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

donlin's picture

Very clear and honest. It’s refreshing to have a well known reviewer that doesn’t love everything. Sounds like there may be a rush to get the last 650’s.

Maybe's picture

The 660S is rather boring. I was excited at first but I got Sennheiser'ed.

Performance was to be expected given that the HD 700 and the HD 650 use transducers with almost identical TS parameters.
I'm sure if you were to stick an HD 599 driver inside a 650 it would sound similar aswell.

Used HD 580s for 150€ with new pads seem preferable.
However, I think I'll buy the 660S grills for my 650. Looks neat.
The closed HD 800 is next on the list. Then the new Baby Orpheus. And maybe they'll do something completely new in 10 years.

MikeC20's picture

I am a headphone enthusiast but I started with my obsession with High Fidelity home audio, the headphone to me is 100% based on trying to get the best sound possible while being the most convenient.

I still havent fallen in love with any bluetooth so I am fine dealing with the discomfort of cords, but I have been wanting the HD650 but with the inconvenience of needing an amp to really enjoy makes it something that I would never use as I would rather listen to my home audio if I have to be stationary.

My currenty heavy in use headphones are the se535, HD558, AKG 553 MKII and one of my favs the AKG K240 (what a high value headphone!).

My question is, is the 660S is a significant upgrade over the 558/598 headphones? If so I may have a must buy.

MikeC20's picture

Great job as always Tyll, thank you for doing such great work over the years!

metal571's picture

Actually any of the 600, 650, and 660S would be an upgrade over the 558/598 series. The only thing you will lose is soundstage width, and also the signatures are a bit less bright and aggressive in the upper mids and treble region. Detail however is significantly improved vs the 5x8 stuff.

jaredjcrandall84's picture

Based upon reviews, yes it is, but apparently the 650 is better than the 660s, so go there instead.

metal571's picture

Did you buy brand new pads for your 600 and 650 units immediately before doing this review? If not, that has enormously colored your perception of these models' sound signatures. It is well known that older pads will give your 600 and 650 that "smoother" and more "organic" sound that you mentioned. As soon as the pads are replaced, the sound is significantly changed. I would like to hear full follow-up impressions and see measurements of all of these 3 units once you have replaced all pads, if that was not done already, for the sake of everyone who is going to buy pairs of these headphones brand new.

ericw's picture

Agreed. Rin Choi measured a HD 650 with old and new pads, and switching from old to new pads increased 5-6kHz by more than 5dB, and generally everything above 1kHz got tilted up.

see: (scroll down to "Discussion")

I'm not sure if those measurements have been repeated elsewhere?

metal571's picture

Solderdude also looked into this a bit, talking about older style 650 pads vs newer ones, in addition to pad condition.

crazywipe2's picture

All the Headphones Tyll uses for comparison are in almost brand new condition. He is more than 25 years working with headphones giving a huge contribute to all the Headphone World. Do you really think he messed up the review because of the pads? Man, this is ridiculous and disrespecful, you are not talking with a noob.

Tyll Hertsens's picture
The 600/650 I use are relatively new, maybe 4-5 years old. You have to remember I only use them for comparative purposes, so I would guess both pair have less than 40 hours on them. The pads appear in very good shape.
zobel's picture

grainy A moderate texturing of reproduced sound. The sonic equivalent of grain in a photograph. Coarser than dry but finer than gritty.

What causes a transducer to produce grain?

amartignano's picture

Although I don't ear this increased graininess when comparing my 660S to my 600 (indeed I ear the 660S to sound more "organic") I really appreciated this sincere review, as always.

gibtg's picture

Pads, schmads ... There's clearly evidence in the measurements that the treble responses differ from the past designs and I'd trust Tyll's ears to determine how those changes will effect the overall presentation of the headphone. That's why I'm here!

