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Published: Oct-24-2017

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NO SMOOTHING is applied to the shown plots. Most measurement sites have some smoothing applied which ‘irons flat’ sharp peaks and ‘wiggles’. I do not use smoothing because some info about sound quality is lost when plots are smoothed.

Aside from a small correction of the microphone itself also some correction in the lowest frequencies is applied to the plots to compensate for the perceived loss of bass when using headphones. This is described HERE in more detail.
A ‘horizontal‘ frequency response curve on the shown frequency response plots on this website thus indicates a perceived ‘flat’ tonal signature.

ALL measurements are made with a good SEAL on a flatbed measurement rig.
The shape of your head, bone structure, pad size, pad ‘softness,  (compliance), hair or no hair and or wearing glasses may (drastically) change the frequency response of some headphones, so… your personal experience may differ substantially from these plots.

Frequency response (tonal balance) is the most sound-determining aspect of headphones. A horizontal line shows audible neutral response in the plots on this website. Deviations in different severities at different frequency bands have an effect on the sound character.
The bigger the deviation the stronger the effect.

Below an aid to help determining the sound character of headphones with relation to the frequency response.


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Sennheiser HD660S


The Sennheiser HD660S is an open over-ear dynamic headphone. Its designator (HD660S) positions it between the HD650 and HD700, nearest to the HD650.
When looking at the street price of the HD650 it will not be a surprise that the HD660S will be somewhat more expensive. Certainly right after the launch in Sept. 2017.
The S behind the typenumber merely indicates it comes with Symmetric cables (XLR and Pentaconn).

The pads are slightly different from those of the early HD650 as these pads are slightly more tapered toward the ears/skin. The contact area thus is smaller. When new this may result in a perceived higher clamping force but as time goes by it will comply sooner to the contours of the face and thus possibly create a better seal.

The construction is similar to that of the HD650. The color scheme and headband lettering is different. Also the grille is slightly different and has an embossed square on it with the Sennheiser logo. This gives it a more modern look and think it is quite nice.

The headphone is quite comfortable on the head but the clamping force is quite high, similar to that of the current HD650.
The supplied cable plugs into the cups with the exact same plug as that of the HD6x0 series. It is supple and very low in microphonics (= mechanically conducting sounds, like touching or rubbing against clothes, to the earcup / ears).
You can use the familiar after-market cables if you are of the opinion that such helps with sound quality.

Due to its somewhat lower impedance (half that of the HD650) you have a 4dB more volume at the same volume control setting. Not a big difference though.
Not enough to drive this headphone to more than decent levels from phones etc.


Type: Over ear, open
Isolation: poor
Usage: Home
Driver type: dynamic
Pads: replaceable velours
Foldable: No
Headphone cup connector: flat ‘Sennheiser’ connector.
Cable entry: dual sided
Cable: 3m 6.3mm TRS to flat Sennheiser connector + short 6.35mm to 3.5mm adaptor cable +  4.4mm Pentaconn balanced 5-pin connector.
Driver size: 38mm
Max power rating: 0.5W
Max Input Voltage: 9Vrms
Max drawn current: 60mA rms
Max. S.P.L.: 122dB
Impedance: 150 Ω
Efficiency: 96dB/1mW  (104dB/1V)
Weight: 260 g.
Clamping force: medium/high
Accessories: manual, 3mm cable with 6.3mm TRS, 6.35mm TRS to 3.5mm TRS adaptor cable + 3m 4.4mm Pentaconn balanced 5-pin connector cable.

Sound description:

The sound is a lot like the HD650 but also differs. It is a very balanced and tonally ‘correct’ headphone. One of the happy few in this price bracket.
It differs with the HD650 in other areas than the tonal balance which is more similar than not.
Bass on the HD660S is noticeably ‘tighter’ and better defined. The HD650 can be slightly ‘soft/wooly-ish/warm’ compared to the HD660S. Mids are slightly cleaner and realistic sounding with ever so slightly less/different ‘clarity/presence’ (3kHz presence) perhaps.

Mids sound a little ‘cleaner’ and more ‘dynamic’ and maybe a tad more ‘forward’.
Some people ‘turn around’ the effect of dynamics and use the word opposite to reality.
When music is compressed you will hear more smaller details than when not compressed. You need to turn up the HD660S a bit more to hear the same amount of small details so it actually compresses less than the HD650.

One has to realize that ‘finer details’ are not found in the upper treble (related to rise-time) but are actually in the 2kHz to 5kHz range, where the ears are most sensitive.
The HD660S has about 3dB less signal there which is audible. For that reason the HD600 and HD650, both having a higher level there, can be found to show slightly more ‘finer’ detail or the other way around the HD660S lacking somewhat..
This too has an effect on the perceived dynamics. The HD650 performs better in this department. The small ‘bump’ at around 1.2kHz, however, makes it slightly more forward sounding. So in essence it sounds a little more ‘forward’ and a tiny bit less ‘detailed’ in the mids. These differences are small.

