Sennheiser HD 660 S Headphones

I love headphones. I mean, how else can you finish off your day listening to some of your favourite tunes without waking up everyone in the house? The only caveat is that you have a sufficiently capable amplifier and a set of headphones with enough muscle to wrestle the sonic wonders out of your CD, LP and FLAC, WAV, AIFF and other files. Which is why I was excited about receiving the latest Sennheiser audiophile model, the HD 660 S, to spend hours relaxing while engaging in the review process.

The HD 660 S is a reference-class open-back, dynamic headphone replacing the long standing HD 650. They employ new and improved transducers utilising a specially-manufactured precision stainless steel fabric designed to improve control of the diaphragm movements as well as extremely light aluminium voice coils to improve impulse response. Sennheiser says that the transducers are hand-selected for precise (±1 dB) left/right matching which improves accuracy and spatial presentation. They are supplied in a nicely crafted hinged box with precision-shaped foam so you can rest assured that when you’re not using them, your headphones will be stored safely.

Sennheiser quotes a frequency response for the HD 660 S that covers the 10Hz to 41,000 Hz (-10 dB) spectrum while the impedance is stated to be 150 ohms. The HD 660 S is capable of solid Sound Pressure Levels (SPLs) with the official specification being 104 dB at 1V 1kHz with low distortion (Total Harmonic Distortion + Noise) of less than 0.04% (1 kHz, 100 dB).

There are two detachable cables included – the traditional 6.35mm stereo plug (with a 3.5mm converter) and a 4.4 mm Pentaconn balanced stereo plug which is handy if you happen to have a headphone amplifier that can accommodate it. Sennheiser seems to want to expand the target audience to include those listening on-the-go to portable audio players. I think they could improve the value proposition here by including a shorter 3.5mm terminated cable (instead of an adapter) and offering the balanced connector as an optional extra. The cables appear to be high quality and, to their credit, did not exhibit any significant ‘handling noise’.


My wife stated that “they look funky” as I unpacked them – another box ticked as I’m unlikely to get laughed at while wearing them.

With everyone tucked up in bed and the house to myself I plugged them into my Perreaux e250i 40th Anniversary Edition integrated amplifier (which has a particularly good headphone amplifier stage). The HD 660 S with their 150 ohms nominal impedance proved easy to drive but did need a little more volume than my AudioQuest Nighthawks to reach a comparable listening level. Ultimately it was a non-issue, as they could be driven to levels high enough to aurally incapacitate most mere mortals.


Since it was still relatively early, I fired up Bruno Mars 24K Magic and my immediate impression was a smooth, clear midrange, top-end sparkle and a spacious sound-field. The HD 660 S had the familiar firm head grip that I’ve experienced from other Sennheiser headphones. Had I the inclination (or perhaps I should say the ability) to strut some funky stuff on a nearby smooth surface I have no doubt that the HD 660 S would have stayed firmly in place. As cool as this is, it means that no matter how caught up in the music you get, you are unlikely to forget you’re wearing them.

Onto some Angus and Julia Stone from their Down the Way album and the sound was up close and personal. Compared to the Nighthawk it was front row seats rather than mid-way back. At the time I didn’t have an HD 650 or other Sennheiser headphones on hand for direct comparison but the overall sound presentation matched my recollection of previously auditioned Sennheiser models. The HD 660 S captured a fantastic amount of detail. I noticed on the track “Big Jet Plane” for the first time a slight distortion from the left channel with Angus' voice around 26 seconds in. I’m not sure how I've missed that before but sufficed to say that the HD 660 S is not forgiving and will reveal much within your music. I will leave it up to you to decide if this is a good thing. Or not.


I can't escape the feeling that a lot of popular music produced in recent years has been produced with a bass emphasis making for a bottom heavy type of sound. This may work well for a wide range of less expensive (and less capable) components, but on proper gear designed to emulate real life it can be fatiguing.

Gimme Some BASS Man!

Listening through my Chord Mojo (a truly fantastic piece of gear if ever there was one) the sound was even more detailed and involving. The cymbals on King Crimson’s “The ConstruKtion of Light” from their recent Live in Vienna album positively sparkled through the HD 660 S. You could almost feel the sticks hitting the metal, and the drum skins being struck.

