Transparency note: This research-based review uses Sennheiser’s official HD 660S2 page and specifications. We did not perform hands-on listening, amplifier matching, balanced-output comparison, comfort or durability testing. Claims about warmth, intimacy and extended bass are manufacturer descriptions, not our measurements.
Quick answer: The Sennheiser HD 660S2 is a 260 g, open-back home headphone with 38 mm transducers and 300-ohm impedance. It includes detachable 6.3 mm and balanced 4.4 mm cables. The package is flexible for desktop hi-fi, but it offers virtually no isolation and needs a source with sufficient voltage.
Key specifications
- Dynamic open-back over-ear design
- 38 mm transducers
- 300-ohm impedance
- 260 g without cable
- 8 Hz–41.5 kHz stated frequency response
- 104 dB SPL at 1 kHz and 1 Vrms
- THD below 0.04% at 1 kHz and 100 dB
- 1.8 m cables with 6.3 mm and balanced 4.4 mm plugs
What changed in the S2 concept
Sennheiser describes the HD 660S2 as using a revised coil and vented magnet system to extend low-frequency output while controlling distortion. The move to 300 ohms aligns it with the HD 600 and HD 650 rather than the lower-impedance original HD 660S. Those are engineering changes; their subjective value requires comparison.
The published 8 Hz lower limit is not proof of stronger audible bass by itself. Frequency-range endpoints omit level tolerance and tonal shape. A full response graph, pad condition and fit provide more useful evidence than the lowest printed number.
Open-back use and isolation
Open earcups allow sound to pass in both directions. This can support a less enclosed presentation, but it also makes the HD 660S2 unsuitable for commuting, shared offices and tracking near open microphones. People nearby will hear your music, while room noise will remain audible.
Use it in a quiet room where leakage is not a problem. If isolation is essential, a closed-back model is the correct category rather than an equaliser setting or stronger amplifier.
Amplification and balanced connection
The 300-ohm load benefits from a source that can supply adequate voltage. Many desktop amplifiers and competent audio interfaces can do so, but output capability should be checked rather than assumed. A 3.5 mm adapter is included for the 6.3 mm cable, yet an adapter cannot compensate for a weak output.
The 4.4 mm cable supports balanced headphone outputs. Balanced operation can provide more voltage from some devices and separates channel returns, but the connector itself does not guarantee better sound. Compare level-matched outputs and use the connection that gives clean headroom.
Comfort and ownership
At 260 g, the HD 660S2 is light for a full-size wired headphone. Velour pads and ventilation may help during long sessions, though clamp, glasses and head shape remain individual. Pads compress with use and should be treated as replaceable acoustic components, not only cosmetic parts.
Both 1.8 m cables detach at the earcups and are more desk-friendly than a 3 m living-room cable. The supplied pouch protects against dust but not heavy impact. Sennheiser’s modular family design supports replacement pads and cables, which improves long-term value.
Who should buy it?
Choose the HD 660S2 if you want a serviceable open-back Sennheiser, already have a capable desktop source and can use its 4.4 mm or 6.3 mm connections. Skip it when isolation, portable use, a microphone or minimum price matters more than connection flexibility.
Alternatives and verdict
The HD 650 is often the value alternative, the HD 600 is the reference-focused sibling, and the DT 770 Pro is the closed studio option.
The HD 660S2 offers a coherent desktop package: 300-ohm engineering, low weight, two useful cables and replaceable wear parts. Its advantage over cheaper siblings cannot be established from specifications alone. Compare all three Sennheiser models on the same source before paying the premium.
Primary source: Sennheiser HD 660S2 official product page and specifications.