Excellent review once again Tyll!

pete111's picture

"But I'll also steer buyers towards the HD 600—or Massdrop HD 6XX if it becomes available again—as the better sounding alternative."
Letting people know that the 6xx are indeed available on Massdrop now, yes with a delay, but still... a 650 for 200 bucks...

zobel's picture

Is there a relationship here, really?
Aren't the factors that determine impedance in dynamic drivers
1) length of voice coil wire (L)
2) gauge of voice coil wire (AWG)
3) type of conductor in VC wire (what metal)
4) strength of magnetic field in VC gap (B)
5) acoustic and mechanical resonance (Ir)
6) acoustic and mechanical impedance (Imm)
7) voice coil geometry (underhung vs overhung)

Knowing of several examples of sound being improved by increased
sensitivity / lower impedances, rather than the opposite, I don't believe that design criteria (Sensitivity / impedance) is a determining factor in driver performance. Think, for one example, Sennheiser Amperior vs HD 25 II, even in aluminum for both. Requiring a separate special amplifier is becoming less and less popular.

Not to say that better amplifiers won't improve the sound of headphones, any headphones, sensitive or not. Not only are we moving away from chip amps in phones through a wire to the cans, but now it is the rage to be wireless, which involves inevitable signal degeneration. Compressed, lossy music files via Bluetooth to sensitive IC powered cans is the future, very sadly. It isn't the sensitivity of the cans that is the problem though.

Pdxsnap's picture

The introduction of this model does seem a bit odd. It could/should have been a slam dunk in just improving a little on a classic to build on its cult following. If they had just made a unit just appreciably bettter than the 600/650 there would be a large market in devoted followers in upgrading.
I wonder how much of the desire to drop impedance is the reason for the miss fire? I also wonder if there were heated debates within Sennheiser regarding the impedance and direction of this model. Time will ‘perhaps’ tell.

zobel's picture

For many of the reasons Tyll pointed to,these cans don't really cut the mustard anymore. None of them. Biggest problem...bass rolloff. Next to that, the veil..lacking low treble and high mids.

This is a failed attempt to address the real problems with these cans. I disagree that the Sennheiser HD 6XX Massdrop was such a deal. Personally I think AKG had a better can through Massdrop with the AKG K 7XX.

The fact that the housing of the drivers makes such a huge difference in sound points to how much differently we hear them with our various sized ears. I bet I fill a whole lot more of the cans cavity with my flappers than most do. The size,and shape of ear canals makes huge differences as well as hair, glasses (sometimes), head size (fit) it isn't too surprising that pairing drivers to the entire enclosure involved (cans and the individual wearing them) is critical and accounts for the varied responses from individual to individual.

Good review Tyll....very disappointing that after what, 17 years? Sennheiser hasn't made more progress.

potterpastor's picture

I consider the He-1 progress

pete111's picture

I have to say that after your comment I was quite surprised to look at the AKG's 7xx frequency response... AKG are not exactly known for their deep bass. I agree with you that this roll off at 100 is kinda... Well there's something missing. What I was gonna say tough is that to get this bass, in a sub 500$ range, you have to either go closed, or planar, with the compromise associated with both these design and to me the 6XX is still very good value. But I was just thinking today that... Yeah, great classic, very good sound but I keep coming back to something with subs. My other cans are He-400is and Th-X00, which are both to me better than a 6 serie senn. but still more than twice the price of a Massdrop 6XX

zobel's picture

That Massdrop version of AKG's 7XX has improved bass over previous versions..another good result for Massdrop. It measures down 5 dB at about 17 Hz...not too shabby at all. It has all the clarity and detail, if not a tad extra presence, which is from 3.5 kHz to 6 kHz, (not as Tyll has it) that is sorely lacking in the HD 6XX series.

For example the HD 600 are down 5 dB at 40 Hz,(17 Hz for AKG) and down 10 dB at 5 kHz, compared to the AKGs which are flat at 5 kHz. It adds up to much better bass, and even better detail and presence. $200.00...a great deal.

