Sennheiser HD 660S2

Since 1997 and the HD 600, Sennheiser’s 6XX series has carved out a niche that few lineages in audio can claim. The HD 660S2 takes up the torch from the HD 660S with a targeted promise: to correct the lack of bass that accompanied its predecessors, without sacrificing the clarity and midrange precision that made this family’s reputation. Developed in Germany and manufactured in Ireland, this open-back over-ear headphone is designed for high-quality, stationary listening.

A thoroughly reworked transducer

The most significant change between the HD 660S and the HD 660S2 is on the inside. Sennheiser has redesigned the aluminum voice coil of its 38 mm dynamic driver to make it lighter, which improves transient response: the headphone reacts faster to transients, those brief, sharp attacks you hear on a snare drum hit or a pizzicato double bass.

Impedance goes from 150 ohms (HD 660S) to 300 ohms, returning to the value of the HD 600 and HD 650. This technical choice is no accident: magnetic force and electrical damping increase, contributing to a cleaner signal. The surround of the driver (the flexible part that connects the diaphragm to the frame) has been softened, lowering the resonance frequency from 110 Hz to 70 Hz. Concrete result: sound pressure doubles in the lowest frequencies. The lowest note on a piano (the A at 27.5 Hz) or a kick drum hit between 45 and 60 Hz gain a physical presence that the HD 660S struggled to reproduce.

Sennheiser’s proprietary ventilated magnet system uses stainless steel mesh grills instead of the foam inserts found on previous generations. This change of material guarantees tighter manufacturing tolerances and better durability, while optimizing airflow around the driver. Total harmonic distortion remains remarkably low, under 0.04% at 1 kHz and 100 dB.

A warmer sound signature, without losing detail

The frequency response covers a range from 8 Hz to 41,500 Hz. In practice, the HD 660S2 sounds warmer and fuller than the HD 660S, which was often described as analytical, even a bit lean in the lower spectrum. The midrange remains the strong point of the 6XX series: vocals, strings, and brass are reproduced with remarkable transparency and texture. The treble, boosted and smoothed above 5,000 Hz compared to the previous model, gains clarity without becoming aggressive.

This headphone is not fatiguing. Its relaxed tonality and natural rendering allow for long listening sessions. This is something the HD 660S2 shares with the HD 650, while offering better resolution and a slightly more open soundstage. However, despite the improvements in the low end, the bass performance does not rival that of planar magnetic headphones in terms of firmness and sheer impact. This is an inherent characteristic of the open-back dynamic design: the bass is clean and extended, not physically punchy in the literal sense. On jazz, classical, folk, or rock, the result is convincing. On electronic music or hip-hop with a strong sub-bass component, fans of visceral sensations may find the rendering a bit too polite.

Open design and everyday comfort

The external design changes little compared to the HD 660S. You still get the lightweight plastic frame, the metal grills on the back of the ear cups that reveal the driver, and the bronze accents on a matte black background. The whole unit weighs 260 g, a featherweight that quickly disappears on the head.

The velour earpads, soft and breathable, surround the ear without excessive pressure. The padded velour headband consists of two foam sections separated by a central gap. The clamping force is a bit firm out of the box, but it loosens over time and ensures good acoustic positioning. Practical point: the earpads are replaceable, and spare parts are widely available. Be aware that progressive foam wear slightly shifts the sound signature towards a more mid-centric rendering.

The open design offers no sound isolation. Sound leaks out freely through the grills, and external noise comes in unfiltered. This headphone is meant for a quiet space: a living room, a closed office, a studio. It is not suitable for use on the go or in a noisy shared environment.

Connectivity and powering

The HD 660S2 comes with two 1.8 m dual-entry cables (one connector per ear cup). The first terminates in a 6.35 mm jack plug (single-ended), the second in a 4.4 mm Pentaconn plug (balanced). A 6.35 mm to 3.5 mm mini-jack adapter is also included.

