Audio T Verdict
The Sennheiser HD 660S2 take everything that made the original HD 660S such an accomplished headphone and give it something many listeners wanted more of: properly extended low bass. Crucially, Sennheiser has achieved that without simply turning up the mid-bass and changing the fundamental character of the 600 Series.
The 38mm transducers retain the natural midrange, intimate presentation and smooth treble that make this family so easy to listen to, while the revised 300-ohm voice coil, more compliant surround and improved airflow allow the S2 to reach considerably deeper. Bass has more weight and authority, but remains detailed and controlled rather than overwhelming the rest of the music.
They deserve good amplification, and their fully open construction means they are very much headphones for a quiet listening environment. Give them both, however, and the HD 660S2 are a wonderfully engaging alternative to the more neutral HD 600 and smoother HD 650, with balanced 4.4mm and conventional 6.3mm cables included straight from the box.
Important: Fully Open-Back Headphones
The HD 660S2 deliberately allow sound to pass freely through the rear of the earcups. This is fundamental to their spacious, natural presentation but means they provide very little isolation from outside noise and will leak sound into the room. They are intended primarily for quiet home listening rather than commuting, offices or shared spaces where other people need silence.
Sennheiser HD 660S2 Open-Back Headphones
The Sennheiser HD 660S2 are premium open-back hi-fi headphones developed as the successor to the HD 660S and as a modern continuation of the long-running HD 600-series philosophy.
At the heart of the design is a pair of 38mm dynamic transducers manufactured at Sennheiser's production facility in Ireland. A revised ultra-light aluminium voice coil raises the nominal impedance to 300 ohms, while changes to the transducer surround and airflow allow significantly greater low-frequency output than the original HD 660S.
The result combines the detailed midrange, natural timbre and long-term listening comfort associated with the 600 Series with deeper and more physical sub-bass reproduction.
- Premium open-back hi-fi headphones
- 38mm high-performance dynamic transducers
- Transducers manufactured in Ireland
- Designed in Germany
- Revised ultra-light aluminium voice coil
- 300-ohm nominal impedance
- More compliant transducer surround
- 70Hz transducer resonance frequency
- Optimised airflow around the transducer
- Vented magnet system
- Steel-mesh acoustic damping
- 8Hz to 41.5kHz frequency response
- 104dB SPL at 1V RMS
- Less than 0.04% THD
- Extended sub-bass response
- Natural, warm tonal balance
- Smooth treble
- Open and intimate stereo presentation
- Soft velour earpads
- 260g headphone weight
- Detachable dual-sided cables
- 1.8m 6.3mm cable supplied
- 1.8m 4.4mm balanced cable supplied
- 6.3mm to 3.5mm adapter supplied
- Carry pouch supplied
- Replaceable earpads, cables and headband padding
A Modern Take on the HD 600-Series Sound
The HD 660S2 share obvious design DNA with the HD 600, HD 650 and original HD 660S.
The fundamental goal remains natural music reproduction rather than using exaggerated bass or treble to make a dramatic first impression.
Where the S2 differs is in the amount of useful energy available further down into the lowest frequencies.
More Sub-Bass, Not Simply More Bass
There is an important difference between extending bass response and simply boosting the upper-bass region.
A broad bass boost can make headphones sound immediately fuller, but it can also obscure vocals and lower midrange detail.
Sennheiser instead revised the mechanical and electromagnetic behaviour of the HD 660S2 transducer so that it can reproduce more information in the lowest octaves while retaining the familiar tonal balance of the series.
Double the Low-Frequency Sound Pressure
Sennheiser states that the revisions produce approximately double the sound pressure at the lowest frequencies compared with the original HD 660S.
This gives instruments with genuine low-frequency content greater scale and physical presence, including:
- Kick drums
- Bass guitar
- Double bass
- Low piano notes
- Electronic bass
- Orchestral percussion
- Organ
- Film-score effects
Lower 70Hz Transducer Resonance
The transducer surround has been made more compliant than the one used in the original HD 660S.
