[mythtv-users] Impedance mismatch?

[Simon Hobson]

Dick Steffens <dick at dicksteffens.com> wrote:

> I have the audio line out on the back of my MythTV box connected to the
> Aux input of my stereo. Works great.
> 
> I added my Blu-ray player's audio output to the audio line in of the
> computer. It adds noise. Is this likely an impedance mismatch or could
> it be some other issue with the Blu-ray player?

I'll hazard a guess that the BR player has a 2 wire mains lead ? If so, then see if you can find a convenient screw in the metalwork* and run a ground wire from that to a similar screw on the computer case - I suspect it'll clear up some or all of the noise.

Why ?
With ungrounded equipment like that, the filters (or even stray capacitance) makes a weak potential divider that raises the "common" point to around half the mains voltage (so 120V AC in the UK !). Once you connect a lead between that and the earthed computer, this common point gets earthed via the audio line - with a small amount of "mains" current passing through the audio cable screen (and then the computer circuit board).


* Specifically, you are aiming to earth the unearthed metalwork.