Problem with electret mic and a circuit that powers it

[danesh]

loading on electret mic

i have design a circuit which would be bidirectional. This path would be used to flow the receive and transmit audio signal and also would used to power um my electret mic. I have notice whn i connect elektret mic to this line the signal ac level reduce. Can any one help me

 

[paulux]

Re: loading on electret mic

THe mic can be modeled as small equivalent resistor and capacitor so it loads your circuits. You may try to use resistor to bias the mic and then ac-coupled the mic signal for further processing.

 

[xxargs]

Re: loading on electret mic

i have design a circuit which would be bidirectional. This path would be used to flow the receive and transmit audio signal and also would used to power um my electret mic. I have notice whn i connect elektret mic to this line the signal ac level reduce. Can any one help me

Your electret mic loads (heavy) your signal-bus and is difficult to make solution in easy way...

- telegraph and telephone industrie try this solution in beginning of 1900 and need find out to make workarounds ;-)

In most case you need Low noise OP-amp with recommend impedances for best response and lowest noise from your mic and, isolate your common LF-bus to NOT feeding mic with ac-signal from other sources on your bus...

---

I don't know your topology, but bidirectional soloution (on 2-wire) is always tricky to make good and most case including fork or hybrid circurits to splitting up from 2-wire common bus to 4-wire in terminate side input/output (mic/earphone) and make good balace for good isolation between input and output (not hearing itself talking).


see litterature on telephone...

 

[flatulent]

Re: loading on electret mic

You can use active or passive hybrid circuits to split the two directional signals to two each two wire circuits. You have to be careful on the isolation of the two paths or you can get oscillations.

With the passive hybrid the termination of the fourth port in the line impedance has to use complex RC networks. One each R and C is not enough. Two each will drastically improve things.

Another option which is much cheaper for short cable runs is to use cables with two twisted pairs inside the outer jacket.

 

[paulux]

Re: loading on electret mic

Hi flatulent & xxargs,

Could you kindly provide schematics examples or reference documents of the circuit you described for further elaboration of the situation?

Thanks for your advices.

 

[artem]

loading on electret mic

Just t uderstand how it (hybrids) works :
**broken link removed**

 

[danesh]

Re: loading on electret mic

this would be my block diagram

 

[xxargs]

Re: loading on electret mic

You can use active or passive hybrid circuits to split the two directional signals to two each two wire circuits. You have to be careful on the isolation of the two paths or you can get oscillations.

biggest problem in most case with active hybrid and gain on signal is make stability outside wanted bandwidth and frequence range, very easy to make oscillating around 10 - 20 kHz in situations 300 - 3400 Hz working very well...

With the passive hybrid the termination of the fourth port in the line impedance has to use complex RC networks. One each R and C is not enough. Two each will drastically improve things.

And bigger bandwidth, longer distance you need, your more complicate (higher orders) and more well tuned RC network (line ekvivalent) needs for still good isolation...

Another option which is much cheaper for short cable runs is to use cables with two twisted pairs inside the outer jacket.

I prefer last solution with separate mic and earphone/tx pair, but mic needs driver/buffer if distanse is more a few meters....

And cheap and possible screened Kat-5 cables working well for analog transmission.

With right drivers with complex C//R impedance out and terminate impedance, you can send 20 Hz - 20000 Hz pretty long way on CAT-5 cable without diskant loss.


/xxargs

 

[paulux]

Re: loading on electret mic

Is there any circuit including both mic & low noise amplifier? What considerations do we have when optimizing the noise of this system?
Any comments are welcomed. Thanks

 

[xxargs]

Re: loading on electret mic

Is there any circuit including both mic & low noise amplifier? What considerations do we have when optimizing the noise of this system?
Any comments are welcomed. Thanks

mic-amplifier inside cheap soundcard give more noise than elecret mic itself in many time (depend of very noise Power supply from computer). if you want high dynamic, you must have noise under control and possible rice to line level before transporting on long cable...

in Public adress-world using balanced line in most case to depress noise and inteference for mic and interconnection between equipment.

but if you only want S/N enougt for playing network game with headset, you not need spend time on high quality input...



xxargs