On 23/04/2021 10:47, Tim Streater wrote:
On 23 Apr 2021 at 09:17:58 BST, Peter Able wrote:

Just to give an example of this, this PC is connected by twin, screened
cable to my hifi. It works fine - until I pull the jack out of the PC,
when I get a loud hum. Why - when it is, after all, just a couple of
metres of shielded wires?

The amplifier has a relatively high input impedance, so only a little
induced hum current manifests as quite a high voltage (Ohms Law).
Generally PC LINE OUT sockets expect to see about a 32 ohm load. I could
totally fix this by connecting a 33 ohm resistor between each of the
pairs' screen and inner. Just too damn lazy, I guess !

It's gonna be the jack, then, innit?

No, not at all

What happens if you use your fingers to
short the contacts on the jack? I'd bet that the hum goes away.

No it doesn't. In fact the hum increases - even when I have all three
parts of the plug squeezed between my thumb and finger. This makes
sense. I referred to this issue in my final paragraph. My thumb and
finger resistance if far greater then 33 ohms - so I increase the size
of the hum "aerial" considerably and offer very little reduction in the
line impedance. Result, more hum.

I'd also bet
it reduces as you move the jack away from the vicinity of the PC. The jack is
picking up hum from the PC.


No it doesn't. The induction is overwhelmingly in the 2 metres of
cable, not in the 0.02 metres of the jack plug - and it is not coming
from the PC. Leave the cable just lying around the room - and switch
off and unplug the PC - no difference.

PA