My laptop has the audio jack right next to a USB port, and it has happened that sometimes if I try to plug it in without looking, I’ve jammed it into the USB port instead, underneath the pins. Usually, I realize my mistake upon having a bzzzt bzzzt buzzing or electrical sound coming from my earphones. My question is, why? I mean clearly, electricity is running through it but how come it gets translated into that buzzing sound by the speakers
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If the power pins in your USB port touch the signal or ground contacts of your headphones, that electricity is going to transmit down your headphone cable like any other.
The problem here is that analog audio signals are essentially low voltage AC (alternating current) waveforms, generally ranging from 20Hz to 20kHz, or rather, oscillating 20 to 20,000 times per second. Out of your headphone port, it’s probably on the order of millivolts, from positive to negative wave peaks. The power coming from your USB is DC (direct current). There is no oscillating, except maybe some 50-60Hz ripple from your mains power, and it’s on the order of 5V. Which means the electromagnets that receive the signal in your headphones, and use it to oscillate the magnets that are glued to the speaker cones inside the headphone cups, are being *way* overdriven, which sounds terrible, and could be damaging the voice coils of the headphones. And maybe even your ears if it’s particularly loud.
I’m sure I don’t have to add that repeatedly jamming a 3.5mm TRS plug into a USB port is also probably hard on the port.
TLDR; headphones -> small signal AC, bus power -> larger signal DC
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