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Cardo S-640 Bluetooth Headset

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Review posted January 2008 by Cedric Bosch

For some people, over-the-ear Bluetooth headsets just don’t cut it. Not only are traditional headsets bulky and awkward to wear, many people are wary of sporting a shiny gadget with flashing LED’s while out in public. The Cardo S-640 eliminates all of these problems by relocating most of the necessary electronics to a small black clip that can be worn anywhere you please. Despite the S-640’s diminutive size, it packs high-quality audio and a long battery life. The S-640 is a pleasure to wear, too. If you like wearing earbud headphones but can’t stand the bulk of a Bluetooth headset, you’ll love Cardo’s latest Bluetooth offering. The Cardo S-640 comes with a 1-year limited warranty and carries an MSRP of $59.99.


The S-640 is remarkably small, measuring a slim 1.77 x 0.70 x 0.91 inches. At 0.58 ounces, the headset is a featherweight, too. It’s slightly more complex than an average Bluetooth headset, though, because the device is split up into two parts: the microphone and the speaker. While it’s not quite as easily pocketable as a standard one-piece headset, the S-640 compensates by offering far more freedom and comfort. By moving the bulk of the device’s weight off of your ear and onto a clip, Cardo made the S-640 a delight to use for hours at a time. The earbud is no different than those on a pair of headphones and is so light, it’s easy to forget you’re even wearing it at all. In addition, Cardo includes a set of foam earbud covers for extra comfort.

Cardo S-640

Pairing and Features

There are only two buttons on the device- one on the earbud for controlling the phone functions and one on the clip for changing the volume. Volume is controlled by a single button, not a two-way rocker, which means you have to cycle through the volume presets with each tap.

The S-640 doesn’t stray far from the standard set of Bluetooth headphone features, but it excels at what it does. Sound quality was good on both ends of our conversation. The earbud provides crisp, clear sound, and is usually loud enough to be heard in busy environments. On the other end of the line, callers heard us with exceptional clarity, most likely because the S-640’s microphone is located closer to the mouth than on other headsets.

The S-640’s LED indicator is also well thought-out. It provides information about the headset’s charging and pairing status when you need it, but calms down when a call is in progress to avoid unwanted attention.

We wish all headsets were as easy to pair as the S-640. To pair the device, you hold down the control button for eight seconds, at which point it enters the “paring mode.” All the phones we tested it with the headset found them quickly, and we were chatting away in no time at all.

Voice Quality and Range

The S-640’s audio quality is excellent in relatively quiet environments, but Cardo’s DSP system could use some improvement. We tested the S-640 in a busy coffee shop and on a public bus. The S-640 does a decent job of removing continuous, uninterrupted background noise (such as the roar of an engine), but struggles to provide clear sound quality when there are pronounced fluctuations in sound levels. That said, the S-640 still provides a far clearer conversation in noisy environments than the traditional “dumb” headsets usually bundled with cell phones. Also, the Cardo did a better job of filtering out background noise on our caller’s end than ours, according to our tests.



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We tested the S-640 on four different phones and found the range to be more than enough for keeping the phone in a backpack or pocket. If you’re the type of person that likes to leave your phone on your desk and walk around, though, you’ll be disappointed to hear that the S-640’s range rarely exceeds 7 feet with a direct line of sight. Here are our observations:

When working with Apple iPhone
The headset had the best voice quality with the iPhone. Apple’s audio processing hardware works well on its own, but sound quality improved noticeably when using the S-640. The range, however, was a bit disappointing- we measured a mere 8 feet before the sound started crackling. All the features worked perfectly, except for voice dialing, of course- the iPhone doesn’t support that feature. In case you’re wondering, it isn’t possible to play music through the S-640.

When working with Sony Ericsson S710a
The S-640 had a far greater range when paired with our Sony Ericsson phone. Sound quality was reliable, and the DSP worked well in all but the noisiest situations. Plus, we were able to move about 15 feet away from the phone before the sound became distorted.

When working with BlackBerry 8800
BlackBerry’s pairing procedure worked like a charm, and we were chatting away in under thirty seconds. We were able to keep a clear audio signal within 12 feet of the 8800, and sound quality only declined slightly in loud situations. All the S-640’s phone functions, including voice dialing, worked smoothly.

When working with Motorola RIZR
We experienced yet another painless setup when pairing the S-640 with Motorola’s basic RIZR phone. Range, however, was the worst out of the bunch- we only were able to move 3 feet away from the phone. Voice quality was rather poor on both ends, too- we experienced frequently garbled noise and our callers said our voices were muffled.  Voice dialing worked well, though we had some trouble getting the S-640 to communicate its “last number dialed” command.

Battery Life

The rechargeable lithium-polymer battery provides around 6 hours of talk time, and 150 hours (around a week) of standby- about average for similarly-priced headsets. The S-640 comes with a small AC wall adapter that charges the headset completely within 3 hours. We were glad to see Cardo decided not to use a charging cradle, as other manufacturers do; instead, the AC adapter plugs directly into the headset itself.


The S-640’s innovative design truly brings a breath of fresh air to the otherwise status quo Bluetooth headset market. The device’s construction is of top quality, the features work perfectly, and the battery life is outstanding. Though the range is somewhat limited with certain phones (notably, the Motorola RIZR), the Cardo S-640 provides an attractive alternative to traditional Bluetooth headsets, sounds great, and is easy to use. If you want a high-quality headset without the pain associated with an over-the-ear device, look no further.

Package Contains

The S-640 headset, world AC charger, user’s manual, and quick start guide.

Technical Specs

-Bluetooth v1.2.
-Profiles supported: Handsfree and Headset Profiles.
-Claimed talk time: Up to 6 hours.
-Claimed standby time: Up to 150 hours.
-Headset size: 1.77 x.0.70 x 0.91 inches.
-Battery: Rechargeable Li-Po.
-Ac adapted: 110-240v.

List Price: $59.99

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