FANKY
Posts: 3
Joined: Tue Jun 07, 2016 11:34 am

Ground noise from Microphone

Tue Jun 07, 2016 2:20 pm

Hi,

I am having a microphone connected to a terrace sound card to my RPI 2 mod B. When i have a monitor connected to HDMI everything is fine. When i disconnect the monitor i get a hum from the microphone. it seems like the hum comes when there is nothing connected to the earth ground.

Is there any power supply fro RPI that has a ground connection?

Any other ideas on solution to this?

jdb
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
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Re: Ground noise from Microphone

Tue Jun 07, 2016 3:21 pm

5V micro-USB power supplies are almost always double-insulated, meaning they float referenced to ground. It may well be that your microphone is high-impedance enough to pick up noise via capacitative coupling to the air.

- Are the leads to the microphone shielded (coaxial preferably)?
- Are they routed near other power conductors or parallel to other grounded cables?
- Can you shorten the length of the wire connecting to the microphone?
Rockets are loud.
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FANKY
Posts: 3
Joined: Tue Jun 07, 2016 11:34 am

Re: Ground noise from Microphone

Wed Jun 08, 2016 8:13 am

Thanks for answering jdb!

- Yes the cable has a shield for the ground of the mic cable.
- No the miccable is not near any power cables.
- No i can't unfortunately not shorten the mic cable in this case. It is apron 5 meters long.

Any other thoughts?

FANKY
Posts: 3
Joined: Tue Jun 07, 2016 11:34 am

Re: Ground noise from Microphone

Wed Jun 08, 2016 11:47 am

Hmmm, if i touch some of the metal parts on the RPI the hum goes away... any thoughts?

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davidcoton
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Re: Ground noise from Microphone

Wed Jun 08, 2016 7:54 pm

FANKY wrote:Hmmm, if i touch some of the metal parts on the RPI the hum goes away... any thoughts?
Two things you could try:
1) Disconnect the shield connection to the microphone. (Do this in such a way that it can be reconnected if necessary.)
2) Provide a ground connection to the shield, either with the shield disconnected from the Pi, or with the shield reconnected thus grounding Pi 0V.

Mains hum is being picked up, probably on the microphone cable. The HDMI ground is enough to stop it, so a permanent ground should do the same. It might cause a ground loop, though, when the HDMI is connected. So the other approach is to try to disconnect the source of the hum. The microphone input should be fairly well balanced (wires routed identically, even if the electrical input is not balanced) so that is a less likely route. So the shield (which is currently floating, because Pi 0V is not grounded) is the most likely source. Isolating it may be enough. Isolating from the Pi 0V and grounding should preserve the screening of the mic connections, just in case the input is not balanced and the hum gets in that way, but will avoid a ground loop with HDMI.

Note: to be ultra safe, check that there is no AC or DC voltage present between the shield and ground before connecting. Use a 1K resistor for a temporary ground connection first, and check the voltage across that, otherwise the hum you are trying to remove may show as an AC voltage.
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flatmax
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Re: Ground noise from Microphone

Fri Jun 10, 2016 1:23 pm

Sounds like you have a ground loop https://en.wikipedia.org/wiki/Ground_loop_(electricity). David's suggestions are great.

Can you record with headphones and no speakers playing ? If so, does the ground loop hum appear in your recordings ?

Matt
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davidcoton
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Re: Ground noise from Microphone

Fri Jun 10, 2016 3:11 pm

flatmax wrote:Sounds like you have a ground loop https://en.wikipedia.org/wiki/Ground_loop_(electricity). David's suggestions are great.

Can you record with headphones and no speakers playing ? If so, does the ground loop hum appear in your recordings ?

Matt
I don't think it's a ground loop, because the problem starts when a ground connection (HDMI) is removed. Loop hum usually behaves the opposite way round -- the problem starts when a ground connection is added. :o
However, trying different input/output options is a good way to proceed, just in case.... 8-) :ugeek:
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openhsc
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Re: Ground noise from Microphone

Tue Feb 12, 2019 8:31 pm

Were you able to solve this issue? I'm seeing this with my setup as well. I tried Pi Zero W / 3A+ / 3B+ and the noise persists.

The USB audio adapter is https://www.adafruit.com/product/1475

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