????????????????

Perhaps you would like to explain how?.

Is every single collector load resistor a 'voltage divider'?, is a single bias resistor feeding the base of a transistor a 'voltage divider'?. Both are simply voltage drops across a load resistor, not a divider - exactly as the electret mike circuit.

A voltage divider requires at least two resistors, one connected to 0V, one to +ve, with the output of the divider coming from the centre tap of the two resistors - usually with a minimum of 5 times the current in the divider, that you can draw for the load.

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Nigel, I want to make sure I have this: In a basic two-resistor voltage divider ("top" end at +v and the "bottom" end at ground, tapped in the center to supply a load), are you saying that good (or prudent?) design usually calls for the load device to draw at least 5 times the current that the two divider resistors flow?
If the load's current draw varies, does the changing current through the "top" resistor cause the center-tap voltage to vary, in turn possibly causing unwanted effects on the load?

I guess I really should get some sort of simulator and play around with these things.

Thanks

 

The resistors act like a potential divider as far as DC is concerned. If you know the DC voltage on the microphone, then calculating the voltage across the resistors can be done using the potential divider formula.

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Hi Ventura,
Just in case Nigels' gone to the pub.

He is saying, 5 times more current flows in the divider resistors R1 and R2.
The load path draws about 1/5th of the divider current.

In this way any small change in load current demand has a reduced effect on the R1, R2 junction voltage.

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Ahhh, OK, Eric. That's how I thought it should be. I wasn't sure if I correctly understood Nigel's description. Apparently I didn't. I'm glad I asked.

Slowly the light of understanding breaks through the fog of my ignorance. I'll keep slugging away at it.

Thank you

 

Ok since its already a topic sort of...
I have a 3 wire MIC...
http://www.radioshack.com/product/in...entPage=search

How would i connect it to a preamp?

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

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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

 

how woul i use this with a 3 wire mic?

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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:

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

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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.

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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?

 

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

http://sound.westhost.com/project93.htm

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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.

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"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?

 

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."

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