Sennheiser HD 660S2 Scene
Now with more depth than its famous ancestors: Sennheiser HD 660S2 for 600 euros (Photo: Sennheiser)

Sennheiser HD 660S2 – the bass-optimized over-ear classic in the test

The recently introduced Sennheiser HD 660S2 is the latest incarnation of the 600 over-ear series from the headphone and microphone specialists from Hanover. At first glance, the changes that the HD 660S2 has undergone compared to its predecessor, the Sennheiser HD 660S, are hardly noticeable. True to the motto “Never change a winning team”, the new model also sticks to the proven, acoustically open over-ear concept of its predecessors. From the outside, it can only be recognized by the now shimmering copper company logo and nameplate; the channel markings of the receiver shells are now on the inside of the headband.

The first practical difference becomes apparent after opening the packaging, which has been designed to be much more resource-friendly than its predecessor: the Sennheiser HD 660S2 now comes with an additional connection cable with a five-pin, 4.4 mm Pentaconn jack plug. This allows balanced operation of the new Sennheiser on appropriately equipped source devices. As with its predecessor, the scope of delivery also includes a practical, short cable adapter with a three-pin jack plug to a 6.3 millimeter jack coupling: This means that the HD 660S2 can be used with almost all common headphone jacks.

Sennheiser HD 660S2 on headphone amp
Primarily intended as headphones for the home, the Sennheiser HD 660S2 can also be used with mobile sound sources (Photo: Sennheiser)

Sennheiser HD 660S2 – Tried and tested improved

The 600 headphone line from Sennheiser, which is as traditional as it is successful, can justifiably claim to be one of the extremely rare sound standards that are appreciated worldwide. This even applies across all industries in both the hi-fi and pro audio sectors. The 600 headphones have always been particularly appreciated for their exemplary sound neutrality with a great wealth of detail, their high (long-term) wearing comfort, their robust construction and their good price-performance ratio. If there is a forgivable weak point in terms of sound, it concerns the low bass range: compared to some other competitors, the 600s tend to show a softer, more restrained side here.

This was the next step in the development of the Sennheiser HD 660S2 compared to its predecessors: deeper, more powerful bass reproduction while fully preserving all the traditional primary virtues. To this end, the Sennheiser developers revised the transducer systems. First of all, modified voice coils are used here. Unlike its direct predecessor, the HD 660S, they no longer have a nominal impedance of 150 ohms, but a nominal impedance of 300 ohms, as with the earlier “classic” HD 600 and HD 650 headphones. Technical background: Higher impedances require a smaller conductor cross-section of the winding. This reduces the voice coil weight, which can lead to improved pulse behavior. The airflow around the converter has also been optimized.

Sennheiser HD 660S2 explosion view
The exploded view suggests that the voice coils of the Sennheiser HD 660S2 do not require a carrier to save weight. This benefits the impulse response (Photo: Sennheiser)

From the measuring laboratory

Due to its higher impedance, the Sennheiser HD 660S2 requires an input voltage that is around 3 decibels higher than its predecessor, the HD 660S, to achieve the same volume level. However, the recorded power is almost identical, so that the HD 660S2 can also be used with mobile sound sources such as high-quality smartphones or DAPs. Pleasing: As the two multitone diagrams show, the overall distortion is low even at high volume levels.

Forward Back
IMD spectrum Sennheiser HD 660S2 @94dBC
As the multitone spectrum shows, the overall distortion of the Sennheiser HD 660S2 is pleasingly low (measurement: J. Schröder)
IMD spectrum Sennheiser HD 660S2 @104dBC
The same applies even for high volume levels in the 104dBC range (measurement: J.Schröder)
Impedance response by magnitude and phase Sennheiser HD 660S2
Impedance profile Sennheiser HD 660 S2: magnitude (red) and phase response (blue) show a typical curve for acoustically open headphones (measurement: J. Schröder)
Forward Back

This is how the Sennheiser HD 660S2 sounds

Good news right at the start of the listening test: Everything that hi-fi fans and professionals have always appreciated about previous Sennheiser 600 models also characterizes the new HD 660S2: Transparency, richness of detail, luminosity, spaciousness and, of course, high wearing comfort. However, the new model did not stop at preserving its old virtues: it was noticeably more powerful and contoured than its predecessor, the HD 660S, especially in the low bass range.

This was very easy to hear on the album Raven by the up-and-coming R&B artist Kelela, for example: when the bass line from the keyboard reached the low C of around 33 Hertz for the first time at around 1:06 min on the track “Let It Go”, this came across powerfully and almost “perceptibly” with the HD 660S2, whereas the predecessor only “breathed” on this tone. The HD 660S2 did not draw its bass qualities from more volume like an overly fat subwoofer. Rather, it impressed with the contoured pressure it developed in the lower registers – without thickening the upper bass range.

Click on the button to load the content from Bandcamp.

Load content

In any case, the extremely cleanly produced album Raven provided several opportunities for the Sennheiser HD 660S2 to show off its deep bass capabilities. He achieved this in spectacular fashion with the menacingly deep descending drone synthesizer in the intro to the track “Fooley” (from 0:08 min) or the percussively played bass synth in the bridge of “Happy Ending” (from 1:54 min). All in all, the HD 660S2 sounded more powerful, somewhat drier and more contoured than its predecessor, the HD 660S, while the latter, with its lighter timbre, came across as more ethereal.


As the test shows, loyal HD 6xx owners don’t have to immediately discard their good piece in favor of the new HD 660S2. After all, in terms of basic performance, it goes in the same open, transparent direction with identical wearing comfort. For uncompromising low-frequency fans or audio professionals who need full control and contour in the low-frequency range when mastering with headphones, the Sennheiser HD 660S2 is highly recommended as a high-quality, practical and affordable choice. Well done, Sennheiser – the HD 660S2 is now the reference in its class at LowBeats.

Sennheiser HD 660S2
Test result: 4.8
Sound Quality
Build Quality


The rating always refers to the respective price category.
Substantial, deep, contoured sound character
Very high resolution and lively playing style
Excellent wearing comfort
Two connection cables plus adapter included

Sennheiser GmbH & Co KG
At the laboratory 1
D-30900 Wedemark

Price (manufacturer’s recommendation):
Sennheiser HD 660S2: 599.99 euros

Technical data

Sennheiser HD 660S2
Design:Stationary over-ear headphones with an acoustically open design
Transducer principle:Dynamic; 38 millimeter diameter with neodymium magnet
Connections Earcups:Left/right for 2-pin Sennheiser flat plug
Connection cable:Length = 1.8 meters, adapter 3.5/6.3 mm included in delivery
Nominal impedance:approx. 320 Ohm
Identification sensitivity:104 dB/1kHz,1Vrms
Special features:Additional balanced connection cable included in the scope of delivery (fitting on amplifier side: 5-pin Pentaconn, 4.4 mm)
Dimensions (W x H x D):17.0 x 23.0 x 10.0 cm
Weight:260 grams (without connection cable)
All technical data
More from Sennheiser:

True wireless in-ear comparison: Apple, Cambridge Audio, Sennheiser, Sony
Test Sennheiser HD 660 S – the affordable high-end headphones
Test over-ear headphones Sennheiser HD 820 – the stroke of genius
Test Sennheiser Ambeo Smart Headset in-ear mic – 3D sound to go

Autor: Jürgen Schröder

Avatar photo
Toningenieur, R&D-Spezialist und das (mess-)technische Gewissen von LowBeats. Kümmert sich am liebsten um Wissens-Themen, Musik und den spannenden Bereich zwischen Studio und HiFi.