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Sennheiser HD660S Review (Headphone)

Rate this headphone:

  • 1. Poor (headless panther)

    Votes: 15 7.2%
  • 2. Not terrible (postman panther)

    Votes: 65 31.4%
  • 3. Fine (happy panther)

    Votes: 103 49.8%
  • 4. Great (golfing panther)

    Votes: 24 11.6%

  • Total voters
    207

amirm

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This is a review and detailed measurements of the Sennheiser HD660 S open-back headphone. It is on kind loan from a member and costs US $400.
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone Review.jpg

You would be hard pressed telling it apart from the similarly numbered Sennheiser headphones which is not a bad thing.

Note: The measurements you are about to see are made using a standardized Gras 45C. Headphone measurements by definition are approximate and variable so don't be surprised if other measurements even if performed with the same fixtures as mine, differ in end results. Protocols vary such as headband pressure and averaging (which I don't do). As you will see, I confirm the approximate accuracy of the measurements using Equalization and listening tests. Ultimately headphone measurements are less exact than speakers mostly in bass and above a few kilohertz so keep that in mind as you read these tests. If you think you have an exact idea of a headphone performance, you are likely wrong!

Fitment on the fixture was easy.

Sennheiser HD660 S Headphone Measurements
As usual we start with the headphone frequency response as comparison to our preference curve:
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone Frequency Response Measurements.png


We have our typical loss of bass response and deficiency in lower treble which likely quashes its spatial qualities. Not sure what is up with that peak resonance above 5 kHz. EQ is mandatory and here is the graph to build from:
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone relative  Frequency Response Measure...png


Distortion is similar to other Sennheisers being dominant in bass:
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone Distortion Measurements.png


SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone THD Distortion Measurements.png


The redesign from HD650 seems to have been to lower the impedance:
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone Impedance Measurements.png


This is half as much, reducing the dependency on high output voltage from the amp (i.e. making it more mobile friendly). Sensitivity is average:
Most sensitive open back headphone review.png


Group delay is messy:
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone Group Delay Measurements.png


Sennheiser HD660S Listening Tests and Equalization
Out of the box sound was very good, proving that you need to EQ (or compare to another neutral headphone) to know what you are missing. After inserting three filters, I was shocked how much the tonality and overall fidelity improved:
SENNHEISER HD 660 S  HiRes Audiophile Open Back Headphone EQ Equalization Filter.png


The bass boost is self explanatory. The middle filter made a substantial improvement to its spatial qualities which was a delight to listen to now. The last filter doesn't make a tonal difference but seems to have the effect of lowering distortion.

I had an easy time driving the HD660S with my RME ADI-2 DAC. It sounded superb (with EQ) up to reasonably loud levels. Above that I thought the bass became less distinct but that was not safe or long term listening level.

With the EQ I sat back and enjoyed the sound with track after track. Turn it off and sound collapses with a more mundane but not annoying tonality.

Comfort was great compared to my much heavier everyday headphone. But after half hour the tips of my ears were starting to become uncomfortable a bit.

Conclusions
The core DNA of this line of headphones from Sennheiser seems to be maintained in this revision. Response is a bit more rough than HD650 but both need EQ anyway. And with it, you have a very top performing headphone.

I am going to put the Sennheiser HD660S on my recommended list.

------------
As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

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  • SENNHEISER HD 660 S Frequency Response.zip
    26.8 KB · Views: 200
OP
amirm

amirm

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Do you think 6xx/650 has a better sound signature, or would you spring extra for these?
I would only get these if you don't have a nice headphone amp and need its lower impedance.
 

Robbo99999

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I don't really see this headphone as an improvement over the HD600/650, in fact it looks worse from both a frequency response point of view & also distortion. HD600/650 have smoother frequency responses and distortion is lower than this headphone. It seems that the HD660s also doesn't have angled drivers or pads to distinguish as different from the HD600/650 either.....which might have otherwise helped soundstage and imaging. (Correct me if I'm wrong about the HD660s not having angled pads or drivers). I voted this headphone quite low.

