Telex Intercom Question. Use of Ear buds?

[Cburg]

We use a wired 1 channel telex intercom. My students would like to try and use earbuds with a mic (like what comes with cell phones) in place of the they ear muff sets. The earbuds test out to have 4 wires, 1-right, 1-left, 1-mic and a common ground. The pinout for the 4=pin XLR on the headset have pin 1 and pin 2 for the mic, and pin 3 and pin 4 for the speaker. No Common ground.

. I suggested using Pin 1 on the XLR as the mic wire, Use Both right and Left wires on Pin 4 of the XLR then use the common ground on Pin 2 and pin 3.

The questions is this. Do you think this will damage the station box? I am thinking it am cause a Hum but not do any really damage.

Does anyone know a better way to do this? Myself, I think even if it works they will not be able to hear very well back stage over the other sounds. But I want them to learn on their own what works and what does not. I just want to be ready if the smoke is about to happy.

Thanks in advance....

Cburg

 

[Chris15]

Depending on your beltpacks, there's a fair chance his just won't work.
Most consumer earphones with mics use an electret mic which needs a bias voltage to function.
Most production headsets use dynamic mics and so the beltpacks were never designed to supply the voltage.
Some newer models do have the option, often by internal jumper, and there have been threads on here in the past on how to wire up a circuit, normally involving a 9v battery to make it work.

Hope that helps point you the right way...

 

[Jay Ashworth]

If your beltboxes come with 3.5mm jacks provisioned, then you *might* be able to get this to work, possibly by changing jumpers or switches inside the box.

If not, then you'll have to fake something up, and my experience has been that if you're trying to emulate the dynamic-mic headsets that are commonly plugged into an A4M or A5F on the beltbox, you will run afoul of the levels on those wires: traditional headsets have 2 shielded pairs, because the levels are hot enough on the receiver that it will cause system-level feedback into the much lower-level mic circuit. Since you're trying to use existing wiring, which does not have those shields, it might get dicey.

 

[EdSavoie]

Depending on your beltpacks, there's a fair chance his just won't work.
Most consumer earphones with mics use an electret mic which needs a bias voltage to function.
Most production headsets use dynamic mics and so the beltpacks were never designed to supply the voltage.
Some newer models do have the option, often by internal jumper, and there have been threads on here in the past on how to wire up a circuit, normally involving a 9v battery to make it work.

Hope that helps point you the right way...

Where did you get 9v? Voltage for the bias on consumer headsets is 1.9-2.9v with nominal voltage being 2.2v

 

[jonliles]

Where did you get 9v? Voltage for the bias on consumer headsets is 1.9-2.9v with nominal voltage being 2.2v

Convenient power supply that will last an extremely long time with the use of a voltage regulator.

 

[EdSavoie]

Convenient power supply that will last an extremely long time with the use of a voltage regulator.

Fair enough! Just wasn't sure what you had meant.

 

[Chris15]

What Jon said...
And with a reasonably vibration tolerant connection, which cannot always be said of multiple AA battery holders, which would be the other easy option.
The more technically minded I'm sure could design it to be powered off the ~30V DC on the party line for a battery free solution, but the cost of new beltpacks might not look so bad by that point compared to 2 boxes not one

 

[Cburg]

Boy, Thanks for all the input. I should have known about the Dynamic/Eletret mic thing. I guess I just wanted my guys to work it out for themselves. Anyway I will look for the power circuit on the forum and see what is involved there. I can throw together a small PCB if circuit is not to big.

So thanks everyone ... now on to the fun game of "Go Search" ..

 

[Chris15]

The search term of beltpack electret at first glance should set you on the right path...