In this particular instance, I have modified a Sony MH1 "Live Sound" headset or earphone so that it doesn't require any adapter to "sound" normal with any device. This has rendered the controls useless, but I will no longer have the strange hollow, ghosting, mono-like sound with incompatible devices. I could have converted the connection from OMTP to CTIA, but that would simply change the devices that the earphones were/weren't compatible with but not eliminate sound problems on all devices.
The process should be similar for any OMTP headset and even CTIA headset if you simply make sure you connect the same pins in both cases. So here are the steps to identify the pins and make the necessary changes. Please feel free to ask questions or leave a comment. I hope this serves useful.
The first step is to open the control section of the headset to allow access to the control circuitry and wiring. This will inevitably vary between headsets. The MH1 proved to be fairly simple, although some care is needed if you wish to put things back together in normal condition.
I was able to open the control casing by using a very sharp pocket knife. I started with the belly of the blade and rolled it into the thin groove where the casing was joined. I rolled the blade forward until the point was in the groove and then gently twisted the blade while applying some pressure until the casing started opening. I then carefully continued this motion while also prying one side upwards until the plastic tabs on one side released from the tabs on the other side (internal tabs you can't see). I started with a very fine screwdriver, but that scarred the casing too much and probably affected the end result negatively.
Here is the opened casing (click for higher resolution images):
Here are the tabs that need to be carefully released. You will see the protruding tabs on one half:
And the indents that they lock into on the other half:
Don't lose the little button if that matters to you. :)
Here is the control board with the microphone visible:
Here is the opposite side of the board with the button contacts visible. Note how fragile the wires are as indicated by the arrow:
Now, for the modification we are going to need a little "helping hands" device. If you have one, use a proper soldering "helping hands" device. For the lazy and poor you can go with a more high-tech solution, such as a zippo lighter and a usb wall adapter (that is technically more high-tech right?). :-P You just want to keep things steady so you can safely and easily solder things together:
Get your soldering iron/solder ready. I set my iron to 600 degrees and use 60/40 kester rosin-core lead solder. If you have an adjustable temperature soldering iron it helps, but it's not required. A good soldering iron will just make the work faster and easier and most likely give you better joints.
Here you can see the mic and ground pins as indicated in the image below. You find these by using a multimeter. Simply set your multimeter to check "continuity" and place one lead on one pin and one lead on the other. If the pins are connected somehow the meter will make an audible tone. The way you differentiate which cable is which, is by understanding a TRS connector.
Let's take a look at the TRS, OMTP and CTIA specifications. Here is a diagram I made depicting the TRS and the OMTP/CTIA connectors:
You can see from the bottom connector drawing that the only difference between OMTP and CTIA is that the ground and microphone wires/contacts are reversed. So to convert one type to the other is as simple as swapping the wires or connections of the ground and mic. This is all an adapter does and also why you only need one adapter to change between either of the two. Note that this should be done at the connector, as there may be two separate mic/ground wires and any mic wires need to be swapped with any ground wires.
But let's ignore that for the moment and just look at the top drawing of a standard audio TRS connector. You have a tip (left channel), ring (right channel) and sleeve (ground). In our situation, we know that the right and left audio channels won't change, so we're left with two contacts - the ground and the only other thing left: the microphone.
The problem with audio compatibility lies in the fact that certain devices are setup so that the contacts inside the jack on the device make contact with those last two sleeve connectors on the TRS type plugs, based on whether they are OMTP or CTIA. If they are backwards, that won't work, because in a TRS audio connector one channel has to be grounded for the signal to work.
So, as I mentioned earlier, I simply set my multimeter to check continuity and probed the pins to see which were connected. The one that results in a tone (connectivity/continuity) has to be the one that is linked to the audio signal, which would be the ground. Probing the ground and either channel will result in a continuity tone. So you can easily find which contact makes two other contacts generate a tone. That is your ground. The only one left is your microphone.
To make things easier (I still used a multimeter to be sure) this board happens to be labeled. mic+ is the microphone contact, mic- is the ground (- is usually ground). And the channels are also listed as +/-.
To complete our modification and make audio work on any device, we simply want to remove the OMTP/CTIA specific "mic" connection, so that "both" pins on the end of the plug (sleeve) act as a ground. This basically makes it a standard TRS plug and allows the ground contact inside any device to match the ground contacts on the jack. To do this you simply solder the mic and ground connections together. Easy as pie. You can do this at the jack or at the controls. It doesn't make any technical difference. On the MH1, it's pretty easy to open the controls and make the change with a simple solder joint. Otherwise you would need to rewire a new TRS plug or cut, swap and solder the wires along the length of the bottom of the cable and seal it electrically and durably somehow.
For our case, simply heat up your soldering iron, press it against the two contacts for the mic/ground until they start to melt. You don't want the wires to come loose, so it is important for things not to move while doing this and to not keep things melted any longer than necessary. Once the contacts are melted, you just need to add some more solder between them until they join into one blob of solder.
You don't need much solder. Just enough to make the connection and keep things clean. If the contacts on a different headset aren't right next to each other, you can easily solder a small wire between the two just the same. In fact, just to be sure, I plugged the earphone into a cheap audio player and touched a wire to both contacts to make sure everything worked before I soldered anything.
Here's the finished solder joint:
Lastly, you need to put the headset control casing back together. My results were mediocre, however if I hadn't started opening the casing with a screwdriver first, marring the plastic a bit, it might have turned out better. Although, I didn't notice any damage to the plastic tabs, so it might just be that I didn't situate the controls perfectly enough in the casing for it to shut completely. Honestly, I'm sick, and I didn't really care at the moment. The tabs snapped together mostly and it felt fairly solid, so I simply wrapped some electrical tape around the casing. If that unwraps, I'll throw a piece of gorilla tape on it.
You can see the fit and finish before taping. Not bad, but could be better:
And finally, the sealed deal. Afterwards I realized that heat-shrink tubing would probably work really well in this case also. It might look a little more uniform than tape and would grasp tighter and more permanently.
That pretty much sums it up. If you wanted to convert one connector format to the other, simply swap the mic and ground connections. Otherwise, joining the two creates a single ground and thus creates a standard TRS configuration. You lose your control functionality, but you gain 100% device compatibility for audio output. Keep in mind, this is for a basic headset control device. Some more complex devices add resistors and other electronic parts to the controls for one reason or another. I can't guarantee success using this method in those type of cases.
If you have any questions or comments please leave them below! :)