This lowers the driver's resonant frequency from approximately 110Hz to 70Hz and allows the diaphragm to move more freely when reproducing deeper bass.
The change is one of the key reasons the S2 can provide greater low-frequency extension without relying on electronic EQ.
Revised 300-Ohm Voice Coil
The HD 660S2 return to the 300-ohm nominal impedance associated with the HD 600 and HD 650.
The revised ultra-light aluminium voice coil provides greater magnetic force and electrical damping, helping the diaphragm start and stop accurately as the musical signal changes.
This contributes to clean transient response and the ability to reproduce subtle variations in texture rather than allowing notes to blur together.
Why 300 Ohms?
High impedance should not automatically be interpreted as either better or worse.
In the HD 660S2, the 300-ohm coil forms part of the complete transducer design and helps Sennheiser achieve the electrical damping and impulse behaviour it wanted.
The practical consequence is that the headphones benefit from a source capable of providing a healthy voltage swing.
Do I Need a Headphone Amplifier?
A good headphone output is strongly recommended. The HD 660S2 can work from a variety of equipment, but they are revealing 300-ohm headphones and deserve an amplifier with sufficient voltage, control and dynamic headroom. A capable integrated amplifier, dedicated headphone amplifier or DAC/headphone amplifier will normally make a better partner than a basic portable headphone output.
104dB SPL Sensitivity
Sennheiser specifies a sound pressure level of 104dB at 1kHz and 1V RMS.
That gives the HD 660S2 greater sensitivity than the HD 600 despite sharing the same 300-ohm nominal impedance.
As always, maximum volume is only part of amplifier matching. Control, noise, distortion and headroom matter too.
Less Than 0.04% Distortion
Total harmonic distortion is specified below 0.04% at 1kHz and 100dB.
Low distortion helps preserve subtle details within a recording rather than masking them with unwanted harmonic information generated by the transducer itself.
Vented Magnet System
Sennheiser has carefully optimised airflow through the magnetic structure surrounding each transducer.
Milled vents in the magnet yoke help air move more efficiently, improving sensitivity and reducing unwanted turbulence around the diaphragm.
The aim is greater precision and speed across the frequency range rather than concentrating purely on bass extension.
Steel-Mesh Acoustic Damping
Steel-mesh damping follows the geometry of the driver and helps regulate the movement of air around the transducer.
Airflow is a fundamental part of any open headphone design because the diaphragm must move freely without uncontrolled pressure changes interfering with its behaviour.
38mm High-Performance Transducers
The 38mm drivers are manufactured at Sennheiser's specialist facility in Ireland.
The same facility produces several of the company's established reference headphone transducers, helping maintain controlled manufacturing tolerances between individual units and between left and right channels.
8Hz to 41.5kHz Frequency Response
The HD 660S2 have a specified frequency response of 8Hz to 41.5kHz.
The headline figures extend beyond the conventional range of human hearing, but the more important point is the driver's ability to operate cleanly throughout the audible spectrum with useful extension at both extremes.
Natural Midrange
Midrange reproduction remains one of the defining strengths of the Sennheiser 600 family.
Voices, guitars, piano, strings and much of the information responsible for recognising the natural character of instruments occupy this region.
The HD 660S2 retain that focus rather than allowing the stronger low-frequency response to overwhelm the middle of the spectrum.
Smooth Treble
The upper frequencies are tuned for detail without unnecessary harshness.
This allows cymbal decay, room ambience, string texture and other fine information to remain audible while helping the HD 660S2 stay comfortable during longer sessions.
An Intimate Open-Back Presentation
Not every open headphone tries to create the largest possible apparent soundstage.
The HD 660S2 retain the more intimate and focused perspective associated with the 600 Series, placing performers clearly within a coherent stereo image rather than artificially stretching everything far away from the listener.
