[SOLVED] Interfacing microphone line (6.9V)

[player80]

Hi,

I have a microphone line on an old intercom (which when disconnected is at around 10V, once the microphone is attached the signal drops to 6.9V). I can pull it to 6.9V with 650Ohm manually.

I have managed to get the receiving audio to a level which I can sample via ADC, but could anyone give me a hint how I could put audio onto a 6.9V line?
A simple DC bias doesn't work in that case..

I'd think about eg. pwm via resistor and mosfet attached to the dc-line? Any better idea?

 

[Audioguru]

An electret microphone has a Jfet inside it that must be powered with about 0.4mA. I power it with a 10k resistor from a filtered 9V. You can replace the microphone with low level audio through a series coupling capacitor to the amplifier input.

 

[player80]

It's not using an electret microphone. it's an analog system with traditional analog microphone. I did some experiments with ltspice and seems like I can match it somehow, I just never used PWM for audio before (I'll have a look at that tomorrow).
The cable comes from the wall (aside of a ground line) and has 10V as mentioned with a relatively high impedance, 6.x volt once the analog microphone is attached.

So far I still think the PWM Audio way is the way to go.

 

[Audioguru]

What kind of a microphone is "traditional"? A dynamic microphone is old but it never has a DC current in it.
A carbon mic is also old and needs DC current but it has the horrible sound of an old telephone.

Many years ago I worked with Pamex huge intercom systems. Its mics were electret and the main controller used PAM (Pulse Amplitude Modulation which is similar to PWM) so that one inside line used 8 channels at the same time selected by "ulrasound time slots".

Modern Class-D audio power amplifiers use PWM.

 

[player80]

What kind of a microphone is "traditional"? A dynamic microphone is old but it never has a DC current in it.
A carbon mic is also old and needs DC current but it has the horrible sound of an old telephone.

Traditional = old, outdated (in my terminology).
It could be such a carbon mic indeed, the quality sounds terrible too yes and the company does not exist anymore either.
I disassembled a dead microphone once but not sure where the parts are anymore.
The audio path is carried on top of the 6.9V with a little swing only (+/-500mA for a rough average when talking, I did not scream into the mic for testing).

Many years ago I worked with Pamex huge intercom systems. Its mics were electret and the main controller used PAM (Pulse Amplitude Modulation which is similar to PWM) so that one inside line used 8 channels at the same time selected by "ulrasound time slots".

Modern Class-D audio power amplifiers use PWM.

It would be interesting if I could get a better quality onto the line than the original system.

 

[Audioguru]

Since the old microphone produced terrible sound then the electronic circuit will probably not have better sound with a new microphone.