Jim Lesurf wrote:
In article , Don
Pearce

Doesn't matter. You only connect to earth for safety.
That's not quite true. You sometimes connect to earth to eliminate
hum. I had a setup that wasn't earthed and when someone approached the
microphone it hummed; when they touched a chassis point it stopped
humming. That wasn't an earth loop but it was capacitative pickup of
mains interference as the ground of all the double-insulated kit was
floating at a hundred volts or so. Earthing the amp cured it. Modern
laptop PSUs have three core mains leads so it probably doesn't happen
on new kit.


That was some other problem, that just happened to be cured by grounding.

Depends on what you mean. A problem can arise if a system with no explicit
grounding link has a leakage path though a relatively large impedance that
then generates an 'error' signal injected into a signal-sensitive path.
(e.g. being the coupling between windings of an unscreened PSU transformer,
particularly if the effective impedances for the two polarities on each
side differ.) Depends on the details of the system, etc.

The behaviour can then be as described. Mains hum until the system has a
decent ground - which then provides a lower impedance path for the induced
hum currents, so suppresses the effect.

FWIW Many years ago I spent time working on ultra-sensitive signal
detection systems for astrophysics[1]. Often used in places with lousy
power and dubious grounding. I encountered a large variety of situations
that caused hum. Often in ways which the standard descriptions ignore or
assume won't happen. :-)

Slainte,

Jim

[1] Worst for this were bolometric (heat) detectors whose resistance
changed a tiny amount when warmed by the radiation from a distant galaxy.
Thus needing you to sense minute changes in current or voltage when such an
element was biassed. At least with heterodyne mixers you could do
everything at RF and avoid mains. :-)


What I really meant was that if you dispensed with grounds altogether,
by running battery-powered, keeping mains hum out is reduced to a matter
of appropriate connections and screening. A ground connection is not a
factor.

d

Don Pearce wrote:

What I really meant was that if you dispensed with grounds altogether,
by running battery-powered, keeping mains hum out is reduced to a matter
of appropriate connections and screening. A ground connection is not a
factor.

Balanced lines help too !

Graham

"Eeyore" wrote in
message
Don Pearce wrote:

What I really meant was that if you dispensed with
grounds altogether, by running battery-powered, keeping
mains hum out is reduced to a matter of appropriate
connections and screening. A ground connection is not a
factor.

Balanced lines help too !

Hey Graham, remember our recent conversations with those mental giants from
an old UK penal colony who could see no purpose for balanced lines in
consumer gear?

What a hoot!

philcud wrote:
Just started to get an earth loop using my laptop as source connected
to amp.
just starte
get 'cricket' type sounds when the laptop is connected to power lead.
problem goes totally when i half remove the power lead,
gets unlistenable when the power lead is removed and laptop running on
battery.

only just started happening, any ideas?

Are you using a headphone output from the laptop set at very low volume?
This can quite easily give high noise from (I assume ) the power line in
the computer. the solution would be to turn up the volume on the
computer and down on the amp.
Or look for a line level output.
Or use a usb sound card.
Sorry if it's too obvious , but it's good to check the easy ones first.

Roger Thorpe

"Roger Thorpe" wrote in message
...
philcud wrote:
Just started to get an earth loop using my laptop as source connected
to amp.
just starte
get 'cricket' type sounds when the laptop is connected to power lead.
problem goes totally when i half remove the power lead,
gets unlistenable when the power lead is removed and laptop running on
battery.

only just started happening, any ideas?

Are you using a headphone output from the laptop set at very low volume?
This can quite easily give high noise from (I assume ) the power line in
the computer. the solution would be to turn up the volume on the computer
and down on the amp.
Or look for a line level output.
Or use a usb sound card.
Sorry if it's too obvious , but it's good to check the easy ones first.


The clue, I think, is in the "gets unlistenable when the power lead is
removed and laptop running on battery", yet "problem goes totally" when the
power lead outer is making contact. That suggest, IMO, that there is no
chassis connection between the laptop and the amp. As Eeyore says that could
be due to the sleeve connection on the 3.5mm jack breaking away from the
PCB, not an uncommon occurrence.