The difference in the low end response between the HD 660 S and the Nighthawk cans however were noteworthy. I mean both are high-end headphones presumably designed for accuracy and detail retrieval, so why the large difference? I figured that I needed to get to the bottom of it.


Chesky Records have long asserted that "if you want to hear how good a speaker or headphone can sound you have to play great sounding recordings" and while I like to use music to which I actually enjoy listening to as a means to assess audio components, I also believe the Chesky philosophy has merit, which is why on this occasion I reached for their Audiogon Presents The Wake Up Your Ears Sampler. I listened to several tracks but the one that sorted out the bass for me was 'Stank', a Jazz piece featuring uncompressed drums played by three percussionists. The HD 660 S sounded crisp and airy and the skins, when struck, seemed to have a few quarks more impact than the Nighthawk, however, the low end that was left 'kicking around the room' was not felt on the HD 660 S as it was on the Nighthawks.

Interestingly, I stumbled upon John Hammond's "Get Behind the Mule" Blues piece which confirmed for me the true difference. On the Nighthawk you could clearly hear him stomping his feet while performing, whereas through the HD 660 S it was more of a ‘tap-tap-tap’. Ultimately it will come down to which presentation you prefer. It could be said that the Sennheiser’s bass was an all-round more accurate rendition of the overall low-end.


I had intended to audition some Wilco to finish off, however I continued the Chesky tangent and ended up enjoying the heck out of CC Coletti's “You Shook Me" (the Led Zeppelin classic). The sound-field was huge and the HD 660 S really let you hear into the recording venue. This recording being free of dynamic range compression, processing and, as Chesky puts it "other forms of studio trickery", allows you to hear what the performance really does sound like.

The Sennheiser HD 660 Ss are great headphones at their price point. They convey all the energy of the performers and have a smooth, clear top-end without ever sounding edgy or harsh. On music produced in the last 10 years they portray a natural and lifelike sounding bass. To state the obvious, everyone's hearing will measure differently, I believe that those whose hearing lacks a little sensitivity on the upper midrange and higher frequencies will find the HD 660 S most appealing, those who are bass heads however, may want more low-end heft.


As an aside, at my office while boxing them up to send back to the distributor I asked one of my colleagues (Pawan) if he had ever heard a set of high-end headphones. He hadn’t, so I unboxed them for a final time and connected them to Mojo. From Tidal, via the iPhone, I selected America’s “Ventura Highway” which I had been enjoying in the car on the commute. The look on Pawan’s face after only a matter of seconds was priceless! I did the same thing with Danie another colleague who stopped by, while Pawan and I were chatting about what he had just heard, and Danie explained that he thought his wife had recently bought him a set of Sennheisers. He put on the HD 660 S, listened, smiled and exclaimed “Oh… mine don’t sound like that! Can I try something classical?” The fun continued…


As is always the case, with a large enough wallet you could always go upwards (there are certainly superb offerings above it within the Sennheiser stable and we can dream about the HE 1…) but if you’re looking for an awesome-sounding and accurate set of headphones while, at the same time, wanting to retain the girth of your wallet you could just go ahead and stop at the HD 660 S.

…Barry Jones

Associated Equipment

  • Speakers — MartinLogan Spire
  • Amplifier — Perreaux e250i 40th Anniversary Edition integrated amplifier
  • SourcesDigital: Lumin T1, Oppo BDP-105D, Sonos ZP90. Analogue: Rega RP6 turntable, Rega Exact-2 cartridge, Rega Aria phono stage & Rega Couple interconnects
  • Processor — DEQX HDP-5
  • DAC/Headphone Amplifier — Chord Mojo DAC
  • Headphones — AudioQuest Nighthawk
  • Cables — Nordost White Lightning, Kimber Cable Timbre balanced and Van Den Hul D-300III unbalanced interconnects, Atlas Compass Digital Coaxial Audio Cable, QED Performance Graphite USB
  • Miscellaneous — Isotek Sirius Power Conditioner, custom made bass traps in front wall corners

Sennheiser HD 660 S Headphones
Price: AU$799.95
Warranty: Two years

Australian Distributor: Sennheiser Australia
+61 2 9910-6700

Sennheiser electronic GmbH
Am Labor 1
30900 Wedemark
+49 (0) 5130 600 0