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The Cardo S-640 rethinks the design assumptions of Bluetooth headsets, giving you a more useful and less obtrusive solution.

If the extra features have value to you, this is a good headset to choose and would therefore be worth the extra cost over the current low priced value leading Jabra BT125.

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Cardo S-640 Bluetooth Headset review

A new approach to BT headset design

Here's an innovative idea that is actually not innovative at all.  A Bluetooth headset that is more (and less) than just a thing in your ear.

As you can see, this unit has an earpiece the same as a regular corded headset, and a clip on control unit that you'd clip to your shirt.  Many people will find this preferable to some of the more inane (and ugly) single piece Bluetooth headsets.

Part of our series on Bluetooth - more articles listed on the right.



I've always quietly laughed at people with Bluetooth headsets stuck in their ears, even when they're not on a call.  It looks somewhere between pretentious and stupid.  It is also semi-dysfunctional - you're blocking your normal hearing in that ear, and it is seldom comfortable for extended periods.

The Cardo S-640 offers a better approach to how you carry your BT headset when not on a call.

With a list price of $65, and a street price of $35 on Amazon, this is a mid-priced headset.

What you Get

The Cardo S-640 Bluetooth headset comes sensibly packaged in an easy open plastic display box.

Inside is the headset unit itself, a multi-voltage (100 - 240V) charger, two earpiece foam pieces, a folded sheet of safety instructions and warranty registration form, and a user guide.

The unit comes with a non-transferable one year warranty that requires you to first send in a registration form.

The unit lists for $64.99, and can be found for less at the usual sorts of electronics resellers such as Amazon (where it is currently $35).

Feature Chart

Use this information to quickly understand the capabilities of the unit and to compare with other units.

Feature                         Comment


List price $64.99.

Purchased from Amazon for $35, Apr 08


Easy to put on and take off

Completely easy, because it is just a simple ear bud that slips into your ear, like an iPod ear bud.

And of course, totally simple to take off again, too.

Easy to use the control buttons

Here was a major opportunity, but unfortunately wasted by Cardo.

With a much greater amount of surface area on the control box, one could have hoped Cardo might have used the area wisely by adding more buttons and indicators to simply the operation of the unit, but - alas - this is not so, indeed Cardo ended up with fewer controls on their unit than most other units.

The control box has a single button to control volume - each push of the button steps the volume through a series of louder and quieter settings.  There is also the typical multi-colored flashing LED.

The 'control' button is actually on the earpiece, and pressing this for varying amounts of time causes it to do various different things.

Along with every other BT headset reviewed to date, Cardo gets a zero score for its control buttons.

Comfortable to wear

The earpiece is reasonably comfortable to wear, but not much more so than some of the single piece all-in-one earpiece headsets.

On the other hand, because it is so easy to put on and take off the earpiece, the comfort issue is limited to how comfortable it is while in use; you don't need to torture yourself unnecessarily by wearing it all the time.

Although the earpiece is lightweight and small, I didn't find it quite as comfortable as I'd hoped.  It is rigid rather than soft, and slightly larger than ideal for my ear (but might be perfect for yours of course).

The unit came with two foam covers to put over the earpiece.  Actually fitting the cover onto the earpiece was very difficult, and once it was done, didn't really seem to make much of a difference to the comfort level.

Can you use with glasses

Yes, the small size of the earpiece doesn't interfere with glasses at all.

Can use with either ear

Yes, and with no need to change anything over from one side to the other.

Securely mounted on ear

Yes.  The light weight of the earpiece makes it easy to fit securely into one's ear, and the main control box is securely clipped to one's shirt without in any way threatening to damage the fabric.

How to carry

The Cardo S-640 takes a different approach to Bluetooth headset design by mimicking elements of a traditional corded headset, with a simple earpiece and a wire running down to a small control box that you clip to your shirt.

When you don't have the earpiece in your ear, you could hang it on the clip, or perhaps stuff the earpiece in a shirt pocket, or take the entire unit and shove it in a pocket.

One comment about this.  At one point I was wearing the headset, with the control unit clipped to my shirt and the earpiece dangling loose.  I was also carrying a laptop bag with a strap slung diagonally from the bag over my opposite shoulder.  The strap interfered with the control unit, and came close to knocking it off my shirt.  So be careful if you're in a similar situation.


