Closer Examination

Undoubtedly the most striking aspect of the Sennheiser HD 660 S exterior are the massive, iconic, perforated ear cups. They reveal the open-back nature of the headphones, as well as the rear of the built-in 38-mm speaker drivers and their complete assembly. It should be noted that the size and shape of the ear cups remains unchanged from the HD 600 and HD 650. Meaning, the ear pads—and cables we'll get to a bit later—are fully interchangeable, which should come as great news to anyone who already owns a certain number of HD 600 or HD 650 accessories and is looking to upgrade. At the same time, it means that owners of the HD 660 S will be able to easily tap into the massive market of replacement ear pads and cables. Everyone wins!


The one thing differentiating the ear cups of the HD 660 S from those of the HD 600 and HD 650 is the Sennheiser logo, elegantly added to the metal mesh.


The frame of the ear cups is made out of smooth and sturdy matte black plastic, and the same holds true for the headband. It will possibly come as a surprise to some that Sennheiser doesn't use more luxurious materials on a pair of $500 headphones, but that's just how it is. The good news is that the plastic looks and feels exceptionally sturdy. I've been enjoying this pair of headphones for a couple of months now, and they still look like I just took them out of the box for the first time.


A robust metal headband connects the left and right ear cup. It also serves as a way to adjust the "height" of the headphones. Unfortunately, there are no markings that would help you adjust the height on both sides equally, so you'll have to do it by feel.


The headphones are quite flexible, which is always nice to see as it usually makes them less prone to breaking if you drop them. Much like almost all home hi-fi headphones, these can't be folded.


Each ear piece has its own cable connector. Sennheiser is using their proprietary 2-prong connectors, the same ones as on many of their older headphones (HD 580, HD 600, HD 650). If you own a cable for any of those headphones, you'll be able to use it with the HD 660 S without any issues. Even though I'm not a fan of non-standard connectors, I do have to admit that these stay in place firmly, and there's nothing you could possibly do while using the headphones that could yank the cable out by accident. Both cables supplied with the HD 660 S are extremely long—a full 3 meters.


The two supplied cables are differentiated by the plug they terminate in. The one most users are going to use is equipped with a standard 6.35-mm plug, which you'll be able to use with just about any sound source that supports a wired connection. Sennheiser kindly supplied the 6.35-mm to 3.5-mm audio adapter, thus saving us from having to buy one on our own. The adapter seems as sturdy as both cables and all corresponding plugs.


The other cable terminates in a 4.4-mm Pentaconn TRRRS balanced connector. There aren't that many devices you'll be able to plug it into directly as this is a fairly new type of a plug, but some of them do support it, such as Sennheiser's own HDV 820 DAC/amplifier ($2,400) or Sony's PHA-2A portable DAC/headphone amp ($570).

Comfort

The ear cushions are made out of velour and are reasonably soft. They're ever so slightly reshaped compared to the ones used on the HD 600 and HD 650, but the difference is easy to miss. The idea is to have them wrap around the ears even better, and they do a great job at that—your ears should fit inside the cushions regardless of how big they are. As already mentioned, the outer dimensions of the ear cushions didn't change, which means that they are interchangeable with those you potentially already bought for your HD 600 or HD 650 headphones. The accessory market for these headphones is absolutely massive; you'll have no trouble finding a pair of ear pads made from a material and in a thickness you may prefer.


The inner side of the plastic headband is covered in a cloth material that sort of feels like neoprene. There's foam underneath it, and it didn't cause any discomfort to the top of my head.


While the materials used on the ear pads and inner headband are comfortable, and the open-back design is inherently better in terms of keeping your ears cooler and less sweaty compared to a closed-back design, I have to admit that I initially didn't find the Sennheiser HD 660 S very comfortable. The out-of-the-box clamping force is quite high. Until it loosens up, the HD 660 S won't be comfortable to anyone with a remotely larger head. Luckily, there are two simple ways of fixing this issue. If you're in a hurry, you can grab the headband with both hands and slowly but firmly stretch it outward, like you want to physically make the headphones "wider". The plastic Sennheiser uses is exceptionally sturdy, so it won't break easily. Regardless, let it be said that you're doing this at your own responsibility. The other, "slower" option is to let your headphones sit on something that's wider than your head. As you can see in the picture above, I used my studio monitors for that and it worked; after about two weeks, the grip of the headband loosened up significantly, and I was able to wear the headphones for multiple hours without any discomfort at all. Talk about a massive change from their original state!