How to connect a MIC to an Oscop?

[zesla]

hello,

just need help about how to connect an electret MIC to an Oscope please?

I have a 1.5V inside battery biased electret mic and a non biased one, So can you hep me how to connect them to an oscop so that they detect the environment conditions please?

Thanks

 

[MikeMl]

A 3 to 9V battery, battery+ connected in series with a 10K to 100K resistor, to electret+. Electret- to battery-. Connect 1X scope probe tip to electret+, the ground clip to electret-. AC couple the scope, and turn up the gain as high as it will go (1mV/div).

 

[zesla]

I tried my biased electret but the ouput is very weak even when the gain of my oscop is at is highest?!

 

[MikeMl]

I tried my biased electret but the ouput is very weak even when the gain of my oscop is at is highest?!

That is why electret mics need an amplifier with a gain of ~1000!

 

[zesla]

so there is not any insided gain circuit with the battery in these kinds of electeret mikes?

 

[Sceadwian]

Sure there's inside gain, but the signal it's measuring is itself incredibly small.
If you define what 'very weak' means it might help, with a properly biased mic you should be getting I dunno maybe 50-100mv spikes if you tap it, that's normal signal levels from an electret mic.

1.5volts for a bias seems really low for an eletret mic Mike's 3-9 volts is what you should be using. My PC sound card's mic jack has about 3 volts as a bias.

 

[Nigel Goodwin]

so there is not any insided gain circuit with the battery in these kinds of electeret mikes?

Not really, just a buffer FET - signal levels are similar to other types of mikes.

 

[Martel]

Normally, electret mikes do have an integrated FET amplifier.

And as MikeMl said you simply bias them with DC through impedance/current limiting resistor and Bingo ! It works ! And they provide relatively high output (> 100mV).

However if you have a bare unamplified electret cell, this is another game.

You will need three things:

1 - an amplifier with a VERY high input impedance (around 100M or 1 G ohm input impedance). Finding a MOSFET is an easy thing, finding a 100M or 1G ohm resistor is more difficult.

2 - A 60V (or so) low noise DC power supply with a 100M or 1G ohm output impedance. You can either use a lab supply with a 100M /1G ohm resistor or build a DC-DC converter to provide the requested voltage (witha ample outpur RC lowpass filtering) and

3 - A high quality (very low leakage) DC blocking capacitor. A small value (1nF) will do the job.

You bias your electret mike cell with your DC supply using the series resistor and
you use the capacitor at the electret cell output to prevent the DC voltage to reach the Hi-Z preamplifier.

Also, use protecting diodes on the preamplifier's input to protect the MOSFET from the DC inrush when you power-on the bias supply.

 

[Nigel Goodwin]

Normally, electret mikes do have an integrated FET amplifier.

And as MikeMl said you simply bias them with DC through impedance/current limiting resistor and Bingo ! It works ! And they provide relatively high output (> 100mV).

Like I said above, the output from an electret is only about the same as other types of mikes - you can certainly get 100mV, but so you can from a dynamic mike as well. For quiet distant sounds they provide uV, just like other kinds of mikes - although to be fair, they are probably more suited for that kind of use than dynamic mikes.

 

[audioguru]

Normally, electret mikes do have an integrated FET amplifier.

No.
They have a single Jfet as an impedance converter. The Jfet is not an amplifier.

And they provide relatively high output (> 100mV).

Only if you scream loudly very close. The output is 5mV when talking in a normal conversation loudness about 10cm away.

However if you have a bare unamplified electret cell, this is another game.

There were some "condenser" mics that were bare but all electret mics have a Jfet. The Jfet is not used for gain.

You will need three things:

1 - an amplifier with a VERY high input impedance (around 100M or 1 G ohm input impedance). Finding a MOSFET is an easy thing, finding a 100M or 1G ohm resistor is more difficult.

2 - A 60V (or so) low noise DC power supply with a 100M or 1G ohm output impedance. You can either use a lab supply with a 100M /1G ohm resistor or build a DC-DC converter to provide the requested voltage (witha ample outpur RC lowpass filtering) and

3 - A high quality (very low leakage) DC blocking capacitor. A small value (1nF) will do the job.

You bias your electret mike cell with your DC supply using the series resistor and
you use the capacitor at the electret cell output to prevent the DC voltage to reach the Hi-Z preamplifier.

No. You are completely wrong since you are talking of a condenser mic. The high voltage is built-into the electret material of an electret mic and its Jfet takes care of matching the extremely high impedance of the diaphragm to the outside world.

Also, use protecting diodes on the preamplifier's input to protect the MOSFET from the DC inrush when you power-on the bias supply.[/QUOTE]