Review: Sennheiser HD 660S

A new addition to Sennheiser?s 600 range of headphones is always a milestone event. The German brand has never failed to produce neutral and dynamic headphones which are praised by users and the specialized press alike. With its new Sennheiser HD 660S, Sennheiser promises to improve upon the Sennheiser HD 650, considered by many to be a reference in its category. To this end, the manufacturer has fitted the 660S with the same drivers as those found on the Sennheiser HD 700.

The Sennheiser 660S is an open-back model fitted with dynamic transducers which cover a frequency range of 10 Hz – 41 kHz. Its sensitivity rating is 104 dB at 1 kHz (1 V) and its impedance at this frequency is 150 Ohms.

Sennheiser HD 660S: why choose this impedance rating?

Sennheiser has chosen this impedance rating deliberately in order to ensure optimal working conditions for the amplifier used to drive the Sennheiser HD 660S. The higher the impedance rating, the less distortion will impact the quality of the audio signal, particularly in the bass register. In addition, a high impedance rating is obtained by using a bigger (yet very thin and light) voice coil, and the magnetic field generated when the headphones are powered allows for more precise excursions of the driver?s cone.

Sennheiser HD 660S: do I need a very powerful amplifier?

The Sennheiser HD 660S has a very high impedance rating (150 Ohms), yet its sensitivity rating is also very high at 104 dB for 1 V. These headphones are thus not particularly hard to drive, and their impedance rating does not make it difficult to enjoy them at very high volume. When it comes down to it, a high impedance rating shouldn’t be seen as a drawback, but rather as a real advantage for a headphone amplifier, as the latter benefits from more favorable working conditions than it would with a pair of headphones with a low impedance rating. Our listening sessions relying on smartphones and digital audio players – inherently optimized for low impedance ratings – were entirely satisfactory.

What?s more, the Sennheiser HD 660S headphones? high impedance rating makes them an ideal match for sedentary hi-fi amplifiers, (Sennheiser, Hafler, Audio Technica?). These models have high impedance ratings, which enables them to offer a linear and smooth restitution with a pair of headphones such as the HD 660S, as well as a substantially greater amount of definition in the bass register.

Sennheiser HD 660S: the benefits of high sensitivity

As is the case for acoustic speakers, a high sensitivity rating offers nothing but advantages for a pair of headphones. Less energy is required to produce the audio signal, which in turn reduces distortion and makes it easier to attain high volume levels. Unlike a speaker ? whose sensitivity rating is measured in an anechoic chamber at a distance of 1 m ? a pair of headphones is measured on a mannequin fitted with an artificial ear canal, inside of which two microphones are placed. The rule of thumb is that these microphones should be positioned where the eardrum would be, at the inner part of the ear canal. An amplifier is then used to send pink noise (wideband, voltage of 1V) to the headphones. The results obtained vary according to two factors: the impedance of the drivers? voice coil (like for a speaker), but above all, the design of the headphones and the distance between the drivers and the listener?s eardrums. Headphones with drivers placed closer to the eardrum have a very high sensitivity rating, while a pair of circumaural headphones whose drivers are about ¾? (2 cm) from the eardrum will have a lower sensitivity rating (due to attenuation resulting from the distance).

It is worth noting that, just like for a speaker, the rating in decibels is indicated for 1 kHz, a frequency to which our ears are very sensitive at any age. This frequency is not representative of the sensitivity rating of a pair of headphones in the bass register, which may be up to 10 dB less. In short, any sensitivity rating should be put into perspective, and it should be kept in mind that it does not determine what the frequency response of a pair of headphones will be.

Sennheiser HD 660S: design

Sennheiser has designed the HD 660S for lightness and wearing comfort. The headphones are a total success in this regard, although this has been achieved by using lots of plastic. Despite this, no mechanical noise was audible during our listening sessions?not even when the cable rubbed against our clothes. The earpads and headband are fitted with velour-lined memory foam. The headband applies just the right amount of pressure to the head.

