Sennheiser HD 660S2 Review

by Bruno Brozovic

Let’s see what the new Sennheiser HD 660S2 headphone model brings us in my new Sennheiser HD 660S2 Review.


Sennheiser HD 660S2 Review

Every launch of a new model in Sennheiser’s renowned and long-running 600 series is a little holiday for fans of audiophile headphones. If you have headphones in your home hi-fi system, you probably considered Sennheiser’s 600 series when deciding to buy. If you are just planning to buy headphones, we are sure that one of the models from this series is on your shopping list.

Finally, if you are a longtime enthusiast of quality sound and audiophile headphones represent a significant, if not the most important part of your hi-fi system, there is a good chance that your collection includes at least one example of these headphones – this speaks volumes about the reputation that this series of headphones enjoys in the head-fi world.

Sennheiser HD 660S2 Specifications

  • Transducer principle: dynamic, open
  • Transducer size: 38 mm
  • Frequency response: 8 – 41,500 Hz
  • Impedance: 300 Ω
  • Sound pressure level (SPL): 104 dB (1 kHz, 1 Vrms)
  • Total harmonic distortion (THD): <0.04% (1 kHz, 100 dB)
  • Ear coupling: Over-Ear
  • Weight: 260g
  • Adapter: 6.3 mm (1⁄4inch) to 3.5 mm
  • Connector: 6.3 mm (1⁄4inch) jack plug, 4.4 mm balanced plug
  • Cable length: 1.8m
  • Magnetic field: 4.5mT

You can download & view the manual here->Sennheiser HD 660S2 Manual

Design, Technical & Package

The Design

If we exclude modern Bluetooth and lifestyle headphones, Sennheiser is known in the audiophile world for its relatively conservative design, and the German way of thinking dictates that form must be subordinate to function. Therefore, the appearance of the 600 series has remained virtually unchanged since the first model, which appeared 26 years ago, and the differences are reduced to details.

Sennheiser HD 660S2 Test


The large over-ear Sennheiser HD 660S2 headphones are equipped with elliptical resonators, which have moderate thickness and a flattened outer surface, so they don’t look too bulky overall. As these are open-type headphones, almost the entire outer surface is a solid metal mesh – the density of this mesh is significantly lower than that of the more affordable 500 series, so the internal structure is visible through it. The cushions are located on the inside of the resonators, which are traditionally covered with a kind of plush material by Sennheiser, with medium-hardness padding.

Like the resonators, the headphone holder is made of well-processed plastic, which is a way to keep the total weight at just 260 grams. However, the mechanism for adjusting the circumference is made using metal strips with clearly defined divisions. At the same time, the hinges are hidden from view but still allow the resonators to rotate solidly around the vertical axis. The inside of the holder is also upholstered with a two-part cushion, which is separated in the central part and equipped with soft filling. In short, the HD 660S2 looks like all other models in the 600 series, and only experts will be able to notice differences in the color of the markings – the latest version brings logos and inscriptions in bronze.

Sennheiser Consumer Audio HD 660S2 - Wired Audiophile Stereo Headphones with Deep Sub Bass, Optimized Surround, Transducer Airflow, Vented Magnet System and Voice Coil – Black
  • Natural, relaxed, high-fidelity sound with extra sub bass: Ultra-light aluminum voice coil with 300-ohm impedance for...
  • Airflow-optimised 42mm high-performance transducers: Improved ventilation due to a uniquely powerful vented magnet...
  • Double the sound pressure at the lowest octaves: You can feel the lowest note on the piano (27.5 Hz) or the kick of the...
  • Exceptional comfort and build quality: The open-back design offers exceptional breathability and the plush ear cushions...
  • Outstanding quality designed in Germany, made in Ireland: With a frequency response of 8 Hz - 41,500 Hz and detachable,...


If the design is practically unchanged, technical solutions are another matter – HD 660S2 is equipped with a completely new speaker unit with a diameter of 38 millimeters (previously 42 mm), with an extremely light aluminum coil. The powerful magnet with optimized airflow for ventilation is responsible for the smallest possible distortions, and the factory claims that the new driver brings significantly better low-frequency range, while retaining the relaxed neutrality characteristic of the 600 series.

