Electret microphone question

How are you supplying power to the microphone?

Hi Tom,

The + terminal of the mic is connected to VCC via a series resistor. That's the configuration they showed on the mic datasheet.

Did you actually measure the DC at the mic? Tom

Yep, I used a multimeter and an oscilloscope to measure the voltage levels.

** Groper Alert !

** If you want to hear amplified room background noises - you MUST listen to the mic via headphones, OR place the speakers in another room acoustically isolated form the first.

There is NO WAY the crude PA system you have created will audibly amplify any such sounds without breaking into a feedback howl.

....... Phil

The opamps used by u is LM6134 IC. try replacing the with LM 324 with is a universal purpose opamps.

Mike should be properly shielded wire, buy 1+1 wire with is available in market ( one is normal and other one will be shielded ) now u will able to hear any sound

Is it really wrong to be using Google Groups?

I actually wanted to hear feedback, but I never got any. At least the feedback would tell me that it is picking up the sound from the speakers.

Thanks! Are there any other terms for 1+1 wire? It's just in case I don't find it in my search.

** You are not a part of usenet at all.

Just a f****ng interloper.

....... Phil

** LOL !!

Grope, grope, grope......

........ Phil

Thanks, Phil!

Try using a larger resistor to bias your microphone. I would try something in the 100K-1Meg range.

You probably have blown your mike with overvoltage. It is rated for 2V operation with that 2.2k, absolute max is 10V. You are operating it with higher voltage I suppose, and blew the internal FET. I would also filter a bit on that line, since any noise is amplified together with the signal.

--
ciao Ban
Apricale, Italy

I used a 100k with the same results.

Hi Ban

The highest voltage setting that I've set my power supply is only 5V. So I don't think that it's blown. What kind of filtering techniques can I use? I already have some bypass caps close to the VCC pin of the IC. I read about a guarding technique this website:

but I don't know much about it, yet.

Also, after looking at the microphone datasheet again and doing some calculation, I got the following numbers:

Referring to 1kHz, at 30 dB re 20 uPa, the mic ideally outputs 4.74 uV. At 80 dB re 20 uPa, 1.58 mV. These numbers are lower than my original assumption. How would I handle signal levels like this properly besides using low noise op amps?

So you take a step back, and take out the microphone. Inject an audio signal where the microphone would be (make sure to use a coupling capacitor). If you don't hear the tone, then you know something else is wrong. If the point where the microphone is supposed to go is high enough impedance, you should be able to touch the point with a finger, and hear some hum.

If the injection doesn't work, then go to the output of the first stage, and inject a signal there. If you hear something, then you know the problem is ahead of that point. If you don't hear something, you know the problem lies later in the circuit.

And so on.

Break things down into small sections, and make sure they work by themselves. It's easier to figure out a problem the fewer components you have to deal with.

Michael

I'm a bit wary of that "Vcc/2" voltage fighting against the static DC output of the first opamp. Those 1M resistors are very high and the opamp bias current will cause some shift away from 2.5V. If the static output of the

2nd opamp is about 2.5V (a volt either way say), then no problem. Otherwise, I'd stick a 10uF cap in series with the 1k resistor between the opamps and just for luck I'd stick a 10uF cap on the output of the second opamp. john

--
Posted via a free Usenet account from http://www.teranews.com

You do realize there's nothing in there that's either going to be damage, or requires such a high value resistor?

There's the actual electret element, and then there's an FET to buffer the signal. The electret is really high output impedance, and the FET doesn't load it down, but provides a lower output impedance.

Take one apart, and you'll likely find an actual garden variety FET. That's what I found when I took one apart, I was actually surprised that it was some recognizeable device and not some blob of epoxy.

You aren't biasing the microphone with that resistor. An electret microphone is charged when manufactured and requires no external charge or bias. The resistor is there to power the FET buffer.

HOw often do you see a circuit that runs off low voltage and yet the voltage is run through such a large value resistor? You don't see it much. You're likely to starve the circuit with such a high value resistor and the low voltage.

And if you run up that resistor, you are back to a relatively high impedance output from the microphone, even if the internal FET buffer works okay with that large value resistor.

No, most resistors to power electret microphones are far lower, no more than 10K and likely lower.

As for the other poster suggesting the microphone is blown, there is virtually no reason that that could happen. There is no reason to put in an FET that is so finicky that it won't run at reasonable voltages.

ANd of course, long before one should be wondering if the microphone is bad (and it can easily be checked by powering it up and feeding it's output to an existing amplifier through a coupling capacitor), the circuit itself could be faulty, or something been's wired wrong.

Michael

Hello Michael:

Yep, I did this, too. Even before I put the mic, I fed a 1kHz signal to the input and got a nice 1kHz output waveform. I then added the microphone and was able to hear the output if I tap or blow on the mic. However, when I took a headphone connected to a walkman and held it above the microphone, I was only able to hear a faint sound from speaker even when I turned the speaker volume all the way up. I was expecting to hear a louder output from the speaker since I can hear the headphone output with my ears. I also had the gain of the amplifier high. But then if I turn it all the way up, I get some feedback from the speakers.