Wired Headsets: A Tutorial on Connectors, Cables & Pinouts

Headset DevicesPrior to smart phones life was much simpler…at least with respect to the wiring for headsets used with portable music players vs cell phones. A recent project has me dealing with some wired headsets. I thought it worth sharing some of the things that I’ve noted along the way, along with just a bit of the history.

What I’m describing relates to the various ways in which the common headset connector has been used over time and across a number of application contexts, including:

  • Music players
  • Desk phones
  • Smart phones
  • Computers

In the beginning there was the pocket transistor radio. When Sony got around to it some transistor radios featured a 3.5mm (1/8”) 2-conductor jack. This was intended to receive an earphone for private listening. It was also called a “mini-jack” since stereo headphones for listening to music used the full-sized, 1/4”  phone plug.

It’s unclear if the creation of the earphone feature was motivated by the desire to listen to the radio after bedtime, or avoid aggravating the Mrs with the play-by-play of the baseball game.

…time passes, until…

The Walkman Cometh

An old school portable music player, like an iPod or my Sansa MP3 player, had a single 3.5mm 3-conductor jack. In fact, this type of audio jack was basically universal. It came into prominence with the first Sony Walkman, which was introduced in 1979.

You can still see these sort of connectors described as 3.5mm TRS, which describes the three conductors as “tip”, “ring” and “sleeve.”

3.5mm 3-conductor-TRS

As these devices were used exclusively for listening to music, the pin out of the 3.5mm connector was quite simple:

  • Tip = Left channel signal
  • Ring = Right channel signal
  • Sleeve = Ground

Since all devices used this wiring standard, basically any headset could be used with any player. Life was good.

Operators Are Standing By

Well, it was basically good…unless you were involved in telephony or two-way radios. Then it wasn’t yet very good. At least not as far as this 3.5mm 3-conductor wiring is concerned.

A headset for use with a telephone, or even a two-way radio of some sort, doesn’t require separate left/right connections. It needs to be wired in a fashion that considers the mono playback and the microphone.

Grandstream GXV3275

Many consumer DECT cordless phones use a 2.5mm 3-conductor connector for a wired headset. While many business class telephones us an RJ-9 type connector for a headset, some manufacturers do use 2.5mm or 3.5mm 3-conductor connectors. Looking around my bench I can find a couple of examples; the Cisco SPA-508G SIP desk phone has a 2.5mm 3-conductor headset jack, and the Grandstream GXV-3275 has 3.5mm 3-conductor headset jack.

Thankfully, these are wired in a fairly standard manner:

  • Tip = Microphone signal
  • Ring = Earpiece signal
  • Sleeve = Ground

So now we have two standard ways of using this connector, varying only by application. Everyone is happy, even the call center staff.

Then along comes the smart phone revolution to wreck it all.

Smartphones Add Complexity

Since a smart phone can be used for both listening to music and making phone calls a 3-conductor connector is no longer adequate. In order to have both stereo music and a microphone signal a 4-conductor connector is required. Enter the 3.5mmm TRRS Connector!

3.5mm 3-Conductor-TRRS

Where things get interesting is in how the conductors are used. There was some initial disagreement about the wiring. Nokia & Lenovo Mobile adopted one approach (OMTP) while Apple and the rest-of-the-known-universe adopted another (AHJ).

In truth, the OMTP standard came first, it was Apple who mixed things up by insisting upon going another way. Happily, it seems that the CTIA eventually defined the AHJ approach as an actual standard.

Here’s the wiring scheme for Apple’s AHJ headset connector:

  • Tip = Left earpiece signal
  • Ring = Right earpiece signal
  • Ring = Ground
  • Sleeve = Microphone signal

Nokia has somewhat thrown in the towel on its OMTP standard by developing a way for their second generation of Lumia phones to sense what kind of headset is plugged in and act accordingly. So there can be some sense of peace, at least in the realm of mobile phones.