This can work particularly well with:
- Vocals
- Small jazz ensembles
- Acoustic music
- Chamber music
- Rock and alternative music
- Live recordings
Fully Open Earcups
The acoustic mesh at the rear of each earcup allows sound generated behind the diaphragm to pass freely into the surrounding room.
This reduces the internal reflections associated with closed-back enclosures and allows the transducers to operate with minimal acoustic restriction.
Why Open-Back Headphones?
When used in a quiet environment, open headphones can provide several important benefits:
- Natural reproduction of recording ambience
- More air around performers
- Reduced enclosed-cup colouration
- Greater sense of space
- More natural long-term listening
The inevitable compromise is that very little external noise is blocked and your music remains audible to people nearby.
Built for Long Listening Sessions
The HD 660S2 weigh approximately 260g without their cable.
Large around-ear cups distribute pressure around rather than directly on the ears, while the open construction allows considerably more airflow than a sealed headphone.
Soft Velour Earpads
Velour-covered cushions provide a soft and breathable contact surface around the ears.
This material is particularly well suited to an open headphone designed for extended home listening rather than maximum acoustic sealing.
Replaceable Earpads
Sennheiser continues to provide replacement HD 600-series earpads with their accompanying foam discs.
This is important because pads naturally soften and compress with years of use, and changing their thickness can alter both comfort and the acoustic relationship between the driver and ear.
Replaceable Headband Padding
The headband cushion can also be replaced separately.
Alongside replaceable earpads and cables, this supports long-term ownership rather than treating the headphones as a disposable product once the most frequently worn components eventually age.
Two 1.8m Cables Included
The HD 660S2 are supplied with two detachable cables, each approximately 1.8 metres long.
One terminates in a conventional 6.3mm stereo jack, while the other uses a 4.4mm balanced connection.
This gives the headphones unusually broad amplifier compatibility straight from the box without requiring an additional balanced cable purchase.
6.3mm Single-Ended Cable
The conventional cable terminates in the full-sized 6.3mm stereo connection commonly found on:
- Integrated amplifiers
- Dedicated headphone amplifiers
- DAC/headphone amplifiers
- AV receivers
- Professional audio interfaces
4.4mm Balanced Cable Included
A second cable terminates in the increasingly common 4.4mm Pentaconn balanced connector.
This allows the HD 660S2 to connect directly to compatible balanced headphone outputs without purchasing an additional cable.
Does Balanced Automatically Sound Better?
No. Whether the 4.4mm output provides a worthwhile advantage depends on the amplifier design. Some headphone amplifiers provide greater voltage or their optimum performance through the balanced output, while others perform equally well through a conventional connection. The amplifier itself matters more than the word "balanced".
6.3mm to 3.5mm Adapter
A short adapter allows the conventional 6.3mm cable to connect to equipment using a 3.5mm headphone socket.
The 300-ohm impedance still needs to be considered, however. A socket being physically compatible does not guarantee that the device behind it has enough output capability to drive the HD 660S2 properly.
Dedicated HD 600-Series Cable System
The HD 660S2 use the familiar dual-sided Sennheiser HD 600-series cable arrangement, with an individual connection to each earcup.
Replacement 6.3mm and 4.4mm HD 660S2 cables remain available separately, along with other compatible HD 600-series cable options.
Not an HD 500-Series Cable Connection
The HD 660S2 do not use the single-sided twist-lock connection found on models such as the HD 550, HD 560S, HD 569, HD 599 and HD 620S. HD 500-series cables and the HD 500 BAM boom microphone are therefore not compatible with the HD 660S2.
HD 660S2 or HD 600?
HD 660S2
- Fully open-back
- 38mm transducers
- 300-ohm impedance
- 8Hz to 41.5kHz response
- 104dB SPL at 1V
- Less than 0.04% THD
- Extended sub-bass
- Warmer, more intimate presentation
- 260g
- 1.8m 6.3mm cable supplied
- 1.8m 4.4mm balanced cable supplied
- Carry pouch included
HD 600
- Fully open-back
- 42mm driver assembly
- 38mm diaphragm
- 300-ohm impedance
- 12Hz to 40.5kHz response
- 97dB SPL at 1V
- Less than 0.1% THD
- Classic neutral reference balance
- Strong focus on midrange accuracy
- 260g
- 3m conventional cable
- Long-established reference design
Which Should You Choose?
