[LAU] ground-loops -- was Re: USB audio interfaces >= 8

Fons Adriaensen fons at linuxaudio.org
Wed May 16 21:19:43 UTC 2012

On Wed, May 16, 2012 at 01:00:10PM -0500, Chris Caudle wrote:

> Which of the equipment (video monitor, computer, active speakers) have
> three wire power connections with safety ground connected, and which (if
> any) have only two wire connections.  Equipment in the US is typically
> double insulated and so does not require a safety earth connection, but I
> don't know if that also applies on equipment shipped for use on European
> 240V power distribution.

Most equipment here (Europe) has an earth/ground connection, the
exception being 'wall warts'. And some of those (e.g. the one for
my new Samsung phone) are so small that I wonder how safe they
actually are.

But there are other potential problems. The LABEL studio at the
Casa della Musica has eight Quested SR8 active speakers. Since they
were to be wired to an analog bantam patch bay (which means their
input signal can be reconnected while they are switched on) I checked
the ground connections - pin 1 of the XLR input to chassis and power
input ground. And on three of them that test failed. So I had a look

There was a yellow/green wire from the ground connection of the
power input socket, the other end of which was soldered to an eyelet
clamped between the main PCB and one of the plastic posts supporting 
that PCB. And in three of the eight cases that eyelet didn't make
any contact with the metal bolt holding everything together. Moving
the eyelet to the top of the PCB (so is was clamped below the head
of the bolt) solved the problem.

Now Quested is not a low-budget name, so I was quite surprised to
find that their QA overlooked such a basic thing. I wrote to them
reporting the problem - and never got any answer.



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