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May 2018

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Sennheiser HD 660 S Over-Ear Headphone Review
If it ain't broke, fix it anyway.
Review By Dave Hanson


Sennheiser HD 660 S Over-Ear Headphone Review


  Updating a much-loved product like the Sennheiser HD 650 is a tricky task. But having spent nearly 75 years in the audio business, Sennheiser understands the importance of reading the tealeaves. The personal audio industry is in a period of tremendous growth and no single segment is growing faster than the portable devices category.

So with that, Sennheiser made a bold move. While many companies would have left well enough alone, Sennheiser took one of their most popular products, the somewhat hard-to-drive HD 650, and updated it with and easier to drive design. The result is the new Sennheiser HD 660 S ($499).

While the original could be a bit fussy with amplification – holding back some of its potential until you plugged it into the perfect tube amp – the HD 660 S skips all the fuss and sounds good with just about anything. And while there's a bit of a cost in terms of overall scalability, the headphone provides an intriguing option for an increasingly mobile audience.

But that's not where the differences end. I'll be honest, I didn't exactly love the original HD 650. But I must say, I actually enjoy the HD 660 S. I found that there were several nuances around the edges where Sennheiser found room for improvement. So what changed? Let's take a closer look.


Sennheiser HD 660 S Over-Ear Headphone Review


Sweet Music And Subtle Differences
The overall tonality of the HD 660 S is sweet, smooth and relaxing. The frequency response levels pretty much mirror those of its predecessor, the legendary HD 650, with a slightly warm tilt that doesn't totally neglect neutrality. They definitely sound similar, but the HD 660 S seems to have a little more refinement overall.

The most significant difference I noticed was a bit more crispness in the HD 660S's attack and decay properties, which I like. The HD 650 always sounded a little bit sluggish to me and had a tendency to blur together fast musical passages, rather than giving notes a clean exit and entry point. I find the edges to the HD 660 S to be much cleaner. Don't expect electrostatic-level incisiveness, but I don't feel that this area is a handicap anymore, which automatically vaults the headphone up a level for me, personally.

One area where I noticed this was in the bass, which came across to me as slightly cleaner in the HD 660 S. For example, you can actually enjoy heavy metal on the HD 660 S, where as the HD 650 had the tendency to get a little sloppy with fast double kick drums and the deepest notes of a down-tuned bass guitar.

In terms of presence, the bass is solid, but if you're looking for something to truly satisfy with hip-hop and EDM, the HD 660 S probably isn't going to satisfy you as well as a Campfire Cascade ($799), Emu Teak ($499) or Fostex TH-610 ($499). For rock, blues, jazz, country and the like, the HD 660 S is just right.


Sennheiser HD 660 S Over-Ear Headphone Review


The bass gives the music a solid amount of weight without being excessive or anemic. Sub-bass is stepped down a bit from the midbass and rolls off a little early compared to a lot of the more current headphones that have been released since the original HD 650's heyday. Still, there is something comforting and familiar about the HD 660 S's bass presentation. It is a classic sound that is just “right”. In terms of reliability as a reference/mixing tool, I think that the headphone definitely has some merit in terms of both performance and tuning. It should yield fairly predictable results with systems that don't have a lot of sub-bass extension.

Another gripe I had with the original HD 650 when I owned it was that there seemed to be a little excess energy or distortion in the middle midrange that always irritated my ears. It was a bit of shouty-ness between maybe 500Hz and 800Hz. I believe this was a problem with earlier HD 650 models that was fixed in the later iterations of the headphone, but I didn't hear it here either.

The midrange overall is excellent, with terrific balance across the board. Vocals are lush and sweet, and tough to top for the humble price of $499. I think to truly top them, one would have to stretch the budget a bit for the MrSpeakers AEON Flow (open or closed $799), the Audeze LCD-2C ($799) or the ZMF Atticus ($999) – depending on what kind of midrange one prefers. That being said, the HD 660 S is not far behind at all, and for the cost savings, it might be a worthy sacrifice, considering just how close they get.

The treble continues the HD 650's tradition of being smooth and inoffensive, giving just enough energy so things have a touch of sparkle, but not so much that it gets sibilant or strident. It runs a near-perfect balancing act, landing typically harsh notes with a sort of velvety softness without robbing the music of its natural sweetness. Bravo, Sennheiser, for really nailing the sweet spot here.


Sennheiser HD 660 S Over-Ear Headphone Review


The treble has really nice airy extension to it, though there is just a little bit more grain to it than some higher end models like Sennheiser's own HD 800. Running out of my Glenn OTL tube amp, I felt that my 300 ohm headphones (HD 600, HD 800, ZMF Auteur and Atticus) were all able to achieve a more grain-free, transparent sound. In other words, I don't think the 150 ohm HD 660 S has quite the same level of scalability as the others.

