Sennheiser HD 660 S Over-Ear Open Headphones Page 2


Sound Quality
On a gross level the HD 660 S sound is similar to the HD 600 and 650. Zooming in a little closer I'd characterize the HD 650 as the warmest of the bunch and the HD 660 S the brighter of the trio. Overall, I'd characterize the HD 660 S as a little lean.

To my ears this comes from two things: The HD 660 S has less of a broad upper-bass centered hump it's virtually flat from 100Hz to 1kHz, and the bass falls off gradually below 100Hz. But I also hear a more forward presence region from 1kHz to 3kHz. To my ears the HD 650 (in stock form) does warrant the "veil" designation; the HD 600 a little; and the HD 660 S not hardly at all.

Imaging on the HD 660S does appear to be a bit deeper and wider, but it's mostly quite like the other two headphones, which to my ears is good reasonably wide, and not particularly deep. The HD 660 S also appears to have more impact. You'll notice I used the word "appear" in both those descriptions. There's a reason for that...

The biggest difference I hear when switching from the HD 660 S to one of the other two, and I heard it immediately, is the fairly significant sense of a smoother and more refined sound of the previous cans. Switching back to the HD 660 S I hear it as grainy more more edgy in comparison. I'm quite saddened by this.

The HD 600 and HD 650 have for well over a decade been considered one of, if not the best headphone available. They are legend! Right on the HD 660 S box it says, "The Legend Continues." Sadly, I think they are mistaken. Right off the bat, this graininess takes it out of the family's rightly earned status.

But it gets worse. One of the defining characteristics of the HD 600/650 is a wonderfully smooth response. I had the good fortune once of sitting right in the circle with the Muir String Quartet and I was simply breathless with the absolutely smooth nature of the sound. Reproduced violins can take on a screechy character; in real life they are not at all. The HD 600/650 does a great job of portraying this smoothness. Very good treble response should give the impression of smooth detail. So many people mistake a bright, sizzly response for good detail. It just ain't so.

The smooth detail of the 600/650 allow them to "scale" well, bringing out the advantages of upstream gear. I tried the the three headphones with my HeadRoom Max, Bottlehead Crack, and Simaudio 430HA amplifiers. The differences were readily apparent with the 600/650, but was much less so with the HD 660 S. The grainy character got in the way.

While the HD 660 S was a bit brighter than the other two, it also lacked the nuanced resolution and dynamic balance of the 600/650. Low level details were lost, and grosser detail seemed compressed into a similar level of response. My measurements show these cans as far more similar than they actually sound to the ear.

To make matters worse, the design of the HD 660 S driver makes them less able to be modified by DIYers. Ugh.

What the...
I just don't understand what Sennheiser was trying to accomplish with these headphones. The intro to their product page says right off the bat:

The new HD 660 S lets you enjoy sophisticated audiophile sound in even more listening situations. The successor of the legendary HD 650 excels with the improved performance of its new transducer design. Thanks to its lower impedance, it delivers reference-class sound also when connected to hi-res mobile players, thus offering much greater versatility.

A) The HD 660 S is only a dB or two more voltage efficient, so there's only a modest difference in volume from a phone. On its product page, Sennheiser claims 104dB/Volt for the HD 660 S; in its manual, Sennheiser say the HD 650 has 103dB/Volt sensitivity. Higher-end portable players will all have plenty of voltage to drive a 600/650 to loud levels.

2) If you're going to tout a product as being more useful in portable applications you should include a short cable in the mix. Both included cables are far too large for that sort of thing; neither are terminated in a 3.5mm plug—for the moment at least, still the most common connector on portable devices. Add the 1/4" to 3.5mm short cable adapter and you have a fairly clumsy connection to portable devices.

I've also seen a post where a Head-Fi made an inquiry to Sennheiser about whether the HD 600 or HD 650 would be discontinued. The Sennheiser representative said the HD 660 S will replace the HD 650 in the line-up, and the older model will be discontinued. I've personally always preferred the HD 600, but acknowledge the HD 650 has a lovely sound many might prefer.

I'm bummed.

Were it any other maker producing the HD 660 S I would have been quite impressed with how close they managed to get to the sound of the legendary HD 600/650. But from Sennheiser themselves producing it and with plans to discontinue the HD 650 I can only shake my head in disbelief.

I loath the race to lower headphone impedance in high-end headphones to make them more compatible with portable devices. My experience with the three beyerdynamic DT880 versions with varying impedance gives me the strong belief that higher impedance headphones will outperform lower impedance cans of the same design. Unfortunately I think this holds true with the new lower impedance HD 660 S.

The new Sennheiser HD 660 S lacks the legendary smooth resolve of the HD 600/650, preventing it from scaling well as upstream gear is improved—one of, if not the most, important characteristics of a great enthusiast headphone. If you're not getting improvements on money spent on electronics you're missing out on half the fun.

Yes, I'll give these headphones a soft recommendation...they remain a decent headphone—comfortable, stylish, and close to as good as an HD 600. Maybe the stealth black look and smartphone capable efficiency is enough to tip the balance for some. But I'll also steer buyers towards the HD 600—or Massdrop HD 6XX if it becomes available again—as the better sounding alternative.

Watch on YouTube.

Sennheiser U.S.A. home page and HD 660 S product page.
Head-Fi impressions thread, sponsored thread, and reviews. impressions thread and 600/650/660S analysis thread.

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