[mythtv-users] audio issue - HDMI tv

John Pilkington J.Pilk at tesco.net
Tue Apr 3 10:38:33 UTC 2018

On 03/04/18 08:41, Tim Draper wrote:
>   ---- On Fri, 30 Mar 2018 20:33:33 +0100 Simon Hobson <linux at thehobsons.co.uk> wrote ----
>   > Tim Draper <veehexx at zoho.com> wrote:
>   >
>   > > Touch the ground of HDMI cable to any metal part of the TV and the sound occurs so definitely ground induced.
>   > >
>   > > what options do i have to fix it? seems like internal TV issue and not fussed enough to resolve the issue to go buy a new one. Can i simply run a wire from a ground point down to the mains socket earth pin (3pin UK; green/yellow)?
>   > >
>   > > mains connection is 2way wall socket > with 1 of the sockets to a 6way extension. all the kit connects to the 6way; tv, amp, rpi, laptop etc.
>   > > could a faulty mains socket cause this, or is it device issue?
>   >
>   > Is the noise a low frequency (50Hz) hum ? Which devices have 2 pin (or core) mains and which have 3 pin (or core) ?
>   >
> theres actually no hum. the Lepy amp clicks (sounds like a pair of relays clicking inside) every second or so and sounds very similar to power-on speaker thump. Only does it when HDMI is connected to the TV *AND*  3.5mm/RCA connection to the amp. can touch the metal casing of the HDMI connector to any metal part of the TV and it'll make the noise. I even get a small arc on either the HDMI > TV casing metal or on the sound cards USB cable ground into the rpi too so obviously a volt difference is present somewhere; thus ground loop issue.
> works fine if i only have 3.5mm/RCA to the amp and far more of a typical setup for 99% of users, so i think it's working correctly in terms of product design.
>> Lots of kit only has a 2 wire mains connection - and hence no earth connection. The input filters typically are two capacitors connected L-chassis and N-chassis, which with an unearthed chassis means a potential divider pulling the chassis to half the mains voltage !
>   > Thus it's easy to end up with units earthed through the connecting leads, with obvious problems.
>   >
> Most likely the lepy amp (someone else in the thread mentioned they thought it was that) as all other cables are 3pin. Lepy amp external PSU is L&N with full plastic earth pin.
> iirc i tried swapping the lepy amp with headphones and didnt get any undesirable issues so i'd go with the TV ground is ok and the issue is on the amp side. isolating the audio line (optical is my preferred method due to car-audio RCA isolator experiences) seems like the way to definitely resolve the issue.
>   > Connecting the chassis of each unit to the same earth as they are plugged into should deal with that - and it's a cheap thing to test.

This is getting to sound either expensive, dangerous or both.  The 
classical ground-loop was a millivolt-level thing.  Here you appear to 
have several DC power supplies, probably with 2-core mains leads, with 
at least two having some sort of leakage (maybe capacitative) to the 
live pin.  I would perhaps put your amp psu as the chief suspect.  Be 

Of course an optical coupling would be a 'modern' solution to a hum 
problem, but that doesn't really fit your descriptions.


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