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

Phoniac's picture

I did not post this here - the buggy software did...

potterpastor's picture

Appreciated the heartfelt review.
Hopefully in years to come, some of that HE-1 awesomeness will trickle down to the 600/700 series.

I really enjoy the HD 579, the older brother of the 569

wiinippongamer's picture

Slightly inferior for nearly double the price, thanks Schwachköpfe.

This will surely drive up HD600 prices too.

MattTCG's picture

I agree with some of your review but there are some issues that we are quite far apart on. Just want to say that its a mistake to suggest bending the headband in the middle. You'll have a contingent of hobbyist emailing you after the headband assembly snaps in two. ONLY stretch the metal section...imo.

amartignano's picture


Tyll Hertsens's picture
Sennheiser has a GREAT materials science lab. I've been relieving the pressure of these headphone in the way I described for...what, 20 years. Never oncehave they felt like they were getting anywhere near fracturing or even fatiguing the material. Don't ham-fist it for sure; feel what's going on. But bending the metal headband extensions will change their shape, and that shape is designed for that pocket in the headband.
GNagus's picture

Senn is replacing a headphone for an new and "improved" version instead of complimenting the HD650 with an easier to drive version. I have some theories why:

1) The HD650 isn't selling as well as Sennheiser would like, when there is the 600 selling for less (MSRP prices).

2) The HD700 is not selling well and Sennheiser created a new headphone using HD700 parts from the parts bin (driver, etc)

I'm disappointed, but only that the HD650 will be discontinued.

darkswordsman17's picture

The pad differences could be interesting. Actually, Tyll did you try swapping the 660 pads with 650/600 ones to see if that alone might offer the subtle sound differences, or even just a change?

Another thing that would've been interesting, and granted them not including a 3.5mm cable seems to indicate they don't expect it in spite of their reasoning of making it more efficient, but a comparison out of portable players I think would be fairly important. The thing is, I'd almost guess that they'd actually be worse (with regards to smoothness), but perhaps that's the desire, or the bass blooms up more which is what more people would want. Or maybe after processing that might be typical on such devices, that the treble is enjoyed (basically it helps retrieve some of that lost that causes say the HD650 to sound too veiled)?

zobel's picture

Summary Table
Frequency Range Frequency Values
Sub-bass 20 to 60 Hz
Bass 60 to 250 Hz
Low midrange 250 to 500 Hz
Midrange 500 Hz to 2 kHz
Upper midrange 2 to 4 kHz
Presence 4 to 6 kHz
Brilliance 6 to 20 kHz

zobel's picture

...many,if not most, would call upper midrange (2 to 4 kHz ) low treble, with treble being 4 kHz to 6 kHz...presence and air (or brilliance) from 6 kHz up.

I think we should all use the same (or close) definitions for what we are talking about, otherwise it is just babble.

sszorin's picture

@ zobel - Sub-Bass is 15/20 to 40Hz.
150 Hz and up are lower middle frequencies

zobel's picture
zobel's picture

Sub bass...............1o Hz to 25 Hz
Low Bass...............25 Hz to 50 Hz
Mid Bass...............50 Hz to 100 Hz
Upper Bass............100 Hz to 250 Hz
Low Mids..............250 Hz to 500 Hz
Midrange.............500 Hz to 2000 Hz
Low treble..........2000 Hz to 4000 Hz..(with presence 3kHz - 6 kHz)
Treble............4000 Hz to 10,000 Hz
Upper treble....10,000 Hz to 20,000 Hz..(same as air or brilliance)

Tyll Hertsens's picture

I go with the mothership. John Atkinson has all the ranges buried in the glossary here:

I think the human vocal range as a stand-in for where the mid-range is. For me, 150Hz to around 800-1000Hz. after that, overtones, spittle throat sounds, and consonant/sibilant sounds appear, which I think of as in the treble area.

The character of 'presence' occurs, in my opinion, in how well one can hear these upper glotteral and consonant making frequencies. Essentially, twig and branch snapping stuff. These sounds---maybe 800Hz-4kHz---are where our primal survival system triggers reside.

Emphasis will give you presence; de-emphasis will give you distance---muting through the trees, I'll offer. I consider that treble response.

I see it as:

Bass: 15Hz-150Hz
Mids: 150 Hz-1500Hz
Treble: 1500Hz-20kHz (That third of an octave up there don't hardly count.)

zobel's picture

I think that it will work for all practical purposes..even if not quite the norm, I think, from most of what I've seen elsewhere.

I think the presence range has almost always been listed no lower than 2kHz, and extending to 6kHz (usually), and maybe the reason for that is because those frequencies are above the fundamentals in music far enough to change the distance perceived to them more than simply the quantity of them. Think a distant branch snap, or grass rustling vs a close by one. Low frequencies travel much further than higher ones. We may hear loud rustling or snapping, but our distance cues are frequency based. When using EQ for playback or recording instruments...including the voice, the cues to distance are found in the low treble..3kHz to 6kHz. You can prove this to yourself by playing with the EQ in listening sessions. If upper mids (800 Hz to 2kHz) are raised in level, to most, I think, it only makes those tones louder, not closer. It is all subjective, of course, but this is one area where there is disagreement. No matter, really, if we are all on the same page.

