Sennheiser’s HD 620S Closed-Back Headphones Sound Open And Light

When Sennheiser announced that it would be licensing 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 the new products developed by Sonova AG would be the acid test of the cooperation.

It’s been a couple of years since the deal was done. Since then, Sonova AG has produced a few products under the agreement, including the Momentum True Wireless 4 earbuds and the Sennheiser HD 620S. Any doubts I may have had over the quality of the products and their overall design have been completely dispelled. The cooperation between Sonova AG and Sennheiser has produced new products every bit as good and, in some cases, even better than those created by Sennheiser alone.

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

The Sennheiser 600 Series of headphones has long had a reputation for smooth and effortless sound. The HD 660S2 open-back headphones I reviewed last year showed how good Sennheiser is at producing traditional dynamic headphones. The HD 660S2 have an open and airy sound that’s a pure joy; although, being open headphones they aren’t quite so good at keeping out unwanted sound.

If you want to use a pair of headphones on the move in places where traffic noise and other distractions can spoil the music, you need to opt for a closed-back design to keep unwanted sound out and to stop music leaking out while you are listening in public. That’s where the HD 620S headphones come in. These are dynamic headphones with an open and airy sound like the HD 660S2 but with sufficient isolation to keep the music in and the unwanted sounds out.

The new HD 620S have a high damping factor and use a 150Ω aluminum voice coil which means the headphones do need a high-quality source like a headphone DAC or a Digital Audio player. The result is an exciting and punchy sound with plenty of dynamics across the entire frequency spectrum which stretches from a gut-rumbling 6Hz to a bat-squeaking 30kHz. They have the sure-footedness and agility of an open-back pair of headphones but with all the advantages of a closed-back design.

The chassis used for the new HD620S is like the iconic HD 660S2, although with a slightly smaller ear pad. A steel plate closes off the back volume from the drivers and makes a good isolator while controlling reflected sound waves within the earcup housing. The drivers are set at a slight angle which promotes free circulation of air, mimicking the reaction of open-back headphones. It’s uncanny how well Sennheiser has managed to achieve this.

With the drivers set at a slight angle, the HD 620S produce triangular imaging for the soundstage, with an effect like listening to a pair of loudspeakers in a great audio setup. The result is an immersive sound that’s captivating and well-suited for a wide range of music, from live recordings to classic multi-tracked albums.

The HD 620S are superbly made and finished with a classy, speckled plastic that makes them smart and hard-wearing. The headband is generously padded with a notch for the skull’s vertex and it’s been reinforced with an adjustable metal band to accommodate a wide range of head sizes. The replaceable ear pads are made of superbly soft pleather rather than the velour 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 and the pads never seem to get sweaty or itchy.

The 1.8mm cable is terminated with a 3.5mm stereo jack plug. There’s also a screw-in 6.3mm adapter for listening with 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 earcup. Later this year, Sennheiser says it will be launching a balanced cable with a 4.4mm Pentaconn connector and I’d love to try that 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 fit that’s not far off being perfect. At 357g including the cable, they aren’t the lightest but somehow, they don’t feel heavy. Now let’s find out how they sound using the FiiO M23 digital audio player streaming hi-res audio from Tidal.

When Sennheiser says the HD 620S are a closed-back design that sound like open-back headphones, that’s no idle boast. I don’t recall ever reviewing a pair of closed-back headphones with so open and airy a sound. There’s none of that boxy and claustrophobic quality found in so many traditional closed-back headphones. Even the heat that tends to build up in closed-back headphones appears to dissipate. Making 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 a pair of 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 bass lines. The sound never wallows or becomes unfocused, no matter how energetic the tempo gets.

Meanwhile, the midrange is as pure and uncolored as can be. No one makes dynamic drivers quite like Sennheiser. The investment in the Tullamore plant 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 like. The focus of the soundstage never wavers or misses the mark. Neither does the imaging become labored or robotic. The balance across the entire frequency spectrum is as good as it gets with a pair of closed-back headphones. I doubt it can be bettered.

Verdict: If you love the airy and natural sound of a pair of open-back headphones but want to listen to music on the move or in places that aren’t as quiet as a dedicated listening room, the Sennheiser HD 620S are a superb choice. With all the laid-back and unforced sound of open headphones, but with all the isolation of a closed-back model, the HD 620S will serve up audiophile-quality sound wherever you are. 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 move, the HD 620S offer value and quality in spades.

Pricing & Availability: The HD 620S headphones are available now from select retailers and at sennheiser-hearing.com priced at $349.99 / £299.99 / €349.90.

Tech Specs:

• Transducers: Dynamic.
• Diameter: 42mm with 38mm 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).