Sennheiser HD 660S: accessories included

The Sennheiser HD 660S headphones come with a soft carrying case, a 3 m long unbalanced cable, a 3.5 mm cable with a jack-to-mini-jack adapter and a balanced cable with 4.4 mm connector.

Sennheiser HD 660S: test conditions and listening impressions

Most of the time, we listened to the Sennheiser HD 660S with the FiiO Q1 Mark II DAC amplifier and the iFi Audio iSilencer. We also listened to the headphones with different audio players and smartphones, all of which gave us a similar impression. The FiiO Q1 II offered advantages including a balanced output, gain control and even a bass boost which could be activated or deactivated. As always, we listened to a selection of FLAC files (CD/HD) as well as music streamed from Deezer (320Kbps MP3 files).

The Sennheiser HD 660S completely won us over. This was no small feat, as we had just reviewed the excellent Focal Elear and Focal Clear headphones. Truth be told, for half or even a third of the price, the Sennheiser HD 660S delivers a totally gratifying sound restitution. Forget about the Sennheiser HD 650; the 660S outperforms its predecessor and corrects the emphasis on mids (1-3 kHz) that was its weak point (shared even by some of the brand’s much more expensive models), while offering an even more convincing restitution of lows. During our listening experience, there wasn’t anything out-of-bounds, and the three registers were seamlessly integrated. We dug up the most poorly balanced recordings we could find, and the 660S systematically delivered a pleasing restitution.

Sound image

The Sennheiser HD 660S strikes a delicate balance between width and depth, regardless of the source being used and without exaggerating the stereo effect. No effort is required to distinguish between the different sound layers, as the HD 660S offers a very straightforward restitution.

Dynamics

Remarkable in that they are never showy and bring out the best in well-recorded tracks. We rediscovered certain recordings which seemed like nothing out of the ordinary, but were revealed to be captivating by the Sennhesier 660S. While listening to New Order?s Blue Monday, the frenzied impact of the electronic drum kit instantly drew us in, and the Sennheiser HD 660S kept our attention without letting it go for a second. Although the binary arrangement is typical fare as far as recordings from the 80s go, there was an overall sense of cohesion that made for an engaging listening experience. On a different level, David Bowie?s Cat People (Putting Out Fire) demonstrated the headphones? ability to deliver a very generous dynamic range (from 1:48 on) and take the lead when necessary. Once again, the transient response in the midbass and bass registers made for a realistic restitution of the percussion. Bowie?s vocals, as well as the female vocal accompaniment, were magnificently restituted. The drum crescendos were surprising and added a lot of spice to the listening experience.

Timbres

Thanks to well-designed drivers with a very linear frequency response, the timbres were truly accurate. Take Jacques Brel’s Les Bourgeois, for example: accordion, acoustic guitar and harpsichord sounded realistic and smooth. With Melody Gardot?s Your Heart is as Black as Night, no instrument outshone the others, and the fundamental chords were always energetically delivered. With the volume turned up, it was spellbinding.

Lows: coherent throughout, very consistent and balanced with respect to the other two registers in terms of volume and transient response. A gratifying sense of completeness.
Mids: natural and very slightly subdued at 1-3 kHz, doubtlessly in order to avoid adding artificial brightness and to keep the listening experience from growing tiresome. A plentiful amount of details, all of which were judiciously distributed.
Highs: what can be expected from a pair of Sennheiser reference headphones, that is to say, accurate and without added brightness, although there is a boost in energy above 10 kHz. All that?s required to bring a lot of dynamics to the other registers.

Sennheiser HD 660S: conclusion

What we liked: the tonal balance, the energy, the wearing comfort, how easy the headphones are to drive
What we would have liked: we can’t find fault with these headphones, especially considering the value they offer for the money.

Let there be no doubt about it, the Sennheiser HD-660S is an excellent pair of hi-fi headphones which easily competes with models available for twice the price. Sennheiser isn’t pulling any tricks here, and the 660S delivers an honest and impressive sound restitution. Hats off.