I haven't heard the HiFiman,HE 400, but it sure measures well! The AKG looks has much more similarities to it than the Senns.

Your Fostex TH-X100 and the AKG K 7XX also measure remarkably similarly, with the Fostex looking a bit better in the sub bass, and
slightly recessed in the highs than HE 400, and the AKG K 7XX. I imagine they all three represent better values than the Sennheisers 6XX series.

I'm guessing that Tyll never listens with a subwoofer, just his little Harbeths, which roll off like the Sennheisers, so it is understandable that he doesn't miss the bottom octave as much as people with flat systems. I design and build loudspeakers, and have been now for 27 years (seriously...with good measuring tools) and am spoiled by having flat in room response. Which do you prefer your HiFiman or Fostex?

pete111's picture

Thanks, I like them both. I really like the punchy and dynamic sound of the Fostex, definitely have more slam but the Hifiman 400is could be called more neutral and with a larger image. Objectively I think the Hifiman is the better headphone but (even tough hifiman marketing suggest that) they are hard to drive and need a decent amp... Nothing fancy but just something better than the output of a phone. I'm having great result with the Bluewave Get that I've designed and put to market... I know, probably deserve to be flagged for self promotion... but threw it in anyway, I'll take the blame if need be...

Phoniac's picture


amartignano's picture

No progress? You forgot hd800.

amartignano's picture

Moreover many find the hd660s an improvement over 600 and 650.

roscoeiii's picture

I think Tyll makes it quite clear the specific reason he prefers the 650, 6XX, or 600. Of course the relative importance of various aspects of audio gear differs from person to person (for example, soundstage isn't especially crucial to my audio decisions). It is very easy to imagine someone preferring the 660S based on aspects of it compared to the 650/6XX/600 that this review describes.

I very much appreciate how Tyll contextualizes his own personal preferences in his reviews. And in many cases will not review a pair of headphones (bright/treble heavy) that go against those preferences. Sometimes we gravitate towards reviewers whose taste in gear reflects our own. In other cases, we must triangulate our tastes with the reviewer's and hope that the review has enough detailed description to allow us to do that.

I hope that in the comments those who prefer the 660S also extend us the favor of giving their reasons why. Let's not forget the role that personal preferences play in this hobby.

amartignano's picture

I want to specify that I really appreciate Tyll reviews and opinions even when my sensation are different. I wrote a long post about my impressions on the 660S on headfi, and I will not repeat here :p It's not my preferred headphone, I like my hd800S more, and I might also prefer my hd700 on the hd660S. Nevertheless I like the 660S and this time I disagree not only in the preferences (obvious because we are all different) but also in some audio facts, which is strange as I always agree with Tyll on the "facts". For example, like others I find the hd660S to have less grain than the hd600, not more.

amartignano's picture

More specifically, there's a thing of the hd660S that really mesmerized me, that it's not mentioned in the Innerfi's review. It's the wonderful work on timbre and harmonics of cymbals and triangles that the hd660S does. It really surprised me. The hd600 in direct comparison seemed quite "muted" on high cymbals and triangles.

BoyBalastog's picture

I generally assume that because Tyll tends to dislike strong treble that he percieves a hard edge on the upper frequencies as being grainy because of how harsh it sounds to him, rather than because there's any actual obscurity of detail. I get what he means when he says that being detailed and resolving isn't the same as merely having elevated upper mid and treble response, but his own preference towards smooth resolve over bright resolve is evident. Some people, including me, prefer to have actual presence of detail while having a bright edge to the upper frequencies to accentuate it.

But on the other hand, I once did hear an HD700 and I really did hear a grainy fuzz to it, but in a way that was markedly different from the very veiled (to me at least) HD650. I've only tried it once admittedly, but it left a bad impression. So I'm not all too surprised that Sennheiser may have fumbled a bit in trying to stuff the HD700's drivers into a 600 series shell to try and make for a lower impedance model.

Phoniac's picture