The Sennheiser HD 620S closed headphones sound open and light

When Sennheiser announced that it would be licensing the development of its consumer products to Sonova AG, the Swiss-based hearing aid giant, 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 pretty much allayed my fears. However, I knew that the new products developed by Sonova AG would be the acid test of cooperation.

A few years have passed since the contract was signed. Since then, Sonova AG has produced several products under the deal, including the Momentum True Wireless 4 headphones and the Sennheiser HD 620S. Any doubts I may have had about the quality of the products and their overall design have been completely dispelled. The collaboration between Sonova AG and Sennheiser has resulted in new products as good as, and in some cases even better than, those created by Sennheiser themselves.

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

Sennheiser’s 600 series headphones have long had a reputation for smooth and effortless sound. The HD 660S2 open-back headphones I reviewed last year showed just how good Sennheiser is at making traditional dynamic headphones. The HD 660S2 have an open and airy sound that is pure joy; although as open headphones they are not as good at blocking out unwanted sound.

If you want to use the headphones on the go in places where traffic noise and other distractions can spoil your music, you need to opt for a closed design that prevents unwanted sound from escaping and prevents music from leaking out while listening. in public. This is where the HD 620S headphones come in. These are dynamic headphones with an open and airy sound like the HD 660S2, but with enough isolation to keep music in and unwanted sounds out.

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

The chassis used for the new HD620S is like the iconic HD 660S2, albeit with a slightly smaller earcup. The steel plate encloses the rear volume from the drivers and makes a good insulator in controlling reflected sound waves in the shell housing. The drivers are set at a slight angle that promotes free air circulation and mimics the response of open headphones. It’s incredible how well Sennheiser has managed to achieve this.

With the drivers set at a slight angle, the HD 620S creates a triangular image for a soundstage with 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 are superbly built and finished in a sleek, speckled plastic that makes them smart and durable. The headband is generously padded with a notch for the top of the skull and is reinforced with an adjustable metal strap to accommodate a wide range of head sizes. The replaceable ear pads are made of super soft leather rather than 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 get sweaty or itchy.

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

So far so good. The HD 620S are a beautifully made pair of closed-back headphones with exemplary build quality and a fit that is far from perfect. At 357g including the cable, they are not the lightest, but somehow they don’t feel heavy. Now let’s find out what they sound like with the FiiO M23 digital audio player that streams high definition audio from Tidal.

When Sennheiser says the HD 620S are a closed design that sounds like open headphones, it’s not bragging. I don’t recall ever reviewing a pair of closed back headphones with such an open and airy sound. None of that boxy and claustrophobic quality found in so many traditional closed-back headphones. Even the heat that builds up in the closed headphones seems to dissipate. Make the HD 620S a dream to listen to without your ears overheating or feeling uncomfortable.

The sound is one of the best balanced I’ve ever heard from a pair of dynamic and close headphones. Bass is tight and goes down to 6Hz with a controlled and airy punch that vibrates freely, especially when reproducing acoustic basslines. The sound never gets bogged down or out of focus, no matter how energetic the pace.

Meanwhile, the midrange is as clean and colorless as can be. No one makes dynamic drivers quite like Sennheiser. The investment in the Tullamore site has paid real sonic dividends, and Sennheiser’s engineers are at the top of their game right now.

The treble quality is as silky and smooth as you want it to be. The focus of the soundstage never wavers or misses the mark. Imaging doesn’t become labored or robotic either. Balance across the entire frequency spectrum is as good as it gets with a pair of closed-back headphones. I doubt it can be improved upon.

Statement: If you like the airy, natural sound of a pair 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 a great choice. With all the relaxed and casual sound of open headphones, but with all the isolation of a closed model, the HD 620S will serve you audiophile-quality sound wherever you are. The fit and feel of these relatively affordable headphones make them an 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 in spades.

Price and Availability: The HD 620S headphones are available now from selected retailers and senheiser-hearing.com for $349.99 / £299.99 / €349.90.

Technical Specifications:

• Converters: Dynamic.
• Diameter: 42 mm with 38 mm diaphragm.
• Frequency response: 6Hz – 30,000Hz.
• Sound pressure level: 105dB (1kHz / 1Vrms).
• Impedance: 150Ω.
• Total Harmonic Distortion: -0.05% (1kHz / 90dB SPL).
• Cable: Detachable 1.8m.
• Termination: 3.5mm stereo.
• Weight: 326g (without cable).