Balanced connection via the 4.4 mm cable reduces interference and background noise, which can make an audible difference with a compatible DAC or headphone amplifier. The 1.8 m length will be sufficient for most desktop setups, but a little short if the source is far from your listening chair.

With its 300-ohm impedance, the HD 660S2 requires a source capable of providing adequate voltage. A smartphone alone will not be enough to fully exploit the headphone’s potential: volume will be low and dynamics will be constrained. A dedicated audio player, portable DAC, or desktop headphone amplifier will make a clear difference. That said, the sensitivity of 104 dB (1 V) is high enough that even a small portable DAC/amp can already deliver a satisfying result.

What’s in the box

The HD 660S2 comes with the headphone, the two cables (6.35 mm and 4.4 mm Pentaconn), the 3.5 mm mini-jack adapter, and a soft carrying pouch. The packaging is minimalist (recyclable cardboard), which contrasts with the more luxurious presentations of earlier generations in the series, but the essentials are there: two cables and an adapter cover most setups without extra purchases.

Frequently asked questions

Does the HD 660S2 work when plugged directly into a phone?

Technically yes, as long as the phone has a headphone output or a USB-C-to-jack adapter. The sound will be audible, but the volume will remain limited and the headphone will not deliver its full dynamics. A small portable DAC/amp (even an entry-level one) significantly improves the result.

What is the main difference compared with the HD 660S?

The bass. The HD 660S2 produces roughly twice as much sound pressure in the low frequencies thanks to a reworked voice coil and a more compliant surround. Impedance goes from 150 to 300 ohms, and the overall sound signature is warmer. The housing and form factor remain almost identical.

Are the earpads replaceable?

Yes. They can be removed easily and are compatible with earpads from Sennheiser’s HD 6XX series. Replacement parts are available from Sennheiser and third-party retailers.

Can this headphone be used for monitoring or music production?

Yes. The low distortion, good midrange resolution, and detailed treble reproduction make it a reliable tool for mixing and critical listening. The slight peak around 6 kHz can even help in spotting sibilance. It does not replace a closed-back headphone for recording (sound leakage would be picked up by the microphone).

Does the 4.4 mm Pentaconn cable provide an audible benefit?

Balanced connection reduces interference and background noise, an advantage that becomes noticeable with quality sources. With a small portable DAC that has no balanced output, the 6.35 mm cable (with or without the 3.5 mm adapter) will do just fine.

Technical Specifications

Audio Specifications

• Headphone type: open-back circumaural
• Drivers: 38 mm dynamic
• Frequency response: 8 Hz – 41,500 Hz
• Impedance: 300 Ω
• Sound pressure level: 104 dB (1 V)
• Total harmonic distortion: < 0.04% (1 kHz, 100 dB)
• Magnetic field: 2.5 mT

Design and Ergonomics

• Structure: open design with steel mesh grilles
• Weight: 260 g
• Comfort: velour-padded ear pads and headband
• Ventilation: excellent thanks to open-back design
• Aesthetics: understated design with bronze accents

Connectivity and Accessories

• Balanced cable: 4.4 mm Pentaconn plug (1.8 m)
• Unbalanced cable: 6.35 mm plug (1.8 m)
• Adapter: 6.35 mm jack to 3.5 mm mini-jack
• Carrying pouch included
• Compatibility: headphone amplifier, digital-to-analog converter, audiophile portable player, smartphone

Dimensions and Packaging

• Product weight: 260 g
• Weight with packaging: 638 g
• Packaging dimensions: 242 x 182 x 100 mm

Box Contents

• HD 660S2 headphones
• 6.35 mm plug cable (1.8 m)
• 4.4 mm plug cable (1.8 m)
• 6.35 mm jack to 3.5 mm mini-jack adapter
• Carrying pouch

General Information

• Origin: developed in Germany, manufactured in Ireland
• Sound signature: warm and natural with extended bass
• Recommended use: home hi-fi listening, music production, quiet environments
• Technology: ultra-lightweight aluminum voice coil for excellent transient response
• Features: doubled sound pressure in low frequencies, detailed mids, clear and precise highs