MOTM

George, and others,
What used to happen in some older gear and gear with polarized plugs but no
ground is that the neutral sometimes got case connected or otherwise signal
connected by the manufacturer or some other person thinking this was a good
idea.  Non-isolated neutrals are one of the biggest causes of what is called
ground hum or ground loop.  Here is what happens:

You must remember that your neutral and ground are actually connected to
each other back at your mains panel.  This is code and establishes a ground
reference at the mains panel for both neutral and ground.  However, the
neutral conductor carries the identical current to your gear as the hot
conductor in an AC system.  The ground is not supposed to carry ANY current.
Now, what happens to the neutral voltage at the outlet where you plug your
stuff in ??  Voltage rise (above ground) caused by the I2R losses in the
copper wire.  We commonly call this voltage drop.  So, an unloaded circuit
reads hot leg 120 volts to ground, and neutral reads 0 volts to ground.  If
we impose load on the circuit that causes 1 volt of drop across the R of the
wiring, then your voltage to ground on the hot leg is now 119, and the
neutral is 1 volt above ground.  Why is that significant?

As long as ground is isolated, all if fine.  But what happens is that
neutral got connected to ground in your mixer (probably)  So, when you
connect a cable to your MOTM jack, the mixer neutral is now tied to your
MOTM ground.  So, the neutral current required to run anything attached to
this ground network through audio cables now has a parallel path for neutral
current back to your AC main through your ground wire.  That is bad.  And
why does it hum?  Because your lovely audio cables are now carrying AC
current in their shield which envelopes your audio signal and it gets
induced quite well.  Why did it pop?  Because you were equalizing to unequal
voltages when you plugged the cable in and creating a spark (so small you
didn't notice) at the end of your audio cable.

To prove this, plug your mixer back the way you had it.  Now, measure AC
(not DC) voltage from the ground part of a jack on the mixer to a ground
part on your MOTM panels.  See the difference?  I bet you do.  There should
not be.  Because only current in the ground wire can cause this difference.
If you have a voltage difference, you have a neutral isolation violation.

Paul is right about isolation transformers.  They are good for a lot for
things.  However. in this case, an isolation transformer is a patch that
masks the problem in your mixer.  The best thing to do is find the offending
neutral that is not isolated from ground and fix it.

Now, I have seen guys use 3 prong adapters to "lift" a ground and stop hum.
Does it work?  Yes.  Is it a good idea?  No.
Why, because the ground is now allowed to float up and equal neutral voltage
and can become a shock hazard.

I could go on and on about neutrals and grounding.  It is something I know a
lot about.  However, I think this addressed your issue.  Now, I ask you all
to consider two other grounding issues of significant interest.  Grounding
becomes a much larger issue when the currents and voltages increase (as they
do on a power system or during lightning strikes).

Most people think that damage caused to their electronic gear is caused when
lightning spikes the electric and comes into their house.  I won't say that
cannot happen.  But I will tell you that modern arrestors on the
distribution transformer all but eliminate that.  The normal cause of
lightning damage to electric equipment is MULTIPLE earth connections in your
home.  Houses are supposed to have ONE and ONLY ONE earth ground that
connects to the main.  How do they get multiple grounds?  I would say 99% of
them were installed by the phone company.  Some idiot got this great idea to
attach the phone line to a water pipe for ground.  Now, for years he got by
with this because our phone connected to .... well our ear.  But now, it
connects to your TV dish, our computer, etc,  When lightning strikes near
your home, the voltage difference between two grounds separated by many feet
can be thousands of volts.  Ever wonder how cattle near a lightning strike
are killed when not struck.  Same thing.  The voltage difference between
front legs and back legs was enough to electrocute them.  Its called step
potential and results from the lightning current passing through the high
resistance of earth ground.  Ohms law.

Sorry to ramble.  Anyone wanting MORE info on grounding, ground loop
mitigation, wiring AC to a studio, or such can mail me privately.

Larry Hendry

----- Original Message -----
From: GeorgeK <george.kisslak@...>
To: <motm@yahoogroups.com>
Sent: Tuesday, February 13, 2001 10:13 AM
Subject: [motm] Re: Ground Loops


Turns out if I plug any module directly into the amp (bypassing the
mixer) the hum is completely eliminated.  (The mixer does not have a
3-prong plug - time for an upgrade.)  JLH suggested the 900 makes the
mixer hum; I agree.  I'll just bypass it for now.

