Sennheiser HD 660 S Over-Ear Open Headphones

I'll not wax poetic this time regarding the long and storied history and my experiences with the HD 6xx family of Sennheiser headphones. For that, simply go have a look at my "The Very Important Sennheiser HD 580, HD 600, and HD 650" review.

For now it's sufficient to state that the Sennheiser HD 600 and HD 650 are probably the most highly regarded enthusiast headphone in the world, and I highly recommend both. The HD 650 is a bit too warm sounding for me (just a bit), and the HD 600 is my favorite of the two.

The only problems with them is that the clamping force on the head is a little too strong, and they can sound a little "veiled" or distant in the lower presence region around 800Hz to 1500Hz—the HD 650 a bit more so than the HD600. Some will say the HD 650 has more mid-bass bloom, which it does, but it amounts to the same thing relatively speaking.

The clamping pressure can be modified fairly easily: extend the earpads all the way; grip the headband with both hands close to the center of the headband; then rotate your hands putting pressur near the center of the headband so as to stretch them out. The headband is a very high quality plastic and will not break; just be firm and not over-zealous feeling the plastic deform slightly as you do it and repeat until the pressure is right. I've heard others suggest to bend the metal headband extensions; I don't like this idea as the shape of the metal extensions are set to slide into pockets in the headband and I think it's best to let them retain their proper shape.

Numerous modifications have been developed by enthusiasts over the years. I have to review headphones in their stock form so I have only a little personal experience with these modified cans. If you'd like to learn more, I suggest starting your journey with this SBAF thread pointing to wide variety of HD 6xx information resources, and this post indexing a number of the modification threads.

And with that, we'll dive right into the HD 660 S

Sennheiser HD 660 S ($499.95)
The HD 660 S visually has very close family ties to its older siblings and, to me, it's the best looking variant yet. Liveried in simple matte black plastic with new grills that include a raised area with the Sennheiser logo it strengthens my personal conviction that audio gear should be black. Silly, I know, but I think a stealth black look helps put the focus on where it belongs: sound quality, not looks.


In this, and the following series of photos, the HD 600 is at left, the HD 650 at center, and the HD 660S is to the right.

The HD 660 S still has the somewhat tight clamping force, which can be remedied as mentioned above. Once set, the HD 660 S has the same quite comfortable, head-hugging fit and light weight of its predecessors. There are a few subtle differences.

Though the presentation case remains the same, the HD 660 S comes with two cables, both about ten feet long, one terminated in a 1/4" TRS plug, the other terminated in the new 4.4mm Pentaconn TRRRS balanced connector. Also included is a 1/4" jack to 3.5mm plug short cable adapter.


There has been a slight change to the earpad. From what I can gather the earpads are slightly thicker and have a slightly beveled inside circumference. In this Head-Fi post a Sennheiser representative said is was done to make them feel a little more "roomy and pleasant." I went back and forth between my HD 650 and the HD 660 S and felt indeed there might be a small improvement in comfort, but it is subtle.

Let's take a look inside:


As you can see there are a lot of similarities between the ear capsules of these headphones, if fact they are identical in a number of ways. Earpads, grills, and cables are interchangeable. The one big different is the driver assembly, which is completely different and not interchangeable. The diameter of the HD 600/650 driver housing is 1.722", the HD 660 S is 1.737" in diameter.


Close-up of the rear of the HD 650 driver (top) and the HD 660 S driver (bottom).

Obviously, these are quite different drivers. Rumors run rampant that this is an HD 700 driver. I wasn't able to find a definitive comment from Sennheiser, but in this post Jude claims it is a derivation:

The driver is based on the Sennheiser HD 700's driver, but it does not sound like an HD700 -- it sounds like an HD600-series headphone, through and through.

He's fairly privy to inside information, and a good hard look at the two make it fairly obvious they're related.


Rear of HD 700 driver (top) and the HD 660 S driver (bottom).

I don't have an HD 700 here to take physical measurements, but the resemblance is undeniable. The most distinguishing feature to my eyes is the very fine stainless steel mesh behind the outside edge of the driver formed to mimic the shape and ventilate that area evenly. Here's what the Sennheiser HD 660 S product page says:

The HD 660 S features a new transducer design developed by Sennheiser. This results in improved control of the diaphragm movements thanks to a specially manufactured precision stainless steel fabric, which is adapted to the contour of the diaphragm. Extremely light aluminum voice coils ensure the highest impulse fidelity.

Hm. Well they sort of state that it's a new design, but I'm betting that's a bit of poetic license—the HD 700 is a relatively new design. Thing is, impedance and phase measurements are almost identical.


Man, it doesn't get much closer than that. By my reckoning, it seems very likely that the HD 660 S is using the HD 700 driver design with a redesigned outer housing to fit into slightly changed HD 6xx baffle plate.


300Hz square wave response of the HD 600 (top), HD 660 S (middle), and HD 700 (bottom).

The thing to me that's very interesting is that, assuming it is an HD 700 driver, how little the driver performance itself effects the performance of the headphone as a whole. In the 300Hz square wave responses above you can clearly see the HD 660 S response (middle) is very much like the the HD 600 response (top) and is quite unlike the HD 700 response (bottom)...and that's a very good thing. I found the HD 700 a very bright headphone with an uncontrolled treble. Just goes to show you how much the overall acoustic of a headphone is largely due to the acoustics of the ear cup and has less to do with the driver itself.

Alrightythen, let's have a listen.

