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I recently built this mic amplifier circuit (source): audio amp circuit

The output is connected to a speaker, but it's not exactly working as intended: I can only hear myself at the output if I blow hard into the microphone, otherwise not much. However, when I touch the metal casing of the microhpone, it behaves like an antenna and broadcasts what seems to be an AM signal from Quebec. This is fine, I understand that the mic can behave like an antenna, but why does this happen only when I touch it?

I noticed this quite a bit with electronics, particularly when there's transistors involved that touching the electronics has an effect on its behaviour, why is this?

It's also interesting that putting the mic into contact with other insulating materials like my desk or paper has no effect, but only with skin contact in particular it seems to make a good antenna.

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up vote 5 down vote accepted

Your mic always "behaves like an antenna." The capacitance from your body only affects the resonant frequency of the "antenna." The resulting resonant frequency just happens to be in the AM broadcast band. This is why, when you touch the mic, everyone starts speaking French.

When you're not touching the mic, the "antenna" is tuned to best receive some other frequency than the Voice of Les Habs.

See this discussion on how RF gets into audio preamps.

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Thanks, so if I put a variable capacitor in parallel with the mic, can I use that to tune some radio station? – hesson May 4 '14 at 4:59
I don't know the electrical characteristics of your body, nor those of your preamp, so I have no idea what value of capacitor (if any) will work. Maybe you should focus on completing your existing project (building a working mic preamp), and then you can experiment with building radios and antennae. – user35648 May 4 '14 at 5:06
haha good point, I think it's the metal casing that is picking up most of the radio signals so a capacitor in parallel to the mic might not work, but it might if I put it across the casing. I'll keep experimenting, thanks! – hesson May 4 '14 at 5:12

Your body is a huge ungrounded antenna.

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According to the circuit diagram, the OP, while touching the mic case, is indeed grounded (at least to the negative battery terminal or power supply rail). One experiment might be to ground the mic case (e.g. to a cold water pipe) and see if there is a difference. – user35648 May 4 '14 at 5:18

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