Help with Electret Mic connection?

[AtomSoft]

Ok since its already a topic sort of...
I have a 3 wire MIC...
https://www.radioshack.com/product/...kw=microphone&kw=microphone&parentPage=search

How would i connect it to a preamp?

Also can i used some lm386 to make a 1W amp for like headphones?

 

[Ventura]

Audioguru, this electret mic will be in a fixed location, and I'd like the option of placing it some distance from the preamp, if that's feasible. I'm wondering just how long a mic cable I can run to the preamp before I run into problems with interference, impedence losses(?), or whatever. I don't know if I'm limited to 6' or 60'.
And should I use ordinary "audio cable"?

Thanks

 

[AtomSoft]

how woul i use this with a 3 wire mic?

**broken link removed**

 

[Nigel Goodwin]

Thanks for posting that diagram AtomSoft

In that there's only ONE potential divider, that's R2 and R3, and the values 'should normally' be calculated to pass five times the base current that the transistor consumes. This is a 'rule of thumb' when designing potential dividers for this use.

If the load current varies, then the voltage will vary as well, but by a lower amount, because it's 'swamped' by the 5 times current through the potential divider.

Hero999 - being able to calculate the voltage drop across a single resistor doesn't make it a potential divider.

Try having a read of:

https://en.wikipedia.org/wiki/Voltage_divider

 

[audioguru]

That is the project that somebody on another forum complained about:
1) Its mic preamp distorted badly and provided hardly any gain when the battery ran down a little.
2) Its frequency changed when something moved near its antenna or moved away.
3) Its frequency changes as the battery voltage ran down.
4) It sounded muffled when heard on an ordinary FM radio.

I built it to see what was wrong.
1) Its mic preamp transistor became cutoff as the battery ran down.
2) Its antenna is connected to its tuned circuit so its frequency changed when something got near or moved away which added or subtracted capacitance from its tuned circuit.
3) It didn't have a voltage regulator so its oscillator transistor's internal capacitance changed when the supply voltage changed.
4) It didn't have pre-emphasis (treble boost) like FM radio stations have so the treble frequencies were cut by the de-emphasis (treble cut) that all FM radios have.
So I fixed it.

For the 3-wire electret mic you must guess which wire is positive (red?), which wire is ground (black) and which wire is the output (white?). Then don't use R1 to power it.

 

[jfmateos]

I have tried several MICs and resistors, and these are my conclusions:

The electret MIC always sinks the same current; different values of resistors does not seem to affect this current. Dear audioguru: using my mics, there is no different in this current using a 6.8k resistor or a 10k resistor or a 4.7 resistor.

I suppose that the quid is to use a resistor that, crossed by the previous current, will generate the optimal voltage jump to place the mic in its standard operation voltage.

am i wrong?

 

[Nigel Goodwin]

Have a read here, for more than you needed to know!

**broken link removed**

 

[audioguru]

If the FET in an electret mic is a perfect constant current sink then it would have a very high output impedance. The electret mics that I measured had an output impedance of about 5k ohms. So the DC current changed a little when the supply voltage was changed.

 

[Ventura]

"I have tried several MICs and resistors, and these are my conclusions:

"The electret MIC always sinks the same current; different values of resistors does not seem to affect this current. Dear audioguru: using my mics, there is no different in this current using a 6.8k resistor or a 10k resistor or a 4.7 resistor.

"I suppose that the quid is to use a resistor that, crossed by the previous current, will generate the optimal voltage jump to place the mic in its standard operation voltage." jfmateos

The experimenting I did with a cheap Radio Shack electret showed pretty much the same results. With either a 1k or 2.2k current limiting resistor in series with the mic, and applying 3VDC and then 10VDC, the current only rose from .172mA to .194mA with either resistor. I guess the mic is happy as long as it can get what it needs...typically .5mA, but less with some models.

On the other hand, the Rod Elliott article that Nigel's last post linked to describes pumping up the sensitivity/output of electrets by applying 9 volts or more to them.

Fascinating little critters, aren't they?

 

[audioguru]

A higher value supply resistor for an electret mic results in higher sensitivity because then the FET has a slightly higher voltage gain. The supply voltage must be high enough so that the mic does not distort at high levels.

The article has an error when it discusses a mic with a 1.5V supply:
"The signal handling is limited." Correct.
"Relatively low sensitivity.' Wrong.

The article correctly describes the sound level handling is low and is much higher when the supply voltage is higher because "a higher supply voltage allows much more output voltage before distortion."

 

[rumpo]

Electret mics that I have measured had a current of 0.5mA with 5V across them and a current of 0.3mA with 2.5V across them.

Every electret mic draws a slightly different current. Yours is a little lower than most but if it works then it is fine.

I'll preface what I write by saying I'm using some electret mic capsules to see which ones work best for me although I'm not really up to speed on some of the detailed electronics.

I have come across some mics which seem to have a low max voltage. For example there is a table top mic by Audio Technica which is powered by 1.5V silver button cell and which their tech support suggest is not increased.

Then there is a ultra-miniature three-wire mic capsule ("FG series") by Knowles Acoustics which I am looking at that and this specifies 0.9V to 2.2V IIRC. I'm told not to use 5v or anything like that. I think this mic from their FG series uses a different type of transistor than usual.