The perceived weight of the earpiece alone is about 0.1 ounces.  This is light, and because there are no levered parts that can increase the feeling of ungainly weight, it is very easy to wear on the ear.

The total weight of the unit with control box is 0.6 oz, but this is a fairly meaningless number compared to the perceived weight of the earpiece alone.

Ease of Use

Commands intuitive and easy to remember

The commands on this unit are at least as bad as on other units, and perhaps even worse than some.

They are absolutely not intuitive - but decide for yourself as you read the details of how to do things, below.

Volume adjustable

Yes.  But rather than the typical approach of having two buttons, one for 'Volume Up' and the other for 'Volume Down', this headset has a single button that you push repeatedly, with each push rotating through a series of four different volume levels.

This is not as convenient a method of setting volume as on units with two volume controls.

How fast does it turn on

It takes about three seconds to turn on and about another five seconds to complete synching up with a phone and to then take the call from the phone.


A helpful and well written 16 page (page size 4" by 4") black and white user guide (in English only) is provided with the unit, and the same guide can also be downloaded from Cardo's website.

The printed version of the guide was revision level 3, the PDF on the website was revision 11, but I didn't spot any major differences in content.


Via email, website (very rudimentary), and (800) number - (800)488-0363.  Support hours are 8.30am - 5pm, Mon-Fri, EST.

A test call to support involved almost 9 minutes of wait on hold, before a helpful American man took the call and sensibly answered my questions.

Pairing password printed on device

No (it is 0000 so in an emergency you have a good chance of guessing what it is!)


Battery life

Up to 6 hours of talk or 150 hours of standby time claimed.

These timings are about normal compared to other units on sale in April 2008.

Low battery indicator/signal

When the battery gets low, the headset makes a beep every minute, and the LED flashes red.

Battery type

Lithium polymer.

Replaceable battery?


As with other headsets, the chances are by the time the battery has died, you'll probably have bought a new headset.

Battery charging method/time

Another disappointing design that - for no good reason other than stupidity - doesn't use a standard USB type connector between the power supply and the unit.

Charging time is about 3 hours.

A red light on the control/battery box goes on when charging and changes to blue when fully charged.

Multi-voltage charger


Charger weight/size

2.0 oz.

A small brick charger.

Other charging methods


How many pairings can be stored


The manual mysteriously says it can store multiple pairings, but doesn't say how many, and Cardo's support people didn't know, either.

Headset and hands-free profiles?

Both profiles are supported.  Also supports A2DP (Advanced Audio Distribution Profile) and AVRCP (Audio/Video Remote Control Profile) profiles.

Audio profile for computers


Bluetooth compatibility

Version 1.2


It says it is a Class 2, but it is claimed to have a 33' range, which would suggest a Class 3 type device.

Effective range

The unit claims the standard 33' (10m) range, and was still working reasonably well at a 50' distance (with no obstructions between the phone and handset) depending on if my body was between the phone and headset or not.  Best results of course were when my body wasn't in the path between phone and headset.


The warranty is a generous one year warranty.

Free return

Retailer policies will vary.

Noise cancelling/DSP

None claimed.

Sound quality

Sound quality was reasonably good.

When using the headset, I sounded somewhat muffled and quieter to the person at the other end of the call.  Here is a sample recording of me using both the headset and an iPhone so you can hear the difference for yourself.

The sound of the other person's voice was the same volume (after adjusting the volume to the third of the four levels, ie one less than maximum) but also very slightly muffled (almost as good a quality as direct through the cell phone).


Turning on and off

Turning on is simple - press the control button on the earpiece and wait until the LED flashes blue - this takes about two seconds.

Turning off is also simple - press the control button for about three seconds until the LED flashes red once.

Auto connect


Voice tag support

Supported (if also supported on the phone, of course).

Briefly tap the control button, then say the name of the person you wish to call.

Last number redial

Yes (in the Handsfree profile only).

Double press the call control button for a last number redial.

Transfer call to/from phone

This works with the Handsfree profile only, and is somewhat complicated.

To transfer a call back to the mobile phone from the handset, you simultaneously press and hold both the Volume button on the control box and the control button on the earpiece for several seconds until the call switches back to the phone.