Sennheiser HD 660S2 - exploded view of the headphone driver

The changes are also reflected in the technical characteristics, so HD 660S2 brings an even lower limit (8 Hz), but also double the impedance of 300 ohms compared to 150 ohms in the previous model HD 660S. Like all more expensive hi-fi headphones from this manufacturer, the HD 660S2 is equipped with patented two-pin connectors on the bottom of both resonators, which means they can be used in a balanced configuration.


The headphone package may seem modest compared to the price, but it contains everything necessary, and more. In addition to the manual and textile carrying bag, there are single-ended 6.35-millimeter and balanced 4.4-millimeter cables with a length of 1.8 meters, as well as an adapter from 6.35 to 3.5 millimeters – for the predecessor, the cables were made of OFC copper, but we couldn’t find that data for HD 660S2.

Sennheiser HD 660S2 Sound Test

As you can conclude from the label, HD 660S2 should not be seen as a completely new model in the series, but rather as an improved version of its predecessor. The sound signature is therefore quite similar, which implies an extremely neutral coloration and plenty of details in reproduction. If you have never tried headphones from this series, this description may make their sound seem dry and sterile.

However, this is precisely where Sennheiser’s magic lies – despite numerous nuances and laboratory-precise tones, HD 660S2 provides almost unreal layers of notes and makes the music come alive in the listener’s ears. The fantastic separation of each tone does not diminish the fluidity of reproduction and their connection into one coherent musical whole, and the degree of articulation is high. The soundstage is wide enough to clearly and without congestion display the entire symphony orchestra, while with a smaller number of instruments, this space is used for precise positioning and creating a three-dimensional space.

Of course, it is not even worth mentioning the well-known “magnifying glass effect” because HD 660S2 allows for reproduction volume up to the point of comfortable listening, without noticeable distortions and any other unwanted effects – provided that they are connected to a sufficiently powerful and stable amplification.

Sennheiser HD 660S2 Close Up VIew

In the second part of the HD 660S2 testing, I connected them via a balanced connection, and, as expected, the sound became even better – to such an extent that I consider this the only correct way to use these headphones. The differentiation of individual tones is at such a high level that the resolution seems infinite, and the way these headphones outline notes and resolve the most complex sections impresses with a seemingly casual approach and allows for very long listening sessions without fatigue.

Very significant for the overall reproduction experience is the completely dark background, so when listening to songs with pronounced ambient, the tones jump out of the darkness, making listening to music an unforgettable experience. According to my ears, the HD 660S2 has a slightly protruding mid-range, but the overall transparency and precision reduce this to a feature without much significance – because the overall picture is so clear and defined that other parts of the spectrum are equally present and clear.

Sennheiser cites the low-end range as the biggest improvement compared to its predecessors, and in this part of the spectrum, I first noticed a slightly greater mass of tones. Therefore, there is no talk of an exaggerated HD 660S2 bass that will cover everything else, but it is about a relatively small amplification, as indicated by comparative graphs, which can be found below.

Sennheiser HD 660S2 Frequency Graph

All other low-tone features, such as pronounced speed and convincing, comprehensive representation of the most complex sections, are fully preserved, so there is no improvement in one feature at the expense of another.

Finally, like all Sennheiser headphones in the hi-fi segment, the HD 660S2 proved to be extremely comfortable because the combination of soft plush and good ventilation ensures that the ear cups remain unheated even after hours of listening. The overall feeling is contributed by a relatively small weight and optimally measured pressure on the head, and the adjustment of the slider on the headband is done with slightly greater resistance, which we consider a good solution – once properly adjusted, HD 660S2 will not easily move from the ideal position during use.

Sennheiser HD 660S2 Video Review

To Buy Or Not?

As mid-range hi-fi headphones designed in Germany and manufactured in Ireland, the HD 660S2 offers a classic design and is made to last and bring happiness to its owners for years to come – this is evidenced by the replaceable ear pads. The sound of this series has been a reference for numerous audiophiles worldwide for years, and these tested headphones continue to provide a beautiful neutral reproduction, richness of precise details, and unsurpassed transparency, following a well-established path.

Sennheiser HD 660S2 Pro & Cons


  • Design subordinate to function
  • Exceptional ergonomics and comfort
  • Seemingly endless resolution
  • Neutrality and transparency


  • According to my ears – nothing
Sennheiser once again succeded to make headphones which will become a legend. Highly recommended!
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4.3/5 - (3 votes) - Click on stars to leave a review

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