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Topic: Buzz from speakers (Read 1 time) previous topic - next topic

Latency

Sep 05, 2012, 05:12 pm Last Edit: Sep 05, 2012, 05:16 pm by Latency Reason: 1
Hi,

This isn't really related to the arduino but I am hoping someone might explain it me.

I have a speaker system that connects to my laptop (via a 3.5mm headphone jack). When I unplug the jack from the laptop I hear a faint hum coming from the speakers.

If I grab the speaker wire the hum doesn't change but if I grab the plastic part of the plug the hum increases dramatically. Is this because there isn't as much shielding in the plastic part?

And lastly, would someone please explain to me what exactly is causing the hum? Why does my touching the plastic part of the plug *increase* the hum?

Also, aside from turning off the speaker system, is there anything else I can do to reduce/stop the hum? (The reason I ask is that the power switch to the system is in a difficult to reach location).

Thanks in advance!
LT

johnwasser

Sounds like the plug is picking up 60Hz hum from the house wiring. When you hold the plug you act as an antenna and the hum you pick up is capacitively coupled to the plug.

To reduce the hum you can ground or shield the plug.  The fancy way would be to get a stereo headphone jack, connect both channels to the ground terminal, and plug he plug into that.  The cheap way is to cram the plug into a ball of aluminum foil.
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Latency

John, thank you *so* much for your explanation. I just tried the foil and it worked perfectly - no more hum!

The foil is acting as the shield, correct? For some reason I thought that using the aluminum to effectively connect the two portions of the connector would have been a "no no" (by creating a short circuit).

Best,
LT

johnwasser


For some reason I thought that using the aluminum to effectively connect the two portions of the connector would have been a "no no" (by creating a short circuit).


Short circuits are bad for outputs (zero resistance means infinite current if the output voltage is non-zero) but OK for inputs.  Connecting an input to ground provides a nice quiet 0v input.
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Latency

Thanks again John. I think I finally understand this! :)

Best,
LT

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