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I'm trying to build an electret mic amplification circuit. I see some weird spikes by just connecting the microphone like so:

5v ---^^^---------
     10kΩ)       |
                 |    
                 |
          MIC+------------probe signal

          MIC-
             |
-GND---------------------------probe gnd  

(^^^ = resistor)

The problem is that I'm getting some big spikes that I can't explain. The battery is 4xAA at around 5v, microphone is (this), probe attenuation x10, 1mV/div, 5uSec/div. Any ideas what these spikes might be? The specs of the mic say normal voltage 2V and I'm giving it 5V, could the mic be fried? Does seem to work when I speak, just there is hum when I amplify.

signal spike

UPDATE I changed the 4xAA battery supply to a 3.3V linear regulator (UA78M33CKCS) and replaced the 10kΩ resistor with a 2.2k one. I now have around 0.3mA going to the microphone (0.5mA is the max). The spikes are much smaller but still big compared to the signal I get when I speak. The time div in this picture is 20μS/div. My setup

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  • \$\begingroup\$ Can you show a picture of your setup? Could it be that your probes are picking up noise? \$\endgroup\$ – EasyOhm May 14 '15 at 22:58
  • \$\begingroup\$ Try connecting a BFC directly across the electret's supply terminals. \$\endgroup\$ – EM Fields May 15 '15 at 0:25
  • \$\begingroup\$ "there is hum when I amplify" ... are the spikes 10ms apart, by any chance? (or 8.3ms in 60Hz-land) ... and is the hum a harsh sounding hum? Then you are picking up mains interference somehow. \$\endgroup\$ – Brian Drummond May 15 '15 at 10:30
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The standard operating voltage for that microphone is 2.2 volts with a 2k2 resistor in series to a higher supply. Using 5V and a 10k might be a problem though. My advise is to lower the 10k to 2k2 - it is very doubtful if the microphone will be damaged by this because the current into the microphone will still be only a couple of mA. Make sure your battery polarity is correct of course.

If this does not work then it is highly likely that your electret microphone is dead.

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