Converting an electric signal into mechanical movement takes energy. Thus, current needs to flow from the audio amplifier in the CD player to the headphones. In

Converting an electric signal into mechanical movement takes energy. Thus, current needs to flow from the audio amplifier in the CD player to the headphones. In electronic terms: the headphones have a low impedance (AC resistance). Due to a poorly designed circuit in the CD player, noise is introduced when current flows. So the more current is flowing, the more noise. Driving a dynamic load is not hard, it has been solved for a long time. The vendor just chose a poor design, probably to shave a couple tenths of a cent from the material bill. A proper designed amplifier requires very little current (ideally none) on the input. In electronic terms: the amplifier has a high impedance. Thus the amplifier causes very little noise when connected to the cheap CD player. The OP build a better amplifier that causes less noise when current is drawn from it, so using that results in a better quality sound.