Hardware Reviews

Sennheiser HD660S


“The legend continues…”, said Sennheiser, upon the release of their HD660S dynamic open-back circumaural headphone, the successor to the venerable HD650 that has brought aural bliss to music lovers around the world for the past 15 years.  As a long-time admirer of the HD600 series, I was both eager and anxious to discover whether the bold decision to replace one of the most iconic and best selling headphones in history was a wise one.

Sibling rivalry
The HD600 open-back headphone series was established in 1997 with the launch of the HD600, although the ancestry really dates back four years earlier with the creation of the HD580 in 1993.  The HD600 soon established itself as a reference and was lauded for offering an exceptional balance between neutrality and musicality for an attainable price.  The HD650 became Sennheiser’s new flagship in 2003 and was tuned for a warmer and more laid-back presentation.  It went on to achieve cult status in the headphone community and as a result has been the German firm’s second longest running open-back dynamic headphone in its production history.

Released in October 2017, the new HD660S retails for £430, an ostensibly modest price increase over the HD600 and HD650 which are £370 and £400 respectively.  Street prices of the 600 and 650 are however often much lower – indeed either model can currently be purchased for under £300 – effectively making the 660S more than 40% dearer.  While discounts on the 660S are admittedly beginning to emerge, the new kid still faces price-pressure from its own siblings to prove itself as a worthy heir to the 650. 

Upholding tradition
The HD660S shares a very similar design and construction with the HD600 and HD650.  It is essentially identical in shape, structure and weight, uses 650 headband cushioning and very similar but slightly firmer and deeper velour earpads, and thus provides that familiar cosy fit with strong clamping force.  This will be welcome news to fans of the HD600 series, I’ve always felt Sennheiser really nailed the ergonomics in their original design.  It is substantial and durable in the hand yet lightweight and comfortable on the head, and the absence of angles and sharp edges provides a very high level of tactility, which perhaps explains why this headphone has never undergone a fundamental re-design in its two decades of production.  The new model does however sport a few minor cosmetic changes, plus one rather significant technological departure that I will elaborate on shortly. 



The clearest aesthetic difference between the 660S and the 650 is the re-finishing of its plastic structures from gloss titanium (grey) to matte black, which gives it a stealthy and more streamlined appearance.  The matte finish may not be to everyone’s taste as it does draw attention to the headset’s predominately plastic construction.  Sennheiser does however use very high quality plastics – even in its flagship HD800S and HD820 – for both sonic and ergonomic benefits, which are of course of the priorities when selecting appropriate materials.  More subtle alterations include a shrinking of the ‘Sennheiser’ brand on the headband, which is now positioned left of centre.  The punched metal honeycomb earcup housings have also been subject to a minor re-design and now feature a small area with an outward bevel that houses the logo, a rather neat aesthetic touch.

New bloodline?
More significant change is seen on the technology side, as the HD660S surprisingly uses a totally different drive unit to that of the 600 and 650.  According to Sennheiser, it is a new transducer design that provides “improved control of diaphragm movements thanks to a specially manufactured precision stainless steel fabric, which is adapted to the contour of the diaphragm”. Other sources suggest that it is a repurposed HD700 driver, or is at least based it, owing to it sharing a very similar appearance and impedance curve.  If true, this would be quite a controversial departure as the HD700’s tuning is typically favoured by listeners who enjoy a particularly bright and analytical sound with an enhanced sense of width and separation; attributes not usually associated with the smoother, warmer and more cohesive HD600/HD650. What remains unchanged, however, is the 660S being manufactured to the German firm’s exacting HD600 standards, with each pair of drive units matched to very impressive ±1dB tolerances.

Sennheiser states that the HD660’s 150 Ohm impedance “offers much greater versatility with mobile devices”. As portable players are generally good at delivering current but poor at generating voltage, a lower impedance should enable higher sound pressure levels with lower distortion.  Portable device-friendly headphones typically have impedances of less than 50 Ohms, so it is unclear whether halving the impedance from 300 Ohms to 150 Ohms is enough of a reduction to be of significant benefit. Sensitivity does see a modest increase from 103dB/1V (650) to 104dB/1V (660S), though it is again questionable if this will be noticed in normal use.  It is also important to remember that the 660S is still a fully open-back design which of course leaks sound in both directions, and as such its practicability in public places carries the same limitations as its predecessor.

Modern times
The HD660S earcups accept the same detachable 2-pin HPSC connectors as the 600 and 650.  Unlike its siblings however, which come with just the one three metre cable terminated in a gold-plated 3.5mm (HD600) or 6.3mm (HD650) TRS jack, the HD660S ships with two three metre leads.  The first is terminated in a 6.3mm TRS jack like the 650, while the second sports a 4.4mm Pentaconn jack for balanced drive.  A short 6.3mm-to-3.5mm adaptor cable is also included to allow connection to ubiquitous single-ended portable devices.


product detail x2 desktop Sennheiser HD 660 S product image grille


As the HD660S is marketed as a mobile device-friendly headphone, the included cables may frustrate users who would prefer to have a shorter lead terminated in a 3.5mm TRS jack. A three metre lead tethered to a 6.3mm-to-3.5mm stub does seem a rather cumbersome solution for portability.  I do however praise Sennheiser for embracing the new Pentaconn interface, not only equipping the 660S with it but also their flagship HDV 820 amplifier.  While it appears to be the future of balanced connections, it is still in the early stages of adoption.  The inclusion of a 4XLR adaptor would therefore have been useful to enable pairing with desktop headphone amps that provide only the 4XLR interface for balanced drive.

