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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?

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  • \$\begingroup\$ Possibly related, but for sure not a duplicate. \$\endgroup\$ – user17592 Mar 29 '13 at 19:13
  • \$\begingroup\$ It could be the electrical impulses that control your muscles - are you a particularly fidgety guy or do you have some kind of tic that could cause this maybe 4C 4F 4C \$\endgroup\$ – Andy aka Mar 29 '13 at 19:58
  • \$\begingroup\$ @Andyaka not really, and I hear people having the same experience, so it's probably a general thing. \$\endgroup\$ – user17592 Mar 29 '13 at 19:59
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    \$\begingroup\$ It's power line hum. No fancy complicated explanations are necessary. This is very common. In fact, it's a quick way to test a audio input to see if it is live. If it is a very low level input, like a microphone, turn down the volume first as the result could be very loud. \$\endgroup\$ – Olin Lathrop Mar 30 '13 at 13:15
  • \$\begingroup\$ Related (for touching scope): electronics.stackexchange.com/questions/174190/… \$\endgroup\$ – Fizz Oct 25 '15 at 5:28
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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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    \$\begingroup\$ It is called "50Hz hum" here ;o) \$\endgroup\$ – jippie Mar 29 '13 at 19:25
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    \$\begingroup\$ I just checked here, I can get about 4Vrms@50Hz simply touching a 10 megohm scope input. Into 50 ohms, nothing you can see on a scope. This makes a finger a pretty useful piece of audio test gear (unless you're working on valve amps! :-) \$\endgroup\$ – Brian Drummond Mar 29 '13 at 21:27
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    \$\begingroup\$ Can you also explain how the body picks up interference from power lines? I didn't touch any. Is it just because there's a lot current flowing all around us? (Cool.) \$\endgroup\$ – user17592 Mar 30 '13 at 11:31
  • \$\begingroup\$ @Jippie: It is called "60 Hz hum" here. \$\endgroup\$ – Olin Lathrop Mar 30 '13 at 13:13
  • \$\begingroup\$ @OlinLathrop I bet OP prefers "50Hz hum" \$\endgroup\$ – jippie Mar 30 '13 at 13:22
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Your body is acting as an antenna, or more accurately, the secondary winding of a transformer. A tiny AC voltage/current is being induced in your body from the electromagnetic field produced by the mains electricity power lines all around. When you touch the input line of an amplifier, the tiny voltage/current is amplified and a 50 (or 60) hz sine wave is produced at the amplifier's output. This causes the speaker cone to push and pull 50 (or 60) times a second, producing the hum you hear. Anything that can act as an antenna (or transformer secondary), whether your body or a piece of metal or a length of wire, will have the same effect.

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  • \$\begingroup\$ I don't think inductive coupling is the main coupling mechanism in this case. \$\endgroup\$ – Fizz Oct 25 '15 at 5:33
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    \$\begingroup\$ From opensourceinstruments.com/Electronics/A3013/HTML/Hum.html "We have never seen mains hum inside a faraday enclosure, which means that the mains hum we experience is electrostatic." \$\endgroup\$ – Fizz Oct 25 '15 at 5:56
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I think, the humming sound comes from the mains supply line in your house. Try to turn off the circuit breaker in your house and touch the audio amplifier input line, you will not hearing any humming or buzzing sound come out from the speaker.

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Your body is full of electrical impulses, static and also acts as an aerial to all kinds of RF signals, so what you are hearing is the amplification of that, not necessarily at mains frequency. Try grounding yourself (hopefully it's not a live chassis)?

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