Sennheiser USA
1 Enterprise Dr.
Old Lyme, CT 06371
(860) 434 9190

amartignano's picture

And I've also bought headphones despite Innerfidelity reviews and liked them very much with my ears/tastes.

IgorC's picture

Interesting review. Thanks!

My first thought was that 660s had hand-picked 650s drivers (with better THD and FR characteristic.)

Well, it's actually HD 700 driver after all. Makes sense. Probably Sennheiser doesn't want to throw away some HD 700 drivers which don't meet THD requisites (<0.03%) but still suitable for HD660s (<0.04%)

I still remeber that my first audiophile grade headphones were HD 650. :) Awesome.

Impulse's picture

Welp, that settles it, I'd been curious about these for a little while but seeing as I have some decent upstream gear I guess I'll pass.

I've been wanting to hear the difference between the HD650 & my HD600, and/or mod the former while having a sort of reference, guess the time to do so is now before they vanish.

Hopefully they're still $300-ish in January after I'm done traveling for the holidays...

GimmeCans's picture

I want to echo the sentiment expressed by an earlier commenter, that it's good to know that Innerfi is willing to call things the way you see (hear) them, even when it doesn't 'tickle'. Manufacturers shouldn't expect a 'free ride'.

As a longtime HD650 owner and advocate, the review eased the tinge of buyer's remorse I felt when learning of the 660S just after buying the Beyer Amiron which is $100 more. Despite my high regard for Senn's classic, I'd be hard pressed to name an area where the Amiron doesn't outperform the 650 to my ears (maybe the 650's smoothness makes it a little more 'forgiving' on some material.) For those who were considering the now-discontinued 650 or another open can in that range, the Amiron should be on your radar IMO although I will admit the overall signatures are quite different.

GumbyDammit223's picture

Do you want a HD700 to compare against? I'm in Bozo so it would be quick to let you borrow mine.

GimmeCans's picture

Now comes the HD58xx from Massdrop, looking almost identical to the HD6xx. There's only one man who can sort all this out.... Look! Up in the air! It's a bird! It's a plane!

GimmeCans's picture

Oh, and it's $149....

Martin.'s picture

Saw this as well yesterday. Seems very interesting, would like to see someone review it. I also have just bought a 6xx from Massdrop cause of this review.

arteom's picture

Great review. I get the impression that manufacturers are designing their products to play well with low level gear like smartphones. Think they have come a long way with pulling that off while still having a sound that is above consumer level products and more in line with audiophile level. The compromise being how well they scale and the grain you spoke of. I think they are working hard to disguise the grain as 'energy up top'. It is remarkable that they have been able to achieve this at all, don't think we would have seen a product like this ten years ago. That all being said, I won't be trading in my 650 anytime soon.

verbosity's picture

Hy Tyll,

Would you say the 660s is superior to the HD600/650 when driven by a cheap <$300 amp? If so it seems an easy explanation for its existence and positive reception in the forums. If not, who knows... maybe just personal preference of most people deviates from your own in this case?

Michael Ford's picture

Thanks, Tyll. Great review as always. You've given me all the excuse that I need, and have been looking for a long time, to buy another HD650! I'll use one with a CH650S balanced cable on my HDVD800, and the other with the stock cable on everything else. I also have an early HD600 (and 580 Precision) and although I fully understand why you and many others prefer the HD600, my preference is for the HD650. To me it is warmer, and yes, darker, but it has better resolution and soundstage.

One thing I'm certain about is that if Senn do discontinue the HD650, it will immediately go from legendary status to cult status!

ScaryFatKidGT's picture

I think Sennheiser is running into the Gibson/Fender problem... they have been doing things so right for so long either by nailing it right away or through trial and research an actually being great or by customers just expecting that sound or a combination of the two that if they make anything different it doesn't do well. I think Sennheiser should have improved the HD700 with an "HD700 S" instead of making a 660 and then the Massdrop "580" which is more of a 660 in 580 skin.

detlev24's picture

Another quick review with comparison to the elder models; from another point of view. Enjoy!

maitrishah's picture

The Clear is the same price as HD800S, but simply a much better product. If you like it, it‘s fine for you. But to recommend them in 2018 would be irresponsible in my opinion free amazon gift code generator

blogkens's picture

These headphones are really very great.

blogkens's picture

if Senn do discontinue the HD650, it will immediately go from legendary status to cult status! Read more.

Michael Ford's picture

Tyll, for your information, I've just replaced the ear pads on my original (pre Jubilee model) precision 580's with replacement Sennheiser pads (part number 050635), and they are identical to the new pads on the 660 S, i.e. a little thicker and slightly bevelled on the inside. This leads me to assume that all 600 series headphones will now come with pads like this, and it's not a difference specific to the new 660 S.

Best regards, Michael.

Michael Ford's picture

I now see that the Senn rep stated (in the Headfi post that you linked) that the replacement parts would be the new design from now on, but, again, one would assume that means that new HD 600 and HD 650 headphones will also have the new pads. It wouldn't make any sense to manufacture two slightly different shape pads and provide the old type only on new HD 600/650's and not as replacements for those headphones. All pretty irrelevant in the bigger scheme of things, but nonetheless, good to know.....maybe......

poekimalu's picture

Hi, I have LCD2 Classic which I love it, but it gave me a headache most of the time because the weight and clamped to much near my ear. My question is should I get HD660S or HD650 if I decides to swap my LCD2 Classic? I did tried HD660S before but never try HD650. Cheers

charliechandra's picture

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