For a number of years, Sennheiser's HD 650 headphones have been a mid-range audiophile favorite, and now the German audio brand has announced a successor. The similar-looking HD 660 S open-backs have lower impedance for drive compatibility with a number of Hi-Res digital audio players, improved the harmonic distortion and more connectivity options.

Open-back headphones generally offer a more spacious, and often much improved, sound than their more common closed-back cousins, but the design does mean that they don't block ambient noise and also leak music to those nearby, so are perhaps best suited for quiet room home listening. That said, Sennheiser has reduced the impedance of the HD 660 S headphones to 150 ohms (from 300 ohms), meaning they can be driven by mobile Hi-Res audio players from the likes of Pioneer and Fiio.

Mobile users rocking less capable music players or smartphones – and even some living room listeners – may need to pair the over-ear cans with a headphone amp/DAC like the MHA50 from McIntosh or Sennheiser's own HDV 820 model to drive them.

Sennheiser says that its new proprietary dynamic transducer design better controls the stainless steel fabric diaphragm, and when paired with high fidelity aluminum voice coils promises low harmonic distortion and lifelike audio that's described as expressive, yet accurate and transparent.

The new reference headphones have the same 10 Hz to 41 kHz frequency range as their predecessor, but offer 0.04 percent total harmonic distortion/noise and 104 dB SPL. They have similar elliptical ear cups with replaceable ear cushions and thick padding on the headband for long-haul listening comfort.

A detachable oxygen-free copper cable ending in a gold-plated 6.35 mm stereo jack is included in the box, plus another cord ending in a 4.4 mm Pentaconn stereo plug for balanced connection to DACs. There's a 6.35 mm to 3.5 mm adapter included in the box, too.

The HDD 660 S headphones carry a recommended retail price of US$499.95 and will be available later this month.

Product page: Sennheiser HD 660 S

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