MOTM

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