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