I recently built an audio amplifier based on the LM386. When I touch one of the input signals with my finger, I can hear a buzzing sound on the output. I checked with the oscilloscope, and there appears to be some kind of voltage signal on my hand with respect to the amplifier's ground. What is this voltage? By what theoretical principle can I understand this?
Answer
Your body becomes connected to the circuit. Probably, capacitively coupled through an equivalent of about 100pF. Then, several things can happen:
Extra capacitance makes your amplifier oscillate.
Your body acts picks up 50/60Hz interference from power lines (aka "60Hz hum") and introduces it into your amplifier. To see this, poke an oscilloscope probe at yourself and observe the signal.
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