Confused & Disgruntled Operators

One of my recent projects involved using a telecom operator type wired headset with an iPod Touch. Can you see the problem that implies? Despite the fact that the iPod Touch is a media player, not a mobile phone, it features a 3.5mm 4-conductor TRRS jack just like its more costly iPhone brethren.

Of course, my project involved using the iPod to host a SIP client, so I was seeking mobile-phone like compatibility with a call-center styled wired headset.

iPod Touch VXi Headset

Most professional telephone headsets terminate in an RJ-9 jack for use with a telephone headset amplifier or “direct connection” to a proper desk phone.  None terminate in the 3.5mm TRRS connector commonly used on smart phones and faux smart phones like the iPod Touch.

Headset Buddy sells adapters that allow you to use a 2.5mm or 3.5mm, 3-conductor equipped headset with an RJ-9 equipped phone. These can work, but sometimes suffer audio level issues. That’s why Plantronics and their ilk have had such a long-running business making headset amplifiers.

The solution to using a good, telephony style headset with an iPod Touch was to buy a proper, call-center style headset that used a Quick Disconnect (QD) system. The headset, in my case a VXi Passport 20, is terminated in a “QD” connector. VXi then offers various QD-to-whatever cables, allowing the headset to be used with whatever device you have before you.

In this manner a VXi Passport 20V can be mated to a VXi 1095 QD-to-2.5mm direct connect cord for use with the SPA508 or GXV-3275 that I mentioned previously.

The QD scheme gives flexibility in device selection, but it didn’t quite solve my specific problem. You see, VXi doesn’t make a QD-to-3.5mm TRRS bottom cable suitable for use with a smart phone.

It happens that GN/Jabra, Plantronics and VXi each have their own style of “Quick Disconnect” fittings. The intent is to lock you into using their gear.  In looking at Addasound headsets last month I was surprised to find that they currently use a Plantronics type QD connector, but were also planning one of their own.

That said, VXi does offer their headsets equipped with other companies QD connectors! Further, Plantronics makes a QD-to-3.5mm TRRS bottom cable. So I could combine a VXi Passport 20P (the P denotes the Plantronics style QD connector) and a Plantronics MO300 Headphone Audio Cable, allowing the headset to connect to the iPod.

This combination has proven to work well together. They are capable of reliably delivering HDVoice over Wifi. We’ve not yet experienced any mechanical instability of the headset or cable assembly.

Computers, too!

It also happens that people sometimes need to use a telecom headset with a computer. My preference to date has been to use a USB-attached headset like the Logitech .Audio 615 that I reviewed so long ago. A USB headset is obviously easy to connect and often appears as a separate, identifiable audio device to the OS, making its especially simple to setup in the soft phone.

The project at hand had the potential to see the same headset deployed with an unidentified laptop. To accommodate this potential I found two possibilities;

Older and larger laptops, even desktops for that matter, have a pair of 3.5mm 3-conductor jacks acting as separate connection for microphone and speakers. To use the VXi Passport 20P headset with such a computer I required either a Plantronics QD-to-dual 3.5mm bottom cable or a 3.5mm TRRS to dual 3.5mm TRS adapter.

bottom cable vs adapter

The alternative bottom cable was very long and coiled. It reflects the fact that it is often used with desktop computers in call center applications. Thus the added cable length could be handy. The other adapter, while another widget in the wiring, is just a few inches in length. That makes it more appropriate when used with a laptop that would be right in front of the user.

Lenovo X1 Carbon side view with audio

Ultrabooks and smaller laptops, like the MacBook Air and my Lenovo X1 Carbon, have much smaller surfaces available to hold their array of connectors. Thus they may offer a single 3.5mm 4-conductor jacks, just like a smart phone. I found that the very same headset & cable combination I was using with the iPod Touch worked fine when plugged into my X1C laptop.

DIY Only When Forced

As you may guess, everything that I’ve just described relating to my project was not learned in a nice, rational order. I made a plan, bought some gear, found that there were issues, then had to amend my plan to address those issues. Repeatedly.