Thanks for the info.
GK

Here's an interesting example of how current loops can cause trouble that
has nothing to do with the AC mains:

http://www.diysubwoofers.org/projects/cartf/alternat.htm


I love this story. For want of a slightly more expensive jack...


-- 
Terry Bowman, KA4HJH
"The Mac Doctor"

That is a great story and example.  Makes one wonder about the use of
individually shielded cable in an audio snake.  Oh well.  Maybe we better
buy that expensive directional audiophile cable so we are covered.
LH

----- Original Message -----
From: KA4HJH <droscar@...>
Here's an interesting example of how current loops
can cause trouble that has nothing to do with the AC mains:

http://www.diysubwoofers.org/projects/cartf/alternat.htm

I love this story. For want of a slightly more expensive jack...

Heeeey, look at the big brain on Larry!  But seriously, that was a wealth of
information-- thanks!!  I especially loved the electrocuted cow and multiple
earth bit.

So here's a question.  In my studio, I have my computer and audio equipment
all powered from a pair of power strips plugged into the same UPC, which is
plugged into a 3 prong outlet.  (Er, except for the cable modem box, which
is powered from another outlet and has the network wire plugged into the
computer.)  With everything off (except a Waldorf Pulse and possibly the
computer itself-- I forget), every once in a while I hear an intermitten
60hz hum coming from one of the speakers (a powered Mackie).  Its most
likely something like some electrical appliance in the house kicking in, but
I don't know what.  I would have imagined that the UPC would prevent this by
cleaning up the power, but that would appear to not be the case.  Any ideas
what might be happening?  The sound is so quiet I don't even notice it when
things are on and running; its not a problem, so I've never bothered to
investigate.  I'm just curious.  :)

--PBr

-----Original Message-----
From: J. Larry Hendry [mailto:jlarryh@...]
Sent: Tuesday, February 13, 2001 6:00 PM
To: motm@yahoogroups.com
Subject: [motm] Ground Loops - ANSWERS


George, and others,
What used to happen in some older gear and gear with polarized plugs but no
ground is that the neutral sometimes got case connected or otherwise signal
connected by the manufacturer or some other person thinking this was a good
idea.  Non-isolated neutrals are one of the biggest causes of what is called
ground hum or ground loop.  Here is what happens:

Wow JLH, thanks for the core dump, and right on as usual.  No 3-prong 
plugs here.  As I said earlier, I'll just remove the mixer from my 
setup until a *homogeneous* one can be found.  ;)  Otherwise, given 
the dead cow corollary of Ohm's Law, just to be safe I'd have to 
"lift a leg" every time I jack into the mixer!  Now, we don't want 
that!!  (...??)