And to transfer a call from the phone to the headset, no, you don't do the same thing.  Instead you briefly tap the control button on the earpiece.

Call waiting/Three way calling

Not supported.

You would need to use cell phone controls to use these features, through the cell phone, rather than through the headset.

Call reject

This works with the Handsfree profile only.

Simultaneously press and hold the Volume button on the control box and the control button on the earpiece until the call is rejected.

Call answer/end


Generally you will have your headset off rather than on.  So, to answer a call, you could first turn it on, then, once the headset was on and paired, press the button briefly to answer the call.  This however can take a long time - too long for some callers - so it is better to answer the call on the handset then transfer it once the headset has switched on and paired.

If the unit is already on, a short press of the control button will answer an incoming call.

To end a call, a short press of the control button is again needed.

The unit signals incoming calls with a ringing beep sound, in addition to any ringing sound from the phone.  The built in ringer can be switched off (and on again) by two (and three) second pushes of the volume button when you are not on a call.


Yes.  Press and hold the volume button for about two seconds until the LED starts to flash red.  As long as the LED is flashing red, the call is muted.

To unmute, press and hold the volume button for three seconds until the LED flashes blue.


Attractive design

A different approach to design, with a simple 'normal' type ear piece and a separate control box/battery box/microphone that is clip mounted to your shirt.

Flashing indicators on standby

Yes, a blue light flashes once every five or so seconds while the unit is on standby.

This blue light is fairly muted in brightness, but can't be turned off.


The earpiece component is much smaller than other units.

Overall, the unit is perhaps slightly larger than average, but it is not valid to compare the two part design of this with the one piece design of most other BT headsets.


This is an interestingly different design of headset that offers average performance and an average feature set.  With a list price of $65 and a street price of $35 (Apr 08 - from Amazon) it is only slightly higher priced that entry level headsets such as the Jabra BT125, which sells through Amazon for $20 - $25.


Using the Cardo S-640 Headset

When it comes to actually using the headset, its novel design featuring the split unit with separate earpiece and control unit really don't make much difference to the user experience, because you're still reaching up and pressing the button on the earpiece to do everything.

It would have been much nicer to have the control button on the control unit, which is a larger sized unit, easier to hold, and which doesn't involve jamming the earpiece further into your ear each time you press the button.

Most of the commands are impossibly difficult to remember and use, but the simple on, off, answer and hangup commands are all easy enough to remember - they all involve pressing the same button, but for varying lengths of time.

Maybe I was just pressing the button for the wrong lengths of time, but I had problems using the unit, with it sometimes not answering calls, or being off when I thought it was on.

The 13.5" of cord connecting the earpiece to the control box is a good length - not too short as to limit where you have the control box, and not too long as to get tangled up when not in use.

It seems moderately important to keep the control box high up on your shirt and close to your mouth - much extra distance seems to result in a rapid deterioration in sound quality for the person at the other end of the call.

Connecting with phones

Easy.  Once you've paired the unit with a phone, any time you turn the unit on, it will quickly look for and connect with your phone if the phone is also on and in range.  This is convenient and simple.

The unit paired effortlessly to a Motorola Razr V3, a Blackberry 8800, and an Apple iPhone.


The Cardo S-640 looks different to other Bluetooth headsets, and is perhaps more discreet in appearance and use.

But Cardo overlooked the opportunity to add more user friendly controls to the separate control box part of the combined unit, and the net result is yet another 'me too' hard to understand headset.

It has a current (Apr 08) street price of about $35 through Amazon.  This puts it in the middle price bracket.  In terms of sound quality, it is also in the middle of the range of products - not excellent sound, but probably acceptable for most purposes.

Should you spend $35 for this headset when you can get a perfectly good headset (eg the Jabra BT125) for slightly less?  The Cardo offers no extra features that you'd eagerly pay a premium for, merely a difference design and appearance.

Another question to ask yourself if whether you should spend $35 on this headset when you can get another Cardo unit - the S-800 - for almost the same price, and with vastly better sound quality.  Most people will probably opt for the sound quality of the S-800 rather than the innovative design of the S-640.


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Originally published 18 Apr 2008, last update 21 Jul 2020

You may freely reproduce or distribute this article for noncommercial purposes as long as you give credit to me as original writer.


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