The HD 600 remain the obvious choice if tonal neutrality and reference-style analysis are the priorities.
The HD 660S2 move towards a warmer and more immediately engaging presentation, with greater sensitivity and considerably stronger sub-bass extension.
Both use 300-ohm platforms and reward capable amplification, so this is a comparison worth making by listening rather than simply choosing the more expensive model.
HD 660S2 or HD 650?
The Sennheiser HD 650 occupy another distinct position within the 600 family.
They share the 300-ohm impedance and smooth long-term listening approach but have their own established tonal character.
The HD 660S2 offer newer transducer development, greater sensitivity and stronger low-frequency extension, while the HD 650 remain a much-loved alternative for listeners who prefer their particular balance and presentation.
HD 660S2 or HD 620S?
HD 660S2
- Open-back
- 38mm transducers
- 300-ohm impedance
- 8Hz to 41.5kHz
- 104dB SPL
- Less than 0.04% THD
- 260g
- Virtually no isolation
- More sound leakage
- Natural open-back ambience
- 4.4mm cable supplied
HD 620S
- Closed-back
- 42mm driver assembly
- 38mm diaphragm
- 150-ohm impedance
- 6Hz to 30kHz
- 110dB SPL
- Less than 0.05% THD
- Useful passive isolation
- Much lower sound leakage
- Wide presentation for a closed headphone
- 3.5mm cable supplied
Open or Closed?
If you have a quiet room and do not need to worry about anybody else hearing your music, the HD 660S2 provide the full open-back experience.
The HD 620S make considerably more sense if isolation and reduced sound leakage are important.
Audio T Manchester has compared the two models directly, making this one of the more useful comparisons to explore before buying.
Read Our HD 620S and HD 660S2 Comparison
Our Manchester team listened to both models side by side, covering bass, midrange, treble, soundstage and their very different open and closed-back approaches. Read Sennheiser 600 Series Has a New Edition to the Range on the Audio T blog.
HD 660S2 or HD 800S?
The Sennheiser HD 800S sit significantly higher in the range and take a different approach to open-back listening.
The HD 800S are renowned for a much larger apparent soundstage and an extremely revealing presentation, while the HD 660S2 feel more intimate and place greater emphasis on warmth, timbre and low-frequency weight.
The HD 660S2 can therefore be preferable even where budget allows more, particularly if you prefer music to sit closer around you rather than being presented across the expansive stage of the HD 800S.
Made for Music Rather Than Features
The HD 660S2 contain no wireless radio, DSP, app integration or electronic noise cancellation.
The entire design is centred around the passive transducer, acoustic enclosure and the quality of the source feeding them.
That makes them particularly appealing for a dedicated hi-fi headphone system where adding features is less important than extracting more information from recordings.
No Battery to Age
Because there is no rechargeable battery, the HD 660S2 are not subject to the gradual reduction in battery capacity that eventually affects wireless headphones.
Combined with replaceable cables, earpads and headband padding, this makes them well suited to long-term ownership.
Who Are the Sennheiser HD 660S2 For?
- Serious home hi-fi listeners
- Existing HD 600-series owners wanting deeper bass
- Listeners wanting natural vocals and instrument timbre
- People who prefer warm, smooth open-back headphones
- Listeners wanting stronger sub-bass than the original HD 660S
- Owners of capable headphone amplifiers
- Desktop DAC and headphone-amplifier systems
- Traditional hi-fi systems with good headphone outputs
- Listeners wanting a balanced 4.4mm cable included
- People who value replaceable cables and earpads
- Long listening sessions in quiet rooms
- Listeners who want an intimate rather than extremely expansive soundstage
Who Are They Not For?
