Sennheiser HD 660S
The HD 660S is an open-back headphone with a 150-ohm design and a measured THD of less than 0.04%. However, competing products like the Drop HD 66XX offer similar performance at a lower price, limiting its cost-performance advantage.
Overview
The Sennheiser HD 660S is an open-back dynamic headphone that continues the legacy of the HD 600 series. With its 150-ohm impedance and improved transducers, it is designed to be easier to drive with smartphones and DAPs (Digital Audio Players) than its predecessors. It features excellent distortion characteristics with a THD+N of less than 0.04% and a wide frequency response of 10 Hz to 41 kHz. It has now been replaced by its successor, the HD 660S2, making new units limited in the market.
Scientific Validity
\[\Large \text{0.8}\]The HD 660S’s THD+N of 0.04% (at 1kHz, 100dB) does not reach the transparent level (0.01% or less) on the measurement reference table, but it is an excellent figure for headphones. The 150-ohm impedance is a balanced design that is easier to drive compared to high-impedance models while being less susceptible to the effects of damping factor than ultra-low-impedance models. Its sensitivity of 104dB SPL/V is at a standard level. The frequency response is relatively flat in the 20Hz-20kHz band, although there is a slight peak in the high frequencies. Overall, transparency in the human audible range is at a high level.
Technology Level
\[\Large \text{0.7}\]The HD 660S is based on traditional dynamic driver technology. It improves upon the previous generation by refining the transducer to enhance diaphragm control, thereby reducing harmonic distortion. The use of a lightweight aluminum voice coil also aims to improve transient response. However, the fundamental technology is pre-existing, and its technical advancement is limited compared to other driver types like planar magnetic or electrostatic. From an industry-wide perspective, it is a solid design that refines existing technology, remaining at a standard technical level.
Cost-Performance
\[\Large \text{0.5}\]The HD 660S was launched at a price of approximately 499 USD (around 500 USD). However, the Drop + Sennheiser HD 6XX offers similar acoustic characteristics and performance for about 267 USD. The cost-performance is calculated as 267 USD / 500 USD ≒ 0.53, resulting in a score of 0.5. The HD 6XX is based on the legendary HD 650 and delivers performance extremely close to the HD 660S at a significantly lower price. Furthermore, planar magnetic headphones like the HiFiMAN Sundara (293 USD) offer superior low-distortion characteristics at a lower price, making the HD 660S’s pricing less competitive in the market.
Reliability & Support
\[\Large \text{0.7}\]Sennheiser provides a 2-year product warranty and has a global repair and support system. The HD 600 series is known for its long-term durability, and stable operation can be expected for many years under proper use. However, the earpads are consumables and require regular replacement, recommended approximately every 1-2 years depending on use. Although this model is discontinued, Sennheiser’s support continues, and repairs and parts supply remain available.
Rationality of Design Philosophy
\[\Large \text{0.9}\]The HD 660S embodies a rational design philosophy aimed at improving sound quality based on measured performance. In particular, lowering the impedance from 300 ohms to 150 ohms is a rational response to modern demands, considering use with smartphones and portable players. The reduction in THD and adjustments to the frequency response are also based on a scientific approach to pursuing fidelity to the master recording. The solid improvement in performance within the framework of existing dynamic driver technology, while targeting a realistic user base and application, is highly commendable.
Advice
The HD 660S is discontinued, making it difficult to obtain a new unit. While exploring the used market is an option, a more rational choice is to select from current products. If you are looking for sound characteristics very similar to the HD 660S, the Drop + Sennheiser HD 6XX offers excellent cost-performance at around 267 USD, making it a top candidate (primarily purchased directly from the Drop website). For those seeking a more modern sound or even better low-distortion performance, the HiFiMAN Sundara (293 USD), a planar magnetic headphone in the same price range, is a strong alternative. Its successor, the HD 660S2, is also a good choice, but considering the price, the superiority of the HD 6XX is clear.
(2025.8.2)