If it works fine on a USB supply but not on batteries it is almost certainly a lack of decoupling on the supply. I would put a big capacitor on the amplifier's power supply, something in the order of 220uF plus.
If it works fine on a USB supply but not on batteries it is almost certainly a lack of decoupling on the supply. I would put a big capacitor on the amplifier's power supply, something in the order of 220uF plus.