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Balance Microphone amplifier

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I am working in an hospital. there we have a room where doctors are isolated from nurses because of the X-ray.
But sometimes doctors should talk to nurses. this is done by a small condenser Microphone mounted on the ceiling (On Top of the doctor head) and a
pair of PC speakers in the nurses room. the distance between the Mic and Speaker is around 12 meters and I think this is why nurses
can not hear what the doctor say clearly;
I think If I use an amplifier completely close to the Mic and convert its output to a balance output,  then send the balance output to a differential amplifier in the nurses room, then we can have a more clean sound!

What do U think?
Can anybody tell me how can I convert the condenser Mic output to a balance output?

Here's a pic of what I plan to do.



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Hi Shahriar,
The mic is probably an electret type, not a condenser type. Some people call it an electret condenser mic.

If shielded audio cable is used then the distance of only 12m is fine and the shield will prevent mains hum pickup. If the mic is now connected with a twisted pair of wires without a shield then the mains hum will be much much louder than the voices.

What is not heard clearly? Is the sound mostly mains hum pickup, is it muffled without high frequencies, is it boomy with boosted low frequencies, is it tinny without low frequencies, is it distorted because the gain is too high and the amplifiers are clipping, or is it full of room echoes because the mic is too far away from the voices?

I think the problem is mains hum or that the mic is too far away from the voices so that the sound is mostly echoes.

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Now that you are saying, the problem is the mic is too far away from the voices so that the sound is mostly echoes;
just those part of sound which is louder can be heard;
the distance between the electret mic and doctor is about 2-3m

Should we add an AGC circuit?


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A microphone that is in a room that has echoes is supposed to be 0.05m to 0.3m from the person's mouth. 2-3m is much too far away especially when it is pointing at the top of the doctor's head instead of at his mouth.

More loudness and AGC will probably amplify the echoes exactly as much as the direct voices so probably won't make any difference.
Maybe the amplifier has severe crossover distortion and does not amplify sounds that have a level that is less than the loudest sounds.
Maybe the amplifier does not have enough output power so the weakest sounds are not loud enough.

Look at a boardroom conference system. Each person has a mic close by that is directly in front of him pointing at his mouth.

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So what is the solution?

It is impossible to mount the mic somewhere else;
we can change the mic, we can change the circuits but we can NOT attach a mic to the doctor!

What would you do if want to design such circuit?

Is the problem caused by echoes in the room or is it caused by some voices are too loud and other voices are too soft?

It is a stupid idea to use a mic mounted on the ceiling to pickup voices in the room.
Maybe it would work if the room has no echoes because it has acousic tiles on the ceiling and a carpet on the floor.

If a few microphones are located near where the doctors are, a switched mic mixer turns on the loudest one and the doctor directs his voice to one of the mics nearby then it will work. Audio-Technica has some good but inexpensive switched mic mixers that I have used in boardrooms.
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