One could say the slight ‘veil’ some talk about when discussing the HD650 is not present here. Others may say it has a ‘blacker’ background.
Treble is not elevated at all and also not subdued but in the correct amount. I would not say it is ‘better’ than that of the HD650 but with the ‘improved’ dynamics it would be easy to conclude the highs are maybe slightly crisper / better defined.
Think a HD650 with a ‘cleaner’ and more ‘dynamic’ sound. A less ‘romantic’ HD650 perhaps. A bit fresher/modern version of the HD650. Slightly less ‘polite’.
The HD650, however, is a bit smoother/pleasant in the fine nuances.


Below the frequency response of the HD660S (Left, Right)


Channel matching is excellent.  Below 70Hz this headphone drops off gradually.
No deep rumbles on an impressive level but still quite audible.
Bass sounds ‘tight’ and ‘dynamic’ and integrates seamlessly with the mids.
From 70Hz to 4kHz the response is very flat and accurate.
The small ‘bump’ at around 1.2kHz, however, makes it slightly more forward sounding. So in essence it sounds a little more ‘forward’ and a tiny bit less ‘detailed’ in the mids. These differences are small.
The small dip from 1.5kHz to 4kHz may be a little less ‘deep’ in reality compared to the measurement  because the measurement rig does not have a pinna which does alter the response exactly in that area. That small dip is not audible as a dip.
The treble response is exemplary.
From 5khz to >20khHz with very little sharp peaks and no dips.

As this headphone will peak the interest of HD650 owners below some comparative plots with a current (stock) HD650 and a HD660S.

HD660vs HD650 nr 3
The similarities are very obvious. The differences too.
From 10Hz to 1.5kHz the differences are negligible. The HD650 has slightly more ‘presence’ between 2kHz and 5kHz but the HD660S a dB or so more around 1kHz.

That combined with the cleaner sound may tip the scale towards either headphone depending on taste or people gotten used to a certain presentation and regarding it as a reference.
Treble quantity, and to me quality, is about on par. Perhaps the HD650 has just slightly more and ‘smoother’ treble.

Of course this frequency plot can only say something about tonal balance differences and these are very small.
The biggest differences is in dynamic behavior which cannot be shown in this plot.

Lot’s of people are wondering how the HD660S compares to the HD600.
Below the frequency response of the HD660S vs HD600.HD600 vs HD660S
As can be seen the HD600 has slightly less midbass hump but more presence/clarity in the upper mids. Treble is about the same. When one realizes ‘attack’ and ‘presence/clarity’ is in the 2kHz to 5kHz range it is easy to conclude the HD660S is lacking slightly in that area. However, most of the ‘complaints’ about the HD600 has always been people found it a tad too strident. The HD660S ‘fixes’ that by lowering that area.

Below a direct comparison between HD600, HD650 and HD660S but the vertical (dB) scale is not 5dB per division but 2dB so ‘expanded’ in view to show the differences a bit better.HD600 vs 650 vs 660S
As it is hard to see the perceived tonal balance below the exact same plot but with some light ‘psychoacoustic smoothing’ applied.

HD600 vs 650 vs 660S tonal bal

When comparing directly we can see the HD600 is the ‘brightest’ of the bunch followed by the HD650 which really only has a few dB more upper-bass/lower mids in other words ‘warmth’. The HD660S has the ‘warmth’ of the HD650 but less clarity/presence and slightly less treble.
Look at the scale the differences are quite enlarged in this plot.
Below the differences are shown on a scale I normally use (5dB/division).HD600 vs 650 vs 660 5db div

The used driver in the HD660S is said to be based on (= not equal to) the driver of the HD700. Of course the HD700 has a completely different enclosure and sound !
Still it may be interesting to compare the HD700 to the HD660S to see if there is some resemblance there as well. Below the HD660S vs. HD700

HD660S vsw HD700

Resemblance to the HD650 is much greater than that to the HD700.
Still, while there is a different tonal balance there are some aspects of the HD700 found in the HD660S. This is the ‘dynamics’ aspect. The HD700 is more ‘dynamic’ than the HD650 but feels less dynamic.
The HD660S seems to combine the best of both worlds here.
The Dynamics of the HD700 and tonal balance of the HD650 with more ‘power/energy’ to the music.

output resistance / damping-factor

As this is a dynamic headphone the frequency response might be amplifier output resistance dependent when certain higher output resistance amplifiers are used.
To test this the headphone is measured via a low impedance amplifier (0.2Ω) and a high impedance amplifier (120Ω). On a higher output resistance amplifier the output level will be considerably lower. To compensate for this the amplifier is cranked up to the same level (at 1kHz) as the low impedance amplifier. This way the plots are overlay-ed and it is easier to see how the tonal balance changes.
HD660S 120 Ohm

As can be seen the tonal balance changes when connected to a higher output resistance amplifier. You get about 2.5dB more mid-bass. This can make the headphone sound a little more ‘muddy/fat’. Lower bass is not boosted. This headphone should be driven from low output resistance amplifiers. Distortion in the bass also increases so an output Resistance > 10Ω is not recommended.

