Sennheiser HD 660S2: an evolution of the iconic open-back headphones


Mis à jour le 28 March 2023.

The German manufacturer’s open-back headphone family is growing with the new Sennheiser HD 660S2! Launched in 1997 with the Sennheiser HD 600, this headphone range has become a must-have for audiophiles and professionals alike. This new version has the same design as the Sennheiser HD 660S, but differs with a higher impedance and the introduction of a new voice coil for even deeper and more powerful bass. At €599, will the Sennheiser HD 660S2 be the new benchmark for home hi-fi sessions? 

Sennheiser HD 660S2: lifestyle
With a new voice coil and a higher impedance, the Sennheiser HD 660S2 open-back headphones promise even more impressive bass than their predecessor.

Sennheiser HD 660S2: packaging & accessories

The Sennheiser HD 660S2 open-back headphones come with a 6.35mm jack cable, a 4.4mm audio cable and a 6.35mm jack to 3.5mm mini-jack adapter. A fabric pouch is also included.

Sennheiser HD 660S2 + accessories
The Sennheiser HD 660S2 open-back headphones come with a balanced cable, an unbalanced cable and a 3.5mm mini-jack adapter.

Sennheiser HD 660S2: presentation

Audiophile design

Replacing the HD 660S model, the Sennheiser HD 660S2 headphones have a revamped design and promise improved sound. They feature 1.5” dynamic transducers to provide a wider bass response. The headphones are also equipped with a new, lighter voice coil that now has an impedance of 300 ohms, compared to 150 ohms for the previous version. The reworked transducer surround lowers the resonance frequency from 110Hz (for the HD 660S) to 70Hz. Finally, the external grilles are designed to optimally manage the air flow.

“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.” 

states Jermo Koehnke, Audiophile Product Manager at Sennheiser.
Sennheiser HD 660S2: transducer
The Sennheiser HD 660S2 headphones are equipped with an ultra-light aluminum voice coil to achieve more impact in the lows.

Like all the open-back models in the 600 Series, the Sennheiser HD 660S2 doesn’t provide any acoustic isolation. As a result, the sound will leak out of the headphones. It is therefore recommended to use them in a quiet listening area.

Read: Open back vs closed back headphones and IEMs: what are the differences?

Detachable cable

With their high impedance and sensitivity of 104 dB, the Sennheiser HD 660S2 open-back headphones need to be paired with DAP, headphone amplifier or portable audio DAC when possible to enjoy the best musical performance. 

Designed to adapt to many different sources, the Sennheiser HD 660S2 headphones have a detachable Y cable equipped with a 4.4mm balanced connector (Pentaconn). For more conventional connections, Sennheiser also includes a jack cable with a 6.35mm unbalanced connector and a 3.5mm mini-jack adapter cable.

Moreover, the cables now measure 1.8 meters, compared to 3 meters for the previous version. This shorter length is perfect for playback with a USB DAC or a DAP. However, an extension cable would have been good for living room listening sessions, when connecting the headphones to a stereo hi-fi amp.

Guide: why should you use a balanced connection?

Design and comfort

The Sennheiser HD 660S2 open-back headphones have the same lightweight plastic structure as their predecessor, as well as the iconic steel-mesh grilles that allow you to see the transducers. This comfortable model features a velvet-covered padded headband and earpads that evenly distribute the headphones’ 260 grams across the head. The fit is secure and lets you nod your head along to the music without worrying about the headphones falling off.

Sennheiser HD 660S2: lifestyle
The Sennheiser HD 660S2 headphones’ velvety and soft ear pads ensure a comfortable, secure fit without exerting any excessive pressure.

Their refined design benefits from a timeless matte black finish with bronze details for an elegant feel. To highlight their premium aspect, the German brand also points out that these high-end headphones are made in the European Union, more precisely in Ireland. It is a shame that there isn’t a rigid carry case to protect them during transport.

Sennheiser HD 660S2: key specifications

  • Type: open-back, over-ear
  • Transducers: 1.5”, dynamic
  • Frequency response: 8 – 41,500Hz
  • Impedance: 300 ohms
  • Cables: 6.35mm unbalanced (1.8m) / 4.4mm balanced (1.8m) / 6.35mm jack to 3.5mm mini-jack adapter
  • Weight: 260g

Sennheiser HD 660S2: listening conditions

For our review, we paired the Sennheiser HD 660S2 open-back headphones with the Naim Uniti Atom Headphone Edition amplifier using the 6.35mm unbalanced cable. We listened to HD files stored on a USB stick as well as our Qobuz playlists in CD and 24-bit/192kHz Hi-Res quality. We then connected the headphones to the FiiO M7 DAP using the same cable and the mini-jack adapter.

