The HD 620S closed-back headphones from Sennheiser sound open and light

When Sennheiser announced that it would license the development of its consumer products to Sonova AG, a hearing aid giant based in Switzerland, I was skeptical. At the time of the announcement, I interviewed Andreas and Daniel Sennheiser, joint CEOs of the venerable German headphone company. What they told me at the time largely allayed my fears. However, I knew that the new products developed by Sonova AG would be the crucible for the collaboration.

It’s been a few years since the deal closed. Since then, Sonova AG has produced a number of products under the agreement, including the Momentum True Wireless 4 earbuds and the Sennheiser HD 620S. Any doubts I had about the quality of the products and their overall design have been completely dispelled. The collaboration between Sonova AG and Sennheiser has produced new products that are just as good and in some cases even better than those made by Sennheiser alone.

The new Sennheiser HD 620S are closed-back headphones with custom-tuned 42mm dynamic drivers, designed and manufactured at Sennheiser’s Tullamore factory in Ireland. The drivers produced at this factory are some of the best you will ever hear and the investment put into the innovative production line is now paying serious dividends.

The Sennheiser 600 series headphones have long been known for their smooth and effortless sound. The HD 660S2 open-back headphones I tested last year showed just how good Sennheiser is at producing traditional dynamic headphones. The HD 660S2 has an open and airy sound that is pure pleasure; although as open headphones they are not very good at blocking out unwanted noise.

If you want to use headphones on the go in places where traffic noise and other distractions can spoil the music, you should opt for a closed-back design to keep out unwanted noise and prevent music from leaking out while you’re listening in public. That’s where the HD 620S headphones come into the picture. These are dynamic headphones with an open and airy sound like the HD 660S2, but with enough isolation to keep the music in and the unwanted noise out.

The new HD 620S has a high damping factor and uses a 150Ω aluminum voice coil, meaning the headphones require a high-quality source such as a headphone DAC or digital audio player. The result is an exciting and powerful sound with plenty of dynamics across the entire frequency spectrum, stretching from a rumbling 6 Hz to a crunching 30 kHz. They have the security and agility of open-back headphones, but with all the benefits of a closed-back design.

The chassis used for the new HD620S is similar to the iconic HD 660S2, albeit with a slightly smaller earpad. A steel plate seals the rear volume from the drivers and provides a good insulator while controlling reflected sound waves in the ear cup housing. The drivers are placed at a slight angle, which promotes free air circulation and mimics the response of open headphones. It’s uncanny how well Sennheiser has succeeded in this.

With the drivers placed at a slight angle, the HD 620S produces a triangular image for the soundstage, giving the effect of listening to a pair of speakers in a great audio setup. The result is an immersive sound that is captivating and well suited to a wide range of music, from live recordings to classic multi-track albums.

The HD 620S is superbly made and finished in stylish, speckled plastic that makes it smart and hard-wearing. The headband is generously padded with a notch for the cranial angle and is reinforced with an adjustable metal strap to accommodate a wide range of head sizes. The replaceable ear cushions are made of wonderfully soft leather instead of the velor that Sennheiser often uses. The result is a comfortable fit that covers the entire ear in a circumaural design. You can wear the HD 620S for long periods of time and the pads never seem to sweat or itch.

The 1.8 mm cable is terminated with a 3.5 mm stereo jack plug. There’s also a screw-in 6.3mm adapter for listening to hi-fi systems. The cable is detachable and has a 2.5mm jack plug with a clever locking mechanism that keeps it attached to the left ear cup. Later this year, Sennheiser says it will launch a balanced cable with a 4.4mm Pentaconn connector and I’d like to try that with a digital audio player like the excellent FiiO M23.

So far, so good. The HD 620S are beautifully made closed-back headphones with exemplary build quality and fit that’s not far from perfection. With a weight of 357 grams including cable, they are not the lightest, but somehow they do not feel heavy. Now let’s see what they sound like using the FiiO M23 digital audio player streaming Hi-Res audio from Tidal.

When Sennheiser says the HD 620S has a closed-back design that sounds like open headphones, it’s not an empty boast. I can’t remember ever reviewing closed-back headphones with such an open and airy sound. There’s none of that boxy and claustrophobic quality found in so many traditional closed-back headphones. Even the heat that often builds up in closed-back headphones seems to disappear. This makes the HD 620S a dream to listen to without the ears overheating or feeling uncomfortable.

The sound is one of the best balanced I’ve ever heard from dynamic and close-back headphones. The bass is tight and goes all the way down to 6Hz with a controlled and airy thump that vibrates freely, especially when reproducing acoustic basslines. The sound never wallows or becomes unfocused, no matter how energetic the tempo becomes.

Meanwhile, the midrange is as pure and uncoloured as it can be. No one makes dynamic drivers as good as Sennheiser. The investment in the Tullamore factory has delivered real sound benefits and Sennheiser’s engineers are currently at the top of their game.

The high quality is as silky and smooth as you want. The soundstage’s focus never wavers or misses the mark. The imaging doesn’t become laborious or robotic either. The balance across the frequency spectrum is as good as it gets from closed-back headphones. I doubt it can be improved.

Pronunciation: If you like the airy and natural sound of open-back headphones, but want to listen to music on the go or in places that aren’t as quiet as a dedicated listening room, the Sennheiser HD 620S are an excellent choice. With all the relaxed and casual sound of open-back headphones, but with all the isolation of a closed-back model, the HD 620S delivers audiophile-quality sound wherever you go. The fit and feel of these relatively affordable headphones make them the ideal choice for wired listening with a headphone DAC or digital audio player. If you want high-quality sound on the go, the HD 620S offers value and quality.

Prices and availability: The HD 620S headphones are available now from select retailers and sennheiser-hearing.com for $349.99 / £299.99 / €349.90.

Technical specifications:

• Transducers: dynamic.
• Diameter: 42mm with 38mm aperture.
• Frequency response: 6 Hz – 30,000 Hz.
• Sound pressure level: 105dB (1kHz / 1Vrms).
• Impedance: 150Ω.
• Total harmonic distortion: -0.05% (1 kHz / 90 dB SPL).
• Cable: detachable 1.8 m.
• Termination: 3.5mm stereo.
• Weight: 326g (without cable).