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