That being said, I think the HD 660 S does a great job of scaling down with portable amps and DAPs without losing much from an absolute performance standpoint. Running out of DAPs like the Acoustic Research M2 and Fiio X5 III, I felt like the HD 660 S was getting a little closer to its regular peak performance level – maybe at 90% of the maximum, where other headphones (especially the HD 800) were more fussy, maybe giving you 70-80% of their capability.

With the Acoustic Research M2 ($999), the HD 660 S threw a surprisingly open and well-layered soundscape. Width seemed to extend beyond the outside of the cups to mid-shoulder, which I would say is certainly above average. Depth layering is excellent with each instrument having plenty of space to breathe on the well-defined stage. Imaging within the space is very good, and I'm hard-pressed to think of many options under $999 that I would consider definitively more pinpoint with their instrument placement.

The detail levels are strong across the board, with good inner resolution on the instruments and a nice amount of ambient detail from the recording space. Like its predecessor, the details on the HD 660 S are simply present, and can be easily found with a little effort, but they are not pushed up front and in your face like they are on the HD 800 or the Focal Utopia. The listener can explore them at his or her own leisure.


Sennheiser HD 660 S Over-Ear Headphone Review


The New Classic?
On the whole, the HD 660 S is mostly a minimalist approach, with tweaks around the edges to clean up things like attack, decay and drivability. The HD 650 has long been a community favorite, so it's good that Sennheiser left most of the good stuff alone. The only area where they might have missed in this approach is the loss of some scalability with truly high-end gear. As I mentioned above, I think switching from 300 ohms to 150 ohms left a little transparency on the table.

However, Sennheiser was smart enough to see the writing on the wall – portability is a huge purchase driver and it's only growing as the audiophile community expands and diversifies. Updating the HD 650 to be a bit more mobile was a logical decision on their part. I just don't think everyone in the community will necessarily see this product as an upgrade. Those who have been running the HD 650 off of $2K tube amps won't have much use for the HD 660 S.


Sennheiser HD 660 S Over-Ear Headphone Review


But for newer audiophiles and mobile-oriented users it's a different story. If you're a member of that group, you are probably going to be realizing much more of the Sennheiser's performance capability more quickly on entry-to-mid-level and portable gear. For those who aren't already deeply entrenched in the audiophile world, and simply want to spend $500 on a really nice headphone to enjoy without investing fistfuls of cash on peripherals, the HD 660 S is a very, very good thing.

Same goes for audiophiles who want a no-compromise transportable set-up and don't need isolation. For them, the HD 660 S is terrific. As I finish my review, I'm sitting here with the HD 660 S and Acoustic Research AR-M2, 2,000 miles from home, and I'm not missing my desktop rig too badly.

Time will tell if it can be as successful as its classic predecessor. But all comparisons aside, it's a great sounding headphone with a sound that works well for pretty much any kind of music.



Products Used Within This Review
Sennheiser HD 800
Sennheiser HD 600
Campfire Audio Cascade
MrSpeakers AEON Flow Open
Acoustic Research AR-H1
ZMF Atticus
ZMF Auteur
Chord Hugo 2
Wells Audio Milo
Glenn OTL
Meridian Explorer 2
iFi Micro iDSD Black Label
Fiio X5 Gen III DAP
Acoustic Research AR-M2 DAP
DanaCable Lazuli Headphone Cables
DanaCable Onyx Interconnects
DanaCable TruStream USB Cable
WireWorld Equinox 7 Interconnects
Wireworld Electra 7 Power Cables
Wireworld Starlight 7 USB Cable
Schiit Wyrd
Amarra 4 Luxe



Sub–bass (10Hz – 60Hz)

Mid–bass (80Hz – 200Hz)

Midrange (200Hz – 3,000Hz)

High Frequencies (3,000Hz On Up)



Inner Resolution

Soundscape Width Front

Soundscape Width Rear
Soundscape Depth Behind Speakers

Soundscape Extension Into Room


Fit And Finish

Self Noise

Value For The Money




Type: Over-the-ear stereo headphones
Impedance: 150 ohms
Frequency Response: 10Hz to 41kHz 
Sensitivity: 104dB/V
THD+N: <0.04%
Weight: 260g (without cable)
Price: $499




Company Information
Sennheiser Electronic GmbH & Co. KG
Am Labor 1
30900 Wedemark

Voice: (877) 736-6434
Website: www.Sennheiser.com
















































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