Family appreciation
As an HD600 owner for the last 8 years and having auditioned the HD650 on several occasions, I can verify that these ear-speakers are most loved for their engaging musicality.  Despite displaying a gentle roll-off at both frequency extremes, the 600 and 650 offer similarly rich and cohesive presentations. The 600 is the more neutral and agile of the two and has a slightly brighter and forward tuning, whilst the 650’s delivery is more laid-back and warmer with a slightly recessed lower treble, mildly elevated mid-bass and more gradual sub-bass roll-off.  Both models sound incredibly natural and deliver a satisfying measure of ‘analogue tape euphony’ – for lack of a better analogy – that is very easy to listen to and can make other headphones sound somewhat sterile and/or fatiguing in comparison.  

Sennheiser’s HD600 series isn’t without its limitations.  The euphonic quality of the 600 and 650 can be perceived as a “veil” in some configurations.  Also, the tonal cohesiveness coupled with the snug physical fit of the headset – which places the drivers close and parallel to the ears – can present an intimate soundstage that is perhaps at odds with what is expected from an open-back design. It is however possible to mitigate these perceptions with an appropriate choice of partnering amplification.  The 300 Ohm drivers in the 600 and 650 scale exceptionally well and really come to life with amplifiers than can deliver large voltage swings to produce clean and impactful transients, and this helps to dissolve the “soft focus” sometimes attributed to these ear-speakers. High-output tube topologies with low negative feedback are an especially good match and expand the spatial boundaries in all directions, offering highly immersive dimensionality.

Chip off the old block
Listening to the HD660S conjures a strong feeling of familiarity which – given its use of a different drive unit – comes as a pleasant surprise and would appear to validate the remarkable extent to which enclosure design influences the overall tuning and staging.  The 660S very much continues the HD600 family tradition of an intoxicatingly lush soundscape.  In fact, the 600 and 660S share such similar tunings that the casual listener would likely have an extremely difficult job telling them apart. Depending on the source used, I sometimes struggled to reliably differentiate between them and – even when there were audible differences – I could not declare an overall ‘winner’ as it was very much conditional on the partnering amplification.  In a nutshell, the HD660’s performance is consistently excellent, while that of the HD600 and HD650 can span from good to exceptional.

Where the 600 and 650 both struggle to achieve anywhere near their full potential through mass-market sources with limited voltage output capabilities, the 660S is far more effective. With mobile devices and even entry-level standalone amplification, I found a strong preference for the new model, which is both clearer and punchier than the 600 and 650, displaying a more controlled and authoritative low end, better focused upper mids and crisper leading edges. Its elders by comparison present a softer, more relaxed and somewhat diffuse image with constricted dynamics. 



Conversely, while the 600 and 650 really come alive and shine with synergistic amplification – especially high-output tube topologies as mentioned earlier – the 660S does not scale as impressively.  Its ability to retrieve low-level detail and separate out and place in space different instruments still sees improvement, but not to the same extent as its siblings which – in addition to revealing superior micro-dynamic nuance – also exhibit a more natural ebb and flow.  

Driven by decent middle-tier solid-state amplification with plentiful power reserves, I feel the 660S and 600/650 are almost neck and neck in their control, openness and resolve, and the choice ultimately depends on whether listeners favour the slightly crisper and punchier sound of the new model or the slightly smoother and more measured delivery of its seniors.

Despite its lower impedance and higher voltage efficiency, I did not observe a significant difference in sound pressure level between the 660S and 600/650.  The new model sounds 2 or at most 3 dBs louder for the same volume setting.  While any increase in headroom is of course welcome, such a modest gain is unlikely to be of appreciable benefit to owners of sources and/or headphone amplifiers that already struggle to reach levels that are loud enough with the 600/650.  

Sennheiser’s HD600 headphone series has for the last two decades nourished music lovers around the world with rich, balanced and non-fatiguing sounds.  Their new HD660S dynamic open-back very much upholds the family tradition whilst adapting to modern times by making itself accessible to a market of headphone enthusiasts previously unable to fully experience the pleasures of the harder-to-drive HD600 and HD650.  With mobile devices or entry-level amplification, the 660S is the best sounding of the trio and outshines its siblings in clarity, openness and dynamic impact. For audiophiles that have already invested – or are willing to invest – in high-powered amplification that synergises with the HD600 or HD650, these two iconic stalwarts will however scale to provide the more refined listening experience.

N.B. The planned discontinuation of the HD650 has understandably caused considerable angst amongst Sennheiser devotees.  While the 660S shares more sonic similarities to the 600 and 650 than differences, I’d very much like to see all three models remain in production as I feel they each serve different listener preferences very well.  Some convergence on retail prices may be required however, as the current premium the HD660S commands over its elders is difficult to fully justify.


Type: Open back dynamic headphones

Ear coupling: Over-ear (circumaural) 

Transducer principle: Dynamic Open 

Frequency response: 10 – 41,000 Hz (-10 dB)

Sound pressure level (SPL): 104 dB at 1V 1kHz 

THD + N: < 0,04% (1 kHz, 100 dB) 

Impedance: 150 Ω

Connector: 6.35 mm / 4.4 mm Pentaconn / 3.5mm adaptor

Weight (w/o cable): 260g

Price when tested:
Manufacturer Details:

T 0333 240 8185


open back dynamic headphone


Richard Barclay

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