The steel mesh found on the HD 660S drive unit is interesting in design but most impressive when it comes to performance.
 
It completes the ventilated driver, offering consistent control of air displacement to reduce turbulence. As a result, diaphragm travel is pure, letting the music—and only the music—shine in a perfect balance of integrity and expression. The steel composition allows for much tighter production tolerances, ultimately contributing to remarkably consistent characteristics of each HD 660S produced. When combined with the open-back earcups, the sound emanating from each transducer is completely free to expand, just as it would in nature.

Today’s audiophile has more component choices than ever.
 
We optimized the HD 660S for efficiency on a wider range of devices to accommodate the varying levels of power present in output amplification circuitry. At 150 ohms, the HD 660S is the perfect partner for dedicated amplifiers, A/V receivers, and digital audio players (DAPs)—the magnet is more powerful than its predecessors, and thus easier to drive regardless of how your setup changes over time.

Every HD 660S has experienced a matching regimen to mate each transducer to its opposite-channel counterpart.
 
These tolerances are extremely narrow and when bolstered by automated assembly processes, ensure that your HD 660S experience is consistent with those all over the globe. We designed it with thoughtful, lightweight materials invisible to the acoustical system without sacrificing the durability needed for daily, all-day use. With a never-ending supply of music to consume, we wanted every touch point to be soft, luxurious, and durable for those extended listening sessions.

We packed your HD 660S with a versatile set of essential cables, each one impedance-matched with ultra-low capacitance.
 
The ¼-inch / 6.3mm stereo connector offers the audiophile-standard plug for connection to devices such as headphone amplifiers and A/V receivers, digital audio players, analog gear, and more. A balanced, 4.4mm connector opens a pathway to a newer generation of balanced output devices (headphone amplifiers, digital audio players, etc.) that embrace this compact plug’s superior stability over time. A balanced headphone setup offers an even lower noise floor with great dynamic range when connected to balanced source/amp circuit outputs. The listener will thus experience a wider soundstage, tightly controlled transients and impossibly-clear instrument separation. Each cable is 9.8 feet / 3m long to accommodate unencumbered listening on gear deep within your shelves, or favorite listening chair, and a flexible adapter is included to reduce the 6.3mm to 3.5mm.

Hi-fidelity sound
Open-back design
42mm transducers
Luxurious comfort
Detachable cable

Specifications
Article number (SKU)
508826
Frequency response (speaker)
9 Hz - 41,500 Hz
Sound pressure level (SPL)
104 dB (1 V)
Impedance
150 ?
Total harmonic distortion (THD)
< 0.04% (1 kHz, 100dB)
Cable length
3 m
Adapter
stereo jack plug, gold-plated 1/4" (6.35 mm), unbalanced, 3-pin 4.4 mm, balanced, 5-pin
Transducer principle
dynamic, open
Weight
260 g
Connector
stereo jack plug, gold-plated 1/4" (6.35 mm), unbalanced, 3-pin 4.4 mm, balanced, 5-pin
Ear coupling
circumaural
Magnetic field
2.5 mT