Audio Hum from grounding issue

Concat

Hello all,

My setup is very simple right now. RPi3 is connected to a monitor via HDMI, and a cheap home theatre receiver via RCA. I'm hoping someone has some suggestions for solving my issue:

I get a low 60 Hz hum that completely drowns out any audio
When I disconnect HDMI, the humming stops
The hum is present even when RPi is not connected to power
Monitor has 3 prong plug, receiver is 2 prong
RCA is audio only (no video line)

What I think is happening is that the receiver is grounded via RCA to RPi, and the RPi is grounded via HDMI to the monitor, which is grounded at the wall outlet. Is current flowing from my receiver through the RPi to the monitor? Would this cause the hum? I sort of assumed it would only hum if current is flowing through the RCA to the receiver.

I don't really know much about ground loops, but this doesn't sound like an actual loop. More like a chain...

I don't really want to use a cheater plug, so what are my options? Is it possible to ground the receiver even though it doesn't have 3 prongs?

herb_fargus

@Concat https://github.com/RetroPie/RetroPie-Setup/wiki/Sound-Issues#white-noise-fix

mrbwa1

You probably need to look into a ground loop isolator. They make some now that plug into a 3.5mm jack and should do the trick: https://www.amazon.com/ZIOCOM-Isolator-Eliminate-Buzzing-Completely/dp/B019FC6ZQQ/ref=sr_1_3?ie=UTF8&qid=1466087573&sr=8-3&keywords=ground+loop+isolator

Tracing and eliminating ground loops can be a total pain. The isolators are usually an easy fix.

Concat

@herb_fargus said in Audio Hum from grounding issue:

@Concat https://github.com/RetroPie/RetroPie-Setup/wiki/Sound-Issues#white-noise-fix

Thanks. I did see this, but it's not really a hiss I'm hearing. It's a low hum, common with ground issues. I actually have no idea what the "audio dither" is. Sounds like it basically takes away the sound when there is no input signal, so it eliminates background static(?) I guess I could try this and see if the hum goes away.

Thanks for the ground loop isolator link. I was hoping for a way of properly grounding everything. Seems crazy to have everything grounded through the HDMI to my monitor, but I'm not an expert.

Concat

The "white noise fix" had no effect.

I did discover that if I touch an unrelated port on my receiver and ground myself that the hum volume is halved. Definitely a ground issue.

Question is... do I try a cheater plug for my monitor, or a ground loop isolator? I suppose I'll order both. If the isolator works I will just use that.

Concat

Got a USB to female 3.5 mm adapter, and even this has the loud hum!

Exact same scenario... as soon as I remove hdmi cable, the humming stops. When I ground myself and touch another output on the receiver, the volume of the humming is halved.

I guess I will try a ground loop isolator. I did some reading that these will often reduce audio quality, so I thought I'd try USB first. The hum is so loud that I am skeptical an inline noise filter will satisfy me... sigh.

Concat

Not that anyone is following this topic, but I ran a wire from the antenna input on my receiver to the metal casing of the monitor and the hum instantly went away. Reciever doesnt have a ground post, but at least its grounded this way and theres no stray current going through the rca cables.

Might help anyone with a similar solution.