- Commuters needing acoustic isolation
- Listeners wanting Active Noise Cancellation
- People wanting Bluetooth headphones
- Shared spaces where sound leakage would disturb others
- Customers wanting a built-in microphone
- Very low-powered sources that struggle with 300-ohm headphones
- Listeners specifically wanting the widest possible headphone soundstage
- People looking for a compact folding travel headphone
Key Features
- Sennheiser HD 660S2
- Article number 700240
- Premium open-back headphones
- Over-ear / circumaural fit
- Dynamic transducer principle
- 38mm transducers
- Transducers made in Ireland
- Designed in Germany
- Ultra-light aluminium voice coil
- 300-ohm nominal impedance
- Revised transducer surround
- 70Hz driver resonance
- Optimised transducer airflow
- Vented magnet system
- Steel-mesh acoustic damping
- 8Hz to 41.5kHz frequency response
- 104dB SPL at 1kHz / 1V RMS
- Less than 0.04% THD
- Extended sub-bass response
- Double low-frequency sound pressure versus original HD 660S
- Natural, warm tonal balance
- Smooth treble
- Intimate stereo presentation
- Soft velour earpads
- Replaceable earpads
- Replaceable headband padding
- 260g headphone weight
- Detachable dual-sided cable design
- 1.8m 6.3mm cable supplied
- 1.8m 4.4mm balanced cable supplied
- 6.3mm to 3.5mm adapter supplied
- Carry pouch supplied
- No Bluetooth
- No battery
- No Active Noise Cancellation
- No microphone
- No app required
- 24-month manufacturer's warranty
Frequently Asked Questions
Are the Sennheiser HD 660S2 still current?
Yes. They remain part of Sennheiser's current wired, open-back, HD Series and audiophile headphone ranges.
Are the HD 660S2 open-back?
Yes. They use a fully open acoustic design.
Do they block outside noise?
Very little. Their open-back design deliberately provides minimal isolation.
Can other people hear them?
Yes. Open headphones allow considerably more sound to escape into the room than closed-back designs.
What size drivers do they use?
Sennheiser specifies 38mm dynamic transducers.
Where are the HD 660S2 made?
Sennheiser specifies Ireland as the country of origin and states that the headphones are designed in Germany and manufactured in Ireland.
What is the frequency response?
8Hz to 41.5kHz.
What impedance are the HD 660S2?
300 ohms.
Why did Sennheiser return to 300 ohms?
The revised lightweight voice coil provides greater magnetic force and electrical damping, helping improve impulse response while matching the nominal impedance used by the HD 600 and HD 650.
How sensitive are they?
Sennheiser specifies 104dB SPL at 1kHz and 1V RMS.
What is the THD?
Total harmonic distortion is specified below 0.04% at 1kHz and 100dB.
What changed compared with the original HD 660S?
The S2 uses a revised 300-ohm voice coil, more compliant surround and optimised airflow, reducing the transducer resonance and significantly increasing output in the lowest frequencies.
How much more bass does the HD 660S2 produce?
Sennheiser states that the revised design produces approximately double the sound pressure at the lowest frequencies compared with the original HD 660S.
Are they bass-heavy?
They provide considerably greater sub-bass extension than the original HD 660S, but they remain hi-fi headphones with a balanced overall presentation rather than being deliberately bass-dominant.
What is the driver resonance frequency?
Sennheiser states that the revised surround reduces it from approximately 110Hz on the original HD 660S to 70Hz on the HD 660S2.
Do I need a headphone amplifier?
A capable headphone output is strongly recommended. The HD 660S2 have a 300-ohm impedance and can benefit considerably from a good dedicated headphone amplifier, DAC/headphone amplifier or suitable integrated amplifier.
Can I use them with an integrated amplifier?
Yes, provided the amplifier has a capable headphone output.
Can a phone drive them?
That depends on the phone and any external headphone adapter being used. Many portable adapters have limited voltage output, so a dedicated headphone DAC/amplifier is generally a better partner for getting the most from a 300-ohm headphone.