Long time listener's picture

I agree that this looks right:

Sub bass...............1o Hz to 25 Hz
Low Bass...............25 Hz to 50 Hz
Mid Bass...............50 Hz to 100 Hz
Upper Bass............100 Hz to 250 Hz
Low Mids..............250 Hz to 500 Hz
Midrange.............500 Hz to 2000 Hz
Low treble..........2000 Hz to 4000 Hz..(with presence 3kHz - 6 kHz)
Treble............4000 Hz to 10,000 Hz
Upper treble....10,000 Hz to 20,000 Hz..(same as air or brilliance)

Someone on this site once defined "midrange" based on the frequency of middle C on the piano--failing to note that the piano is only a bass/midrange instrument. The keyboard does not present the full range of frequencies shown in Tyll's graphs, but only extends into the upper midrange or lower treble. Thus at about 256 or 260 Hz, middle C actually represents the transition from upper bass to lower mids--as shown in the post by Zobel.

Tyll, the presence region is roughly 2khz to 5khz. It's not for you to re-define according to your own predilections.

I'll be very happy if the above can be used as a reference.

zobel's picture

Bass.........20 Hz to 250 Hz
Midrange....250 Hz to 2500 Hz
Treble......2500 Hz to 20,000 Hz

This represents the range of Human hearing, and divides the frequencies into the old standards of Bass, Mids, & Treble. These have been the widely accepted definitions for many years.

In the table below, the further subdivisions are useful to more accurately describe the audible spectrum, which have developed over time, to describe the ten octaves we can hear as illustrated below. Note also the frequencies that most refer to as ("presence") and ("air" or "sparkle"). It is important that we use these standards in naming the different bands of frequencies in order to have a common language. If not referring to this standard, the only alternative is to talk in terms of the frequencies in Hz, and mention only the numbers.

Sub Bass...........10 Hz to 25 Hz
Low bass...........25 Hz to 50 Hz
Mid Bass...........50 Hz to 125 Hz
Upper Bass........125 Hz to 250 Hz
Low Mids..........250 Hz to 500 Hz
Central Mids......5oo Hz to 1000 Hz
Upper Mids.......1000 Hz to 2500 Hz
Low Treble.......2500 Hz to 4500 Hz ....('presence region'.....
Central Treble...4500 Hz to 10,000 Hz ...3 kHz to 6 kHz)
Upper Treble...10,000 Hz to 20,000 Hz .('air/sparkle' 10 kHz & up)

amartignano's picture

Anyway, I find the absence of listened music for the review a strong defect on this and other reviews (excellent for all other things). I almost never found an headphone always better than another with different recordings.

Phoniac's picture

You first said it correctly in your review, but then come up with thgis wring statement again:

No, the 660s is far from being loud enough on a usual smartphone like an iPhone.

Phoniac's picture

Ok, I give up. This software just too stupid.

coastman25's picture

Thank you Tyll for yet another exemplary review. Being late to the party has its rewards. You are able to comment on the rumours and previous badly made reviews.
I can only assume other reviewers of these phones took too much notice of the Sennheiser hype and swallowed it.
These phones seem to have been designed and made via a committee, with the marketing and sales departments having the biggest say. At best, they are an attempt to be all things to all people and failing to build on the HD 600 heritage. It is as if they do not know the value of what they already possess.
Hats off to you Tyll, for being so exacting and honest with your evaluation.

amartignano's picture

Have you ever heard the 660S? I ask because I have them, and I'm entitled to partially disagree with Tyll impressions. I really appreciate his opinions, but it's not the only "truth" in HiFi. The fact that he disagrees with others, doesn't mean that the others are dumb hypey fanboys.

coastman25's picture

Have you ever bought a fake product? The more you pay for it the less likely you are to question its authenticity. I think its called self justification. If you are happy with the HD 660s then I guess that's all that matters.
Happy listening.

amartignano's picture

Ok, you never heard the 660S.

coastman25's picture

My comments were about the quality of the review and the reviewer not the product. For that I don't need to hear the 660s. I trust the reviewers integrity he didn't just comment about the sound he went into detail about how Sennheiser's misleading hype, the drivers, how they measure like the 700's etc etc.
I have bought 3 headphones without first hearing them, just going by Tyll's reviews and I am not disappointed or have found myself disagreeing with his reviews.

amartignano's picture

The fact is, that many headphones enthusiast, me included, find the HD660S perfectly good as an hair of the hd580/6xx heritage. This is respectable like other opinions. And I strongly disagree with your assumptions about 660S creation and judgement about who likes this headphone. Here we say that you "did a lick"... but you never heard the headphone. To be clear: I always appreciate the quality of Tyll reviews, except for the absence of indications of the music played for the review.

coastman25's picture

So why don't you take up your complaints with the author of the review? I cannot help you. I am happy with the review your not.

amartignano's picture

I was mainly commenting about your post, so I wrote to you. Why write to Tyll about your post? :D

coastman25's picture

Well if you cannot read I guess there is no point in commenting at all!