Cheers,
George

--- In motm@y..., "J. Larry Hendry" <jlarryh@i...> wrote:
> George, and others,
> What used to happen in some older gear and gear with polarized 
plugs but no
> ground is that the neutral sometimes got case connected or 
otherwise signal
> connected by the manufacturer or some other person thinking this 
was a good
> idea.  Non-isolated neutrals are one of the biggest causes of what 
is called
> ground hum or ground loop.  Here is what happens:
> 
> You must remember that your neutral and ground are actually 
connected to
> each other back at your mains panel.  This is code and establishes 
a ground
> reference at the mains panel for both neutral and ground.  However, 
the
> neutral conductor carries the identical current to your gear as the 
hot
> conductor in an AC system.  The ground is not supposed to carry ANY 
current.
> Now, what happens to the neutral voltage at the outlet where you 
plug your
> stuff in ??  Voltage rise (above ground) caused by the I2R losses 
in the
> copper wire.  We commonly call this voltage drop.  So, an unloaded 
circuit
> reads hot leg 120 volts to ground, and neutral reads 0 volts to 
ground.  If
> we impose load on the circuit that causes 1 volt of drop across the 
R of the
> wiring, then your voltage to ground on the hot leg is now 119, and 
the
> neutral is 1 volt above ground.  Why is that significant?
> 
> As long as ground is isolated, all if fine.  But what happens is 
that
> neutral got connected to ground in your mixer (probably)  So, when 
you
> connect a cable to your MOTM jack, the mixer neutral is now tied to 
your
> MOTM ground.  So, the neutral current required to run anything 
attached to
> this ground network through audio cables now has a parallel path 
for neutral
> current back to your AC main through your ground wire.  That is 
bad.  And
> why does it hum?  Because your lovely audio cables are now carrying 
AC
> current in their shield which envelopes your audio signal and it 
gets
> induced quite well.  Why did it pop?  Because you were equalizing 
to unequal
> voltages when you plugged the cable in and creating a spark (so 
small you
> didn't notice) at the end of your audio cable.
> 
> To prove this, plug your mixer back the way you had it.  Now, 
measure AC
> (not DC) voltage from the ground part of a jack on the mixer to a 
ground
> part on your MOTM panels.  See the difference?  I bet you do.  
There should
> not be.  Because only current in the ground wire can cause this 
difference.
> If you have a voltage difference, you have a neutral isolation 
violation.
> 
> Paul is right about isolation transformers.  They are good for a 
lot for
> things.  However. in this case, an isolation transformer is a patch 
that
> masks the problem in your mixer.  The best thing to do is find the 
offending
> neutral that is not isolated from ground and fix it.
> 
> Now, I have seen guys use 3 prong adapters to "lift" a ground and 
stop hum.
> Does it work?  Yes.  Is it a good idea?  No.
> Why, because the ground is now allowed to float up and equal 
neutral voltage
> and can become a shock hazard.
> 
> I could go on and on about neutrals and grounding.  It is something 
I know a
> lot about.  However, I think this addressed your issue.  Now, I ask 
you all
> to consider two other grounding issues of significant interest.  
Grounding
> becomes a much larger issue when the currents and voltages increase 
(as they
> do on a power system or during lightning strikes).
> 
> Most people think that damage caused to their electronic gear is 
caused when
> lightning spikes the electric and comes into their house.  I won't 
say that
> cannot happen.  But I will tell you that modern arrestors on the
> distribution transformer all but eliminate that.  The normal cause 
of
> lightning damage to electric equipment is MULTIPLE earth 
connections in your
> home.  Houses are supposed to have ONE and ONLY ONE earth ground 
that
> connects to the main.  How do they get multiple grounds?  I would 
say 99% of
> them were installed by the phone company.  Some idiot got this 
great idea to
> attach the phone line to a water pipe for ground.  Now, for years 
he got by
> with this because our phone connected to .... well our ear.  But 
now, it
> connects to your TV dish, our computer, etc,  When lightning 
strikes near
> your home, the voltage difference between two grounds separated by 
many feet
> can be thousands of volts.  Ever wonder how cattle near a lightning 
strike
> are killed when not struck.  Same thing.  The voltage difference 
between
> front legs and back legs was enough to electrocute them.  Its 
called step
> potential and results from the lightning current passing through 
the high
> resistance of earth ground.  Ohms law.
> 
> Sorry to ramble.  Anyone wanting MORE info on grounding, ground loop
> mitigation, wiring AC to a studio, or such can mail me privately.
> 
> Larry Hendry
> 
> ----- Original Message -----
> From: GeorgeK <george.kisslak@h...>
> To: <motm@y...>
> Sent: Tuesday, February 13, 2001 10:13 AM
> Subject: [motm] Re: Ground Loops
> 
> 
> Turns out if I plug any module directly into the amp (bypassing the
> mixer) the hum is completely eliminated.  (The mixer does not have a
> 3-prong plug - time for an upgrade.)  JLH suggested the 900 makes 
the

> mixer hum; I agree.  I'll just bypass it for now.
> 
> Thanks for the info.
> GK

That's for sure.  And, much more talk about leg liftin' and jackin' and you
will have to take this conversation to alt.sex.males.only@...
if there was such a thing.
<snicker>
Larry

----- Original Message -----
From: GeorgeK <george.kisslak@...>
just to be safe I'd have to "lift a leg" every time
I jack into the mixer!  Now, we don't want
that!!  (...??)