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Sennheiser HD 660S2: audiophile favorites get improved bass response

Sennheiser has updated its fan-favorite open-back HD 600 series with a new flagship, the $600 HD 660S2, which will be available for pre-order globally from February 7, 2023, and will go on sale February 21, 2023. The company says the HD 660S2 (which replaces the older HD 660S) offer enhanced sub-bass tuning and greater sensitivity across all frequencies.

Man wearing Sennheiser HD 660S2 open-back headphones.

“Our new Sennheiser HD 660S2 offers listeners what they requested most from the headphones’ predecessor,” Jermo Koehnke, Sennheiser Audiophile Product Manager said in a press release. “With precision and power like no other and new sensitivity across all frequencies, listeners will hear details they’ve never heard before, especially at the lower end of the spectrum.”

Sennheiser HD 660S2 open-back headphones.

It has been just over five years since Sennheiser released the HD 660S, and at least from a design perspective, not much has changed. Designed in Germany and hand-built in Ireland, the 660S2 look nearly identical, with the same plushly cushioned headband and earcups, the same hinge-like pivots, and the same metal mesh grilles on the outside. They both weigh 260 grams (about 9 ounces). Save for a new bronze accent color, it can be hard to tell the two models apart.

Inside, much has also remained the same. The dynamic drivers are still 42mm with a 38mm diaphragm, however, Sennheiser says it has reduced the weight of the voice coil and improved overall transducer airflow. The transducer surround drops the resonant frequency from 110Hz (HD 660S) down to 70Hz. It claims that these changes improve impulse response and the nuance of low-frequency passages, resulting in an overall experience that is smoother and warmer than the original HD 660S.

Sennheiser HD 660S2 open-back headphones seen with headphone amp.

A look at the specs further reveals where the two models diverge. The HD 660S2 has a slightly wider frequency range, moving from a low of 8Hz to a high of 41,500Hz, whereas the 660S only made it down to 9Hz. This is presumably where people will hear a difference in bass response. Sennheiser has also abandoned the 660S’s lower 150 ohm impedance for a 300 ohm rating, which the company uses in its other 600-series (HD 600, HD 650).

When asked why this decision was taken after the move to use a 150 Ohm design in the 660S, a company spokesperson said, “While the 300-ohm impedance is a bit tougher to drive from most non-hi-fi devices, that’s not who this is for, as the audiophile would prefer better electrical damping in their listening application.”

Sennheiser HD 660S2 open-back headphones.

Despite the return to a more at-home-friendly impedance, Sennheiser ships the HD 660S2 with shorter cables than its predecessor: you get a 1.8 meter (approx. 6 feet) cable with a 6.3mm unbalanced stereo plug, and 1.8m cable with a balanced 4.4mm plug. The 660S, by contrast, came with 3m cables (just under 10 feet). You also get a 6.3mm to 3.5mm adapter and a protective storage pouch.

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