At one point I thought that I could get myself out of a corner by modifying a handful of QD cables that I had purchased, replacing the molded in place 2.5mm connector with a 3.5mm connector. After making one such cable I decided to just bite the bullet and buy the correct cables!

You are far better off to deploy a molded cable. There’s much greater chance that it will be mechanically stable over a many thousands of connect/disconnect cycles. A cable that I soldered and crimped by hand seemed much more likely to fail, making it a problem perhaps not too far down the road.

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  • Kram

    Michael, thanks for the great run down on audio connections. I realise this is some time since your article.
    I wondered if on your travels you have come across any adapters to allow a 3.5mm audio feed IN to the QD cable circuit (to the listeners headset), which would cut out on audio signal to QD line (on threshold limit). Essentially, the operator of a QD driven phone/headset system could be listening to a second source device through the same headset. On a call being received and accepted by the operator, the QD signal line would override the additional source and mute it. Therefore, on cancelling the phone call (QD line), the additional source audio would again play through (unmute). 🙂 Any ideas? I am imagining some junction box at the QD mid-way connector junction? Thoughts? Cheers.

    • mjgraves

      There’s a limited amount of stuff available with QD connectors, largely because they are manufacturer specific. They offer Y-cords for training purposes, but nothing more complicated.

      Since what you describe switches on presence of signal it sounds like it would be an active device, which probably doesn’t exist. It would most likely be done in software, although adding a computer certainly adds to cost & complexity.

      Sounds to me like a task for a Raspberry Pi or something similar.

  • Dr Romeo Chair

    Great information. Just some input on using a USB sound device on PC: it turns out that the inexpensive C-Media based USB audio interfaces work very well and the common cm108 is one of the few USB audio chips that provides an independent adjustment for side tone (shows up as mic signal in Windows playback volume setting).

    As you probably know, this allows you to adjust how much of your own mic signal is fed back into the headphones, so you don’t feel the need to shout when using isolated headphones. Because the feedback is in the chip’s onboard mixer, there’s no annoying delay like there would be if you tried to do it in the driver or application level software.

    • mjgraves

      Dr Chair – Yes, side tone can be handy, most especially when using noise blocking headphones like my favorites from Etymotic. However, C-Media makes the audio chip sets. One must be careful to know that the USB devices you’re considering uses their chip set. A quick search on Amazon reveals some USB Audio adapters that clearly indicate C Media internals. Also an old PCI sound card. A USB Headset isn’t likely to be so explicit.

  • Robbin

    Michael I was wondering if you have ever ran across a qd g to 3.5 4 post female jack? So you could plug in any standard headphone w mic to the female audio 4 post ? I work in a call center and I think my only option is to get the 4post male and just splice and shrink a new female connector but they allow any head phones and that requires a female connector….

    • mjgraves

      Not QD-to-3,5mm female. However, Headset Buddy has RJ-to-3.5mm female. https://www.headsetbuddy.com/3-5mm-headset-to-universal-rj9-phone-switch-adapter/

      • Robbin

        Yeah… unfortunately my company won’t let us plug in a rj9 the rj9 to qd g is not allowed to be removed. I don’t quite get the difference in the difference but that’s their rule. We can use any equipment that works with qd and they will allow any head set even modded or after market. But all the headsets I like are trust models. I also plan on getting one of thouse small AAA battery powered amps that last 50 hours so I can turn up those soft spoken calls. So… yeah ill just mod one and reshrink heat and add a braid I guess is my only option

  • Interesting, and very useful, I was hooking up one of those cheap Chinese Bluetooth receivers and wanted to add a stereo plug for the input so I could use different amplifiers with it. I purchased some of those cheap Chinese breakout boards with the plug on it, I hooked it up and could not get stereo reception till I found this article that showed me my error, it had the 4 conductor plug and after I changed the wires it now works great.