection of power supply noise).

"UNDERSTANDING, FINDING, &amp; ELIMINATING GROUND LOOPS IN AUDIO &amp; VIDEO SYSTEMS"
http://www.jensen-transformers.com/an/generic%20seminar.pdf