(nice channel matching though)
 

solderdude

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Passive filter for HD660S (and HD58X) gets rid of the treble peak, smoothens the treble.

filter-hd58x.png


Bass in the HD660S is of better quality (less wooly) and stock this headphone is less clear sounding (a complaint folks had about the HD600).

I think it is a nice addition to the classic line-up (when not using EQ) that has a slightly different presentation.
With 110dB/V it is noticeable more sensitive (6dB opposite HD6**) so easier to drive from portable gear.
The price is too high though.

HD660S:
fr-hd660s.png


index.php
 
Last edited:

bunkbail

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I own HD600, HD58X, HD6XX and HD660, I by far much prefer the HD660S than the rest. It sounded the most tonally correct and neutral. I also own a pair of Verum One (the one that Amir reviewed was my unit), it sounded good after EQ but I still prefer the HD660S even at stock settings (I prefer it better with Dekoni Elite Hybrid Earpads though, it has noticably better bass extension). For what it worth, I've heard 4 pairs of HD660S at a local store and they all sounded different than each other. One pair had grating, harsh treble and sounded so wrong. Dunno what's up with that.
 
Last edited:

solderdude

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One of the pair had grating, harsh treble and sounded so wrong. Dunno what's up with that.

Could be a higher treble peak.
There is a new HD660S version out b.t.w.
Alas, my contact at Sennheiser does not respond to questions any more. He told me there were some planned improvements.
 

MayaTlab

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There is a new HD660S version out b.t.w.

Is that a reference to the newer plastic parts, the use of the newer 2021 pads, or something else ?

Alas, my contact at Sennheiser does not respond to questions any more. He told me there were some planned improvements.

In terms of manufacturing the HD6... lineup or more significant changes ?
 

Maiky76

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This is a review and detailed measurements of the Sennheiser HD660 S open-back headphone. It is on kind loan from a member and costs US $400.
View attachment 211584
You would be hard pressed telling it apart from the similarly numbered Sennheiser headphones which is not a bad thing.

Note: The measurements you are about to see are made using a standardized Gras 45C. Headphone measurements by definition are approximate and variable so don't be surprised if other measurements even if performed with the same fixtures as mine, differ in end results. Protocols vary such as headband pressure and averaging (which I don't do). As you will see, I confirm the approximate accuracy of the measurements using Equalization and listening tests. Ultimately headphone measurements are less exact than speakers mostly in bass and above a few kilohertz so keep that in mind as you read these tests. If you think you have an exact idea of a headphone performance, you are likely wrong!

Fitment on the fixture was easy.

Sennheiser HD660 S Headphone Measurements
As usual we start with the headphone frequency response as comparison to our preference curve:
View attachment 211585

We have our typical loss of bass response and deficiency in lower treble which likely quashes its spatial qualities. Not sure what is up with that peak resonance above 5 kHz. EQ is mandatory and here is the graph to build from:
View attachment 211586

Distortion is similar to other Sennheisers being dominant in bass:
View attachment 211587

View attachment 211588

The redesign from HD650 seems to have been to lower the impedance:
View attachment 211589

This is half as much, reducing the dependency on high output voltage from the amp (i.e. making it more mobile friendly). Sensitivity is average:
View attachment 211590

Group delay is messy:
View attachment 211591

Sennheiser HD660S Listening Tests and Equalization
Out of the box sound was very good, proving that you need to EQ (or compare to another neutral headphone) to know what you are missing. After inserting three filters, I was shocked how much the tonality and overall fidelity improved:
View attachment 211592

The bass boost is self explanatory. The middle filter made a substantial improvement to its spatial qualities which was a delight to listen to now. The last filter doesn't make a tonal difference but seems to have the effect of lowering distortion.

I had an easy time driving the HD660S with my RME ADI-2 DAC. It sounded superb (with EQ) up to reasonably loud levels. Above that I thought the bass became less distinct but that was not safe or long term listening level.

With the EQ I sat back and enjoyed the sound with track after track. Turn it off and sound collapses with a more mundane but not annoying tonality.