Casque Sennheiser HD 660S2 + Naim Uniti Atom Headphone Edition
Read: Naim Uniti Atom Headphone Edition review 

Sennheiser HD 660S2: our impressions

Unsurprisingly, the German company has once again produced a very high-quality pair of headphones. With the song Back to Black, Amy Winehouse’s voice was perfectly highlighted. The piano notes filled the background, alongside the cymbals and percussion instruments, which gave the sound more body. With Joe Hisaishi’s track The Flower Garden, the Sennheiser HD 660S2 headphones delivered an extensive, spacious and extremely detailed sound. Every instrument fell into place and truly seemed to materialize on stage in front of us. 

Sennheiser HD 660S2 + Naim Uniti Atom Headphone Edition
The Sennheiser HD 660S2/Naim Uniti Atom Headphone Edition duo was impressive with all musical genres! We enjoyed punchy bass, as well as clear, detailed highs.

This observation was confirmed with every track we listened to, no matter the genre. On the song “Gossip” from the album RUSH by Måneskin, the bass was dynamic and punchy, allowing us to appreciate all the band’s energy. 

When paired with the FiiO M7 DAP, the sound remained warm with deep and well-controlled bass. With Shaka Ponk’s I’m Picky, the vocals, percussion instruments and sound effects came together harmoniously.

Sennheiser HD 660S2 + FiiO M7
Even with the FiiO M7 DAP, the Sennheiser HD 660S2 headphones delivered a clear and detailed sound.

Sennheiser HD 660S2: compared to…

Sennheiser HD 660S: at first, the sound signature seemed to be identical. However, as the listening sessions progressed, we enjoyed details that we hadn’t noticed before. A real success, this second generation benefits from improved bass, and more globally, from greater clarity. 

Grado PS500e: while these open-back headphones earn a point when it comes to comfort, the Sennheiser model offers a better fit, allowing you to move your head without fear of it falling off. The German model delivers sound over a wider frequency range and offers a warmer sound signature. However, the Grado PS500e and their 32 ohms have the advantage of being compatible with the majority of sources, where the HD660S2 are more at ease with powerful headphone amps and DACs. 

Sennheiser HD 660S2 + Naim Uniti Atom Headphone Edition + Grado P5000e
The Sennheiser HD 660S2 headphones provide a richer and warmer sound than the Grado PS500e model.

Sennheiser HD 660S2: who are they for?

With their open design, the Sennheiser HD 660S2 are the ideal headphones for those looking to enjoy their music at home. Every track is accurately reproduced, and the headphones are suitable for the vast majority of music genres. Sennheiser is known for its straightforwardness and linearity. The improved bass of this new generation is a godsend for lovers of rock or electronic tracks. One notes the manufacturer’s genuine desire to offer a pair of headphones with a little more personality while remaining globally analytical – a quality that is dear to the brand.

Sennheiser HD 660S2
With an extended frequency response, richer, more detailed sound and clearer stereo image, the Sennheiser HD 660S2 are an ideal solution for audiophiles and music lovers who want to enjoy all their playlists.

Sennheiser HD 660S2: conclusion

Once again, Sennheiser has knocked it out of the park. The German brand has managed to improve its iconic HD 660S model. The style of the headphones is identical to the previous model, but the performance and comfort are also just as good. Once powered by a headphone amp, DAC or DAP, such as the Shanling M30 DAC or iFi Audio Pro iCAN Signature headphone amp, the Sennheiser HD 660S2 open-back hi-fi headphones deliver a natural and warm sound with deep, extensive bass, detailed mids and crystal-clear highs.

Sennheiser HD 660S2 + Naim Uniti Atom Headphone Edition  + Son-Vidé stand
Sennheiser has managed to improve its iconic HD 660S model with the Sennheiser HD 660S2, which provide an analytical, natural and warm sound.

We liked

  • The dynamic, punchy bass
  • The distinct, precise highs
  • The clear sound
  • The comfort

We would have liked

  • A rigid carry case
  • A 3-meter jack/mini-jack extension cable

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En pleine découverte du monde de la hi-fi et du home-cinéma, Son-Vidé est sans conteste devenu ma caverne d’Ali Baba. Jazz, Rock, Pop, Soul, Rap… Mes goûts musicaux, comme cinématographiques, sont très variés. Pour donner vie à mes playlists préférées, j’ai choisi l’amplificateur Marantz PM-6006, accompagné de la paire d'enceintes colonne Elipson Prestige Facet 14F.

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