The HD 660 S features a new transducer design developed by Sennheiser. This results in improved control of the diaphragm movements thanks to a specially manufactured precision stainless steel fabric, which is adapted to the contour of the diaphragm. Extremely light aluminum voice coils ensure the highest impulse fidelity. The transducers are carefully selected and paired by hand to ensure consistently narrow (±1 dB) tolerances. The HD 660 S has been created to enjoy music in supreme quality and from many different audio sources. Thanks to its low impedance it also offers reference-class fidelity with hi-res mobile players. Naturally, the HD 660 S reveals its full potential when it is connected to the balanced outputs of a dedicated headphone amplifier such as the HDV 820 or to the balanced output of a mobile digital audio player. Accordingly, it comes with a choice of cables for just about any occasion. All cables are made of highly conductive OFC copper, featuring very low handling noise and sturdy para-aramid reinforcement. Everything about the HD 660 S makes listening a pure pleasure - not least its strong yet lightweight open construction. The elliptical ear cups are shaped to follow the anatomy of the ear, while thick padding on the headband and the replaceable ear cushions guarantee hours and hours of listening in supreme comfort.