Comfort was great compared to my much heavier everyday headphone. But after half hour the tips of my ears were starting to become uncomfortable a bit.

Conclusions
The core DNA of this line of headphones from Sennheiser seems to be maintained in this revision. Response is a bit more rough than HD650 but both need EQ anyway. And with it, you have a very top performing headphone.

I am going to put the Sennheiser HD660S on my recommended list.

------------
As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

Here are some thoughts about the EQ.

Notes about the EQ design:
  • The average L/R is used to calculate the score.
  • The resolution is 12 points per octave interpolated from the raw data (provided by @amirm)
  • A Genetic Algorithm is used to optimize the EQ.
  • The EQ Score is designed to MAXIMIZE the Score WHILE fitting the Harman target curve (and other constrains) with a fixed complexity.
    This will avoid weird results if one only optimizes for the Score.
    It will probably flatten the Error regression doing so, the tonal balance should be therefore more neutral.
  • The EQs are starting point and may require tuning (certainly at LF and maybe at HF).
  • The range around and above 10kHz is usually not EQed unless smooth enough to do so.
  • I am using PEQ (PK) as from my experience the definition is more consistent across different DSP/platform implementations than shelves.
  • With some HP/amp combo, the boosts and preamp gain (loss of Dynamic range) need to be carefully considered to avoid issues with, amongst other things, too low a Max SPL or damaging your device. You have beed warned.
  • Not all units of the same product are made equal. The EQ is based on the measurements of a single unit. YMMV with regards to the very unit you are trying this EQ on.
  • I sometimes use variations of the Harman curve for some reasons. See rational here: https://www.audiosciencereview.com/...pro-review-headphone.28244/page-5#post-989169 NOTE: the score then calculated is not comparable to the scores derived from the default Harman target curve if not otherwise noted.
Good L/R match.

I have generated one EQ, the APO config file is attached.

Score no EQ: 44.0
Score Amirm: 49.7
Score with EQ: 97.8
Code:
SENNHEISER HD 660 S Score EQ [email protected] 96000Hz
June082022-155600

Preamp: -10.9 dB

Filter 1: ON PK Fc 21.74 Hz Gain 10.89 dB Q 0.41
Filter 2: ON PK Fc 186.20 Hz Gain -3.73 dB Q 0.84
Filter 3: ON PK Fc 1251.83 Hz Gain -2.06 dB Q 2.42
Filter 4: ON PK Fc 2281.01 Hz Gain 2.91 dB Q 2.30
Filter 5: ON PK Fc 7028.68 Hz Gain 7.06 dB Q 1.11
Filter 6: ON PK Fc 5543.35 Hz Gain -10.97 dB Q 5.32
Filter 7: ON PK Fc 9368.73 Hz Gain 5.18 dB Q 1.57

SENNHEISER HD 660 S Score EQ Flat@HF 96000Hz.png
 

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MayaTlab

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For what it worth, I've heard 4 pairs of HD660S at a local store and they all sounded different than each other. One pair had grating, harsh treble and sounded so wrong. Dunno what's up with that.

There's the question of sample variation, a good chunk of which may come from the pads. As they break down it alters the ratio between the range up to 3kHz or so and the rest above (including in the case of the HD660s the high-Q features around 4-5kHz, which also depend on how it deforms the pinna).

A problem with Sennheiser's pre-2021 pads for the HD6... series not only is that they alter the FR as they age, but that they may not age in a fully predictable manner. Some of my older pads collapsed in an even way, others collapsed unevenly.

I have yet to try the newer 2021 pads but for now I've moved to Dekoni's pads and re-shaped the FR as I like it with EQ and in-ear mics. I have several sets of these and they are more consistent between them than Sennheiser's pre-2021 pads and seem more resilient over time.
 

solderdude

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Is that a reference to the newer plastic parts, the use of the newer 2021 pads, or something else ?
Acoustic changes is what he said they were working on. This was over half a year ago.
No idea how significant, I assumed small improvements.
I hoped to get a sample but there has been no response for months now.
 
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