David.

Eeyore wrote:

Eiron wrote:

Don Pearce wrote:
Doesn't matter. You only connect to earth for safety.
That's not quite true. You sometimes connect to earth to eliminate hum.
I had a setup that wasn't earthed and when someone approached the microphone
it hummed; when they touched a chassis point it stopped humming.
That wasn't an earth loop but it was capacitative pickup of mains
interference as the ground of all the double-insulated kit was
floating at a hundred volts or so.

A (design ?) defect.


Modern laptop PSUs have three core mains leads

Nonsense.

so it probably doesn't happen on new kit.

Equally nonsense. The Y cap bridging the live and secondary sides is still there
(for EMC - LOL).


http://www.maplin.co.uk/module.aspx?...29240&doy=11m7
"An IEC-320 type C5 moulded ‘cloverleaf’ connector on 2m of 3-core cable
with a moulded 13A UK mains plug. Typically used on laptop computers."

A modern laptop in the UK has an earthed chassis.

A five year old laptop doesn't, and if you connect it to
a bunch of old unearthed kit such as an AV amp also connected to
a CRT TV, DVD player, satellite receiver and VCR, all legal and
working as designed, the combined leakage is quite enough to float
the common ground and exposed metal at 100v or so; good for a tingle
when you connect anything. :-)

New kit seems much better though I haven't measured any leakage currents
yet.

--
Eiron.

"Eiron"


A modern laptop in the UK has an earthed chassis.

A five year old laptop doesn't, and if you connect it to
a bunch of old unearthed kit such as an AV amp also connected to
a CRT TV, DVD player, satellite receiver and VCR, all legal and
working as designed, the combined leakage is quite enough to float
the common ground and exposed metal at 100v or so; good for a tingle
when you connect anything. :-)


** So TV antennas and satellite dishes in the UK are not grounded by law?

They are here in Aussie.

Means TV installations are grounded soon as you plug the antenna in.



....... Phil

Eiron wrote:

Eeyore wrote:
Eiron wrote:
Don Pearce wrote:
Doesn't matter. You only connect to earth for safety.
That's not quite true. You sometimes connect to earth to eliminate hum.
I had a setup that wasn't earthed and when someone approached the microphone
it hummed; when they touched a chassis point it stopped humming.
That wasn't an earth loop but it was capacitative pickup of mains
interference as the ground of all the double-insulated kit was
floating at a hundred volts or so.

A (design ?) defect.


Modern laptop PSUs have three core mains leads

Nonsense.

so it probably doesn't happen on new kit.

Equally nonsense. The Y cap bridging the live and secondary sides is still there
(for EMC - LOL).

http://www.maplin.co.uk/module.aspx?...29240&doy=11m7
"An IEC-320 type C5 moulded ‘cloverleaf’ connector on 2m of 3-core cable
with a moulded 13A UK mains plug. Typically used on laptop computers."

A modern laptop in the UK has an earthed chassis.

More likely to have have an earthed PSU as opposed to chassis AFAIK. Would like to see
evidence for your claim.


A five year old laptop doesn't, and if you connect it to
a bunch of old unearthed kit such as an AV amp also connected to
a CRT TV, DVD player, satellite receiver and VCR, all legal and
working as designed, the combined leakage is quite enough to float
the common ground and exposed metal at 100v or so; good for a tingle
when you connect anything. :-)

That's certainly true.


New kit seems much better though I haven't measured any leakage currents
yet.

I simulate them ! Funny waveforms too. ;~)

Graham

Phil Allison wrote:

** So TV antennas ...... in the UK are not grounded by law?

Not old installations for sure to my certain knowledge.

Graham

In article ,
Phil Allison wrote:
** So TV antennas and satellite dishes in the UK are not grounded by law?

No.

They are here in Aussie.

Means TV installations are grounded soon as you plug the antenna in.

I'd hope the aerial input has some form of ground DC - or rather LF
isolation - otherwise you're into an immediate ground loop situation when
connecting to other equipment.

--
*If love is blind, why is lingerie so popular? *

Dave Plowman London SW
To e-mail, change noise into sound.