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Author Topic: Curious about failed audio input circuit  (Read 676 times)
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I tried to built a small pre-amp to use an electret mic as an audio input sensor for a project, but I am not getting the readings that I want from it. Full disclosure, I put the schematic together about a year ago, and only just recently got the PCB made. I thought it would be OK (everything else is working fine), but I didn't do my due diligence on checking it out smiley-razz

My project uses a 3.3V Arduino core, so I put together a really simple pre-amp based on a 2n2222 to amplify the electret mic signal for analog reading. Trouble is, every reading I get is over 1000 (returned by analogRead())! I now know that I maybe should have gone the extra step and used an op-amp, but I'm just curious about why this didn't work.

Its been a couple years since I've had any analog circuitry course work, so I don't quite know what is wrong with the schematic attached. Can someone give me some insight about why I am getting nearly maxed out readings at all times with this?


* audio-schematic.png (7.98 KB, 652x747 - viewed 34 times.)
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Typically you need to capacitance couple the mic signal to the amp stage and then bias the amp stage into it's linear operating range like this example ( which would probably have to be tweaked for 3.3vdc operation)

Lefty

http://www.eleccircuit.com/wp-content/uploads/2010/10/circuit-premic-condenser-into-electrical-signals.jpg

Lefty
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Change R2 to 2k, 10k is too high for most electrets IIRC.
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How are you coupling that signal into the Arduino?. I would certainly hope you have a DC blocking capacitor, 1 - 4R7 uF would be an appropriate range.

Bob
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The other guys are essentially right in their comments. The reason your A/D is reading
near max is specifically because the NPN is not turned on hard enough. If you would read
the voltage on the base you'd see it's very low - somewhere around 1V or less, no
doubt because the electret is pulling it down.

Re d_e's comment, you might not want to AC couple the output because then the signal
to the A/D will be going negative half the time. I would rather fiddle with the R values
[base and emitter Rs], and bias the collector to near Vcc/2, and DC-couple it to the A/D.
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