2.5mm mono to 3.5mm stereo phone headset adapter

March 19, 2012

I’ve spent a lot of time on conference calls, probably too much.  I try not to let that stop me from doing productive work while I’m on them though.  With a good headset, I can participate on a call without disturbing others and have my hands free to do whatever other computer work I have to.

Landline Headset

Typical Mono Landline Headset

One thing that annoys me about most phone headsets is that they are really nothing more than an altered phone handset that you wear on your head.  They are mono, with a single, wimpy speaker, like the one on the left.  Staying on the phone for a long duration with only one ear participating is kind of annoying.

I’d much rather have a more comfortable PC headset that I use for skype conversations.  I purchased a 3.5mm to PC Headset adapter so that I could use my PC headset with my mobile phone (most have 3.5mm jacks).  This adapter worked well, and allows PC headsets to be used with 3.5mm jacks.

3.5mm to PC Headset adapter

Since that worked well, I wanted to do the same thing for my standard telephone, which had a 2.5mm jack.

I started by looking at converting the 3.5mm plug from the adapter I used with my mobile phone to a 2.5mm plug, required by my office phone.

An adapter like this cost only a few dollars, so I ordered it.  This worked, but not quite the way I had expected.  Even though my PC headset had two headphone speakers, sound only came out of one side.  This makes sense, since traditional phones are mono devices.

What I really wanted is the sound to come out of both earphone speakers (right and left) – even if it was the same mono signal going to both sides.

I could have tried an alternate path, like purchasing a PC to 2.5mm adapter, but I suspected that this would produce the same result.  Instead, I started searching for an adapter with 2.5mm plug on one end and 3.5mm jack on the other end.  In addition, the sound on the single audio channel of the 2.5mm plug should be duplicated to both channels of the 3.5mm jack.  After much searching, I could not find such an adapter.  I did find an adapter that did this for audio only,  but did not handle the microphone signal.  I can’t believe such a thing wasn’t available for purchase, and I still think it’s my error in not phrasing the search correctly.  However, it didn’t seem that hard to just make one from parts, so that’s what I did.  I also didn’t find anyone else describing a simple DIY project to handle this, so I figured I’d do that at the same time.

So what I’m trying to build is an adapter that plugs into a 2.5mm traditional mono telephone headset jack, so it should look just like the jack on a standard one-headphone headset:

2.5mm Plug

and on the other side ends in a 3.5mm 4 conductor jack:

3.5mm jack

capable of receiving this 3.5mm 4 conductor plug:

3.5mm plug

What I essentially want to do is map the mono audio channel to both the right and left audio channel of the 3.5mm stereo plug, like this:

Desired connections between plugs.

In my case, this mapping works.  Different devices use different pinouts.  I discovered this largely through trial and error.

The 2.5mm jack I used was purchased at Mouser.   I’m sure there’s a better option for this, as this one was quite difficult for me to work with.  I didn’t find another option, so I made that work. I had to remove the outer ring to get to a decent solder point, then I used a multimeter to figure out what connects to what, as shown:

2.5mm plug, with connections

It was difficult to get the points hot enough to take the solder, but after ruining one plug, I was careful and eventually successful at the second attempt (always buy an extra if the components are cheap).

2.5mm connected

It’s not pretty, but it works.  The jack I purchased was much easier to use given my meager soldering skills.  Its connected to the plug like this:

Jack connection

After the jack’s cover gets screwed on and I put some protective heat shrink tubing on it, it looks like this, and works like a charm.

All done

Update: Eventually, the solder connections on the plug wore out. I searched for ‘2.5mm stereo male plug’ on ebay – plenty of hits and easier to work with. I purchased 20 for $5 of these:


2.5mm male plug from ebay

Connections work out like this:


2.5mm male plug, without cover

and the finished product:


All soldered and finished


8 Responses to “2.5mm mono to 3.5mm stereo phone headset adapter”

  1. Splicer Says:

    Excellent, thank you for this!!!

  2. Ernesto Says:

    Indeed strange there is no standard solution for this. Who likes to have 2 headsets on his desk? Thanks for the informative blog

  3. muxer Says:

    Hi, that article made my day, because i found myself in a very similar situation, where I wanted to connect a 2x 3,5mm computer headset to a 2,5mm DECT handset jack and didn’t find a proper adapter either.

    Funnily enough i also bought that kind of connector without proper solder points first and started wondering, if i’m too stupid to solder a simple audio plug after melting the plastic parts.

    Nice to see, that i’m not alone with that.

    • semicrazy Says:

      ha. Yeah, I don’t know why I didn’t find the right one the first time. Probably bad search terms and not enough coffee.

  4. divmondes Says:

    Nice article, exactly my problem. Before I start with my soldering, are you sure about the pin-out of the 4-pin headset plug? The german wikipedia https://de.wikipedia.org/wiki/Klinkenstecker shows a different pin-out.

    • semicrazy Says:

      This is what worked for me. Give it a try with temporary connections using alligator clips or something first.

      • divmondes Says:

        I finally found the time to put together my adapter and realized that your connections are correct and actually also comply to Type B in that German wikipedia article.
        Thanks for this post.

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