Altoids Tin 1/8" Stereo Mixer using a 3.5mm 4 pole conductor (mic + audio)?

Hey have been thinking on how to modify https://www.instructables.com/id/Altoids-Tin-18-Stereo-Mixer/ to accept a 3.5mm 4 pole conductor as the output while splitting the mic to the inputs. This is going to be something like the Iphone ear buds with built in mic. I just got a pair of amplified 4 pole headphones with mic that I want to use for everything.  Here is my wish list:
3 - 4 pole Audio in (iphone, macbook, ...)
1 - 3 pole Audio in from computer
1 - Mic in from computer
1 - RJ10 (so I can use my headset with my cisco phone)

The audio mixing is basically the same as Rich's write up. But im not sure if I can just split the mic to everything. If I am sacrificing mic volume by splitting to every input I could limit it to the CPU mic and the RJ10.. the ones i really want.

Please let me know any ideas or advice you have.

Here is my purposed schematic:

Picture of Altoids Tin 1/8

I would be really curious to know if anything ever came of this idea -- I have been searching high and low for a mixer that would enable me to use a single headset with my PC, laptop, desk phone, and smartphone simultaneously. No such product seems to exist, but this schematic pretty much nails it.


The biggest concern I see is bias voltage(s) for the electret condenser mics (ECM). Two possible problems:

1) the PC mic requires bias voltage on the connector ring to operate, so as wired you might not get any output. You might be OK because one (or more)  of the 4-pole inputs will supply bias voltage, you'll just have to try it. The standard bias voltage for PC sound cards is 5vdc, but ECMs can usually run on 1.5-12vdc without problems.

2) Each of the 4-pole (TRRS) inputs is supplying a bias voltage (output) on its mic input to power an ECM in a headset. You're tying them all directly together, which might work, or it might do something bad or unexpected.

The bias voltage supplied by different devices can vary. I don't think it would necessarily damage anything (shorting it to ground is how some phones signal the answer/call control) but tying different voltage levels together from a random assortment of devices is unpredictable at best.

To address this you could choose 1 input to be the bias provider for everyone and use a DC blocking cap for all the other mic signals. Or you could block the DC on all and use 1 or 2 AAA batteries to supply your own bias voltage. Or you could just try it and be aware that some odd behavior (or slim chance of damage) might show up by directly tying the mic inputs together.

HTH & good luck with your project,