Below the distortion plot of the HD660S : (only Right channel shown)
Note that this headphone was measured at ears-unlimited-logo where  background noises were present in the demo room. As this is an open headphone distortion levels and spectrum plot performance may thus be better in reality than as shown on the plots.

The distortion products are shown in dB.Dist HD660S R

Below the same data but shown in a percentage scale instead of a dB scale.HD660S dist


Note: ignore the spikes below 100Hz these are ‘hum’ components in the portable measurement rig.
The distortion profile is typical for a dynamic driver. Higher 2nd harmonic distortion in the bass. 2% distortion around 50Hz is not bad but also not a poor value.
The measurement rig has a relatively high 2nd harmonic distortion so numbers below 0.2% will be there in reality above 200Hz.
Distortion from 200Hz to 10kHz is very low for the higher harmonics.

Below the distortion plot of the HD650.HD650 dist

When comparing the 3 models it shows that the HD660S has lower distortion levels
> 200Hz. Well the higher order distortions in any case.
This confirms the ‘cleaner’ mids At 500Hz the HD650 and HD600 are around 0.1% for the 3rd order harmonics where the HD660S is already below 0.05%.

The HD600 distortion plot below.HD600 dist

For the lowest frequencies (till 100Hz)the best of the 3 models appears to be the HD650 which has the lowest distortion of these model. The HD660S is not far behind. The HD600 has higher distortion in the bass area but is otherwise pretty similar to the HD650.
The HD660S is better (ignore the 2nd order as this is the limit of my test rig microphone)
Above 100Hz.

Below the CSD of the HD660S (Left and Right channel are superimposed)The CSD shows an excellent response with no obvious and deal-breaking resonances or lingering. Around 5kHz the HD650 is just slightly ‘better’ in performance.

Below the HD650 CSD plots as a comparison.

I don’t think this small difference is audible though.

By lack of oscilloscope shots (not enough time to measure that) below a step response plot of the HD660S (Right channel)
step HD660S R

The step response clearly shows the lack of ‘bottom end’ as evidenced by the downwards sloping line. Headphones with a good sub-bass extension have a (close to) horizontal line after the initial rise.
The subjective ‘speed’ of a headphone is often said to relate to the highest frequencies a headphone can reach. Nothing is further from the truth. Most of the ‘attack’ in instruments is in the 2kHz to 5kHz range in reality.

There is no overshoot not ringing to speak of. Excellent results and better in this aspect than HD700 and HD800.
There is a small ‘bump’ which is caused by slightly more energy in the 5kHz to 6kHz region. The HD660S has better damping and shorter lived resonances but is slightly more accentuated in the uppermids/lower treble.
Below, for comparative reasons, the step response of the HD650.

step HD650 R


This headphone is top notch when it concerns overall tonal balance and dynamics.
Tonal balance is better than that of the HD800 (and HD700) but doesn’t reach the ultimate quality in ‘realism’ and dynamics the HD800 reaches.
The sound quality, to me, is a small notch above that of the HD650.
Not in tonal balance but in dynamics, punch and ‘power’ in the bass.
I would say a slightly improved HD650 for sure, yet not reaching the sound quality of some other headphones. In its price class you won’t find many headphones that outperform the HD660S/HD650/HD6XX/HD600 in any case.
Not suited for bass heads and Beyer/Grado treble fans.

When contemplating buying a HD650 or HD660S the ‘best’ bet may be to pay a bit more and get the HD660S. It is a slightly ‘better’ headphone in certain aspects but slightly ‘less’ in other aspects.
For this reason I recommend to compare them and have a listen yourself first though, certainly when the price difference is still substantial.
Is it worth upgrading from the HD650 ?
When you like the tonal balance of the HD650 and are wanting a bit less ‘polite’ version of it. A little bit more ‘dynamic’ and ‘lively’ version of the HD650.
A little bit less ‘romantic’ presentation perhaps then it might be worth ‘upgrading’.

Is it worth owning both ? I guess not, they do not differ enough. If you are a headphone horder than … yes.. own both.
Is it better than HD800 on Kameleon (or with EQ) ? No, it can’t touch it, but the price difference is substantial.
If you never liked the HD650 then you can safely skip the HD660S.
If a slightly more ‘dynamic’ and ‘tight’ HD650 is your secret dream … then you could say ‘yes’ to the HD660S.
I can recommend this headphone and confidently say ‘job well done’ to Sennheiser.
Not a huge difference with the HD650 though.
When you are a big fan of the treble smoothness from the HD650 (HD6XX Massdrop) then you may be disappointed by the treble presentation of the HD660S.

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