In article , Bob Latham
wrote:
In article , Don Pearce
wrote:
On Tue, 13 Nov 2007 10:53:08 GMT, Bob Latham
wrote:

Okay, I'll state the obvious. This arrangement was very directional.
Yes, I could tell which way around it was walking into the room with
no prior knowledge and so could everyone else there at the time.

Do you mean you could hear mains hum when it was the wrong way round,
or the musical sound was in some way changed?

There was no hum, it was well screened and properly connected and no
earth loop problem.

The problem here is twofold.

Firstly, that it is unlikely that there was literally "no hum". Almost
certain in a mains-based system that there would be a non-zero amount at
50H and/or harmonics.

Secondly, that low levels of steady LF like 50Hz isn't necessarily heard as
an audible note. It can have the effect of being like a slight 'pressure'
or 'feeling' which can then affect your perception in a way you identify in
another way. Part of the reason is the way our hearing adapts to low level
continuous stimulii. Our systems tend to focus on sudden changes and tend
to acclimatise to steady levels if they are modest/small.

So I'd be surprised if changing which end the screen was grounded made no
difference at all to the hum level. What might well be true is that in both
cases it was too low to be noticed.

The cable direction didn't seem to have any noticeable effect on tonal
balance though it may have made subtle changes I didn't notice. What it
did do was change the sound stage image. In one direction there was no
depth to the sound stage, in the other it was very good. I know I'll not
be believed but this was far from subtle this was seriously obvious.

I have more than once had a similar impression which I've learned to
identify as a sign that there is some low level continuous LF. In some
cases this is hum from the audio system. But in other cases it is something
like rumble or something like a motor in a heating system elsewhere in the
house.

The
impression I got at the time was that the female singer moved about 3
feet backwards or forwards depending on the direction of the cable.

I've never heard anything so obvious before or since, I don't think it
was magic but I would like to understand what was happening. I almost
wish I'd kept the cables now but at the time I didn't wish to be accused
of copper theft from the skip and so put the cables back after the
weekend.

In cases like the one described, changing which end of the cable is used to
ground the screen can alter the hum level. I have no idea if this was the
reason for what you describe, but it seems at least plausible to me as a
possible reason.

In some extreme cases any common ground resistance could also affect the
crosstalk, and thus the image. But I'd regard that as less likely as the
required level of common-mode resistance is probably too high to crop up in
a decent domestic system.

Slainte,

Jim

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