How many cables are supplied?
Two detachable 1.8m cables are included.
What connectors are supplied?
One cable terminates in a conventional 6.3mm stereo plug and the other in a 4.4mm balanced plug.
Is the balanced cable included?
Yes. A 1.8m 4.4mm balanced cable is supplied as standard.
Is a 3.5mm connection possible?
Yes. A 6.3mm to 3.5mm adapter is supplied for use with the conventional cable.
Are the cables detachable?
Yes. They use separate HD 600-series connections to the left and right earcups.
Can I replace the cables?
Yes. Sennheiser continues to provide dedicated HD 660S2 and compatible HD 600-series replacement cable options.
Can I use an HD 500-series cable?
No. The HD 660S2 use a different dual-sided cable arrangement from HD 500-series headphones.
Can I use the HD 500 BAM microphone?
No. The HD 500 BAM uses the single-sided HD 500-series twist-lock connection and is not compatible with the HD 660S2.
Can the earpads be replaced?
Yes. Sennheiser continues to provide compatible HD 600-series replacement earpads and foam discs.
Can the headband padding be replaced?
Yes. Replacement HD 600-series headband padding is available.
How much do the HD 660S2 weigh?
Approximately 260g without the cable.
Are the HD 660S2 comfortable for long sessions?
The combination of relatively low weight, around-ear fit, soft velour earpads and fully open construction makes them particularly well suited to long listening sessions in a quiet environment.
Are they good for gaming?
The open design and detailed stereo imaging can work very well for gaming, but they do not include a microphone and are primarily designed as hi-fi headphones.
Are they suitable for commuting?
No. They provide very little isolation and leak sound to people nearby.
Do they have Bluetooth?
No.
Do they have Active Noise Cancellation?
No.
Do they need charging?
No. They contain no battery.
What is the difference between HD 660S2 and HD 600?
Both are 300-ohm open-back headphones. The HD 600 are the more traditional neutral reference model, while the HD 660S2 provide greater sensitivity, extended sub-bass and a warmer, more intimate presentation.
What is the difference between HD 660S2 and HD 650?
Both share the 300-ohm HD 600-series philosophy and a smooth presentation. The HD 660S2 use a newer transducer design with stronger sub-bass extension and greater specified sensitivity, while the HD 650 retain their own longstanding tonal character.
What is the difference between HD 660S2 and HD 620S?
The HD 660S2 are fully open 300-ohm headphones, while the HD 620S are closed 150-ohm headphones designed to provide substantially better isolation and lower sound leakage while retaining a spacious presentation.
How long is the manufacturer's warranty?
The HD 660S2 currently fall within Sennheiser's 24-month consumer warranty category, subject to the applicable warranty terms and proof of purchase.
What comes in the box?
The HD 660S2 headphones, 1.8m 6.3mm cable, 1.8m 4.4mm balanced cable, 6.3mm-to-3.5mm adapter and carry pouch.
Related Sennheiser Products and Audio T Guides
Why Buy the Sennheiser HD 660S2 From Audio T?
The HD 660S2 sit within a family of headphones where relatively small differences in tuning can matter more than headline specifications. Audio T can help you compare them with the HD 600, HD 650, HD 620S and HD 800S and decide which presentation best suits your music and listening environment.
We can also advise on headphone amplifiers, DACs, 300-ohm system matching and whether your equipment is better suited to the supplied conventional or 4.4mm balanced connection.
- Friendly headphone advice
- HD 600-series comparisons
- HD 600, HD 650 and HD 660S2 guidance
- Open and closed-back comparisons
- Headphone amplifier matching
- DAC and headphone amplifier advice
- 300-ohm headphone guidance
- 4.4mm balanced connection advice
- Replacement cable guidance
- Replacement earpad and headband advice
- Headphone demonstrations where available
- Ongoing after-sales support
Contact your local Audio T store if you would like to arrange a demonstration or discuss whether the Sennheiser HD 660S2 are the right headphones for your system.