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Custom Comm.s headset Qestion

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I am currently working as a lighting board operator for a local theatre and find the standard cans not very commutable for long term usage. So I decide to unplug the current cans and modify the plug on my computer headset to fit the comm. system. I have successively connected my headphones and mic on my headset to the cans input on the comm. unit. but I have an ohms problem I found from information on the old cans that and using a multimeter that my headphones have an impendence rating that is lower that the current cams so I increased this using a resistor in series. But my mic has an impendence that is higher then the current cans. So I am planning to decrees the mics total impendence using a resistor in parallel. So what I want to know is

Is doing any of this going to damage my heaphones/mic or the theatres comm. unit and third is this going to work as I haven’t tested it yet. If you have any suggestions I would be appreciative.


Thanks for any help

Adam Walker

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A little knowledge is a dangerous thing...


"Impedance" is more than just resistance.


Impedance matching is all about getting maximum power transfer between the driving electronics (here cans beltpack) and the speaker, and of course the microphone back to the beltpack mic amplifier.


In a nutshell, every power supply (be it battery, amplifier or the National Grid) has some internal resistance. So when you draw power from it, some power is wasted inside the supply due to that resistance.


It turns out that the maximum possible power transfer is accomplished when the resistance of the device (eg speaker) is equal to the resistance of the supply (eg amplifier).


Therefore, the given values are the ones that give you the loudest microphone (to everyone else) and loudest speaker (in your ear).


If you add resistors then you will make everything quieter, as power is dissipated in the resistor for no effect - you can't hear a resistor!


The correct way to match impedance is to use small transformers - if you open up a DI box you'll find that it is a tiny transformer. More expensive ones have extra bits (powered ones have amplifiers), but the basic concept is the same.


That said:-

PC microphones probably won't work, as the majority are cheap condenser mics and use a PC version of phantom power at 5v - NOT the 48v of professional systems!

Trying it on a cans headset shouldn't damage anything - if it needs phantom it just won't work.

However, assuming it works you may have problems as you will be 'talking' at a radically different volume to everybody else on the system, as most cans systems don't have microphone volume adjustment.


The headphones should work fine regardless of impedance matching, as the power levels are so tiny that the imbalance is almost irrelevant.

You'll have to adjust the volume control again though.

(With big amplifiers this matters as the amps have to work harder to drive lower impedances.)


Please note - throughout this I am assuming that you are putting an XLR-4 connector on your PC headset and plugging it into a cans beltpack or desktop station.

DO NOT plug into the XLR-3 or XLR-6 connector(s), as these carry power and signal between beltpacks/base stations, and will destroy your headset.

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I suggest you buy a lightweight headset if you do not like head vices. That said, in many cases people use head vices for a reason - namely to allow them to hear the other end without the atmos drowning it out. There are a huge range of headsets out there, it is a matter of finding one you like. You can buy a decent PC headset for $30, however a decent cans headset is often $60 or more - and there is a reason for that.
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Hello everybody (First post)


Quick introduction, I'm 19 and got into all this theatre lark when asked by a friend to acompny him follow spotting a local amature panto. That was four years ago, I loved it and have been doing it ever since.

This year I took a week off work to get more involved with the setup and get in (rather than just turning up 15 minutes before the tech starts...). I learnt a lot from it and since have decided to get involved with other local groups to bump up my experience this year, my aim being to operate and design sound (my true passion) on a show one year.


Anyhoo this year we were using brand new comms units with brainbustingly tight single muff's. This got me pondering - as one does up a tower - if an in ear headset would be more comfortable, if practical.


If security folk can use them in high noise enviroments such as clubs, I can't see show noise being a problem with them. And a good quality throat mic such as this one should pick up a whisper. However, as the previous point raised if the impedence isn't correct a quiet signal like a whisper might be lost into the abyss.


Does anyone have any thoughts on this?

Could I simply buy a cheapy one and stick a 5 pin socket on it the end?

Or even better- does anyone know of a professional in ear monitor headset specifically designed for beltpacks?


To justify this hooge post: perhaps Awalker would find in ears even more comfortable than a lighterweight headset?


Chris Austin

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  • 4 weeks later...
I am a sound guy and get annoyed whilst wearing comms as I need both ears to hear properly so I got hold of the canford techspec and a 4pin XLR plug. I found an earpiece that hangs on the ear but lets sound through which was the correct impedance and had an old lapel mic which I'd repaired, connected them to the 4pin XLR and presto a working system that lets me hear both the comms and my sound, works fine! I'd recommend it to anyone. Can't quite remember but the cans should be around 600 ohms which is quite common I think!
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