HUM & PIN #1
By Marty McCann
Copyright 1995
Sooner or later every sound person encounters a ‘ground loop
hum' in their sound system. Most people think that adding a
Peavey PL–2 transformer module to the power amplifier rack
will eliminate the problem, but this is not always the case.
In professional audio, the sound audio signal path should only
be grounded at a single point. This single point grounding
is circumvented when we place the mixing console remotely or
away from the power amplifiers. Our USA National Electrical
Code (NEC) specifies that all electrical equipment that draws
potentially harmful amounts of current from the AC Mains must
have its metal chassis grounded to the electrical systems ‘earth'
ground.
When the mixing console is in a remote position from the power
amplifiers, we now have the sound system's audio signal path
connected to ground at two points. If there are separate racks
of power amplifiers on both sides of the stage, we may have
multiple connections to ground creating paths that also connect
the audio signal to the electrical ‘earth' ground.
The resultant hum is directly related to the differences in
current that flow in the respective electrical circuits, which
all share the same earth ground. If the audio signal path is
only connected to ground at a single point, the differences
in ground currents will not induce any interference from the
power line, so the sound system will not hum.
Many uniformed sound persons remove the grounding pin on the
electrical cord of one or more pieces of equipment deemed to
be the cause of the hum or ground loop. This is in violation
of the NEC and should not be done under any circumstances.
Using the little gray three pin to two pin electrical ground
lifts and not connecting the ground wire to the electrical
ground is ‘not' the correct solution either, and is likewise
contrary to the NEC.
The problem lies in the fact that the balanced audio connections
between the mixer position and the power amplifier positions
have a common audio signal ground that finds its way to the
electrical ground via of the multiple chassis ground connections.
The solution is to lift the audio signal or pin #1 of the balanced
connector at the source of the balanced audio signal line.
In this case, pin #1 should be lifted at the mixer position.
Whichever is the last audio system component that acts as the
source (send) of the balanced line (mixer, EQ, crossover, compressor,
etc.), this source component's send, should have pin #1 of
the connecting balanced cable lifted or ‘floated' from its
chassis.
The shield of the balanced line will still terminate or go
to ground at the receive end or the amplifier rack. This then
eliminates the hum that would be induced if both ends of the
balanced line went to ground. You have eliminated the cause
of the hum by using this correct approach to signal distribution
without violating the NEC. Lifting the electrical ground pin
is always in violation of the NEC.
I recommend that each sound person have a number of short (4
to 6 inches) balanced microphone cords made up with pin #1
lifted or floated in the female side of the connector. This
will allow them to ‘float' the balanced lines on any system
that they encounter without their having to go into the snake
box or the equipment itself to ‘lift' pin #1 from the chassis
ground.
Note: Be careful when cutting the ground wire away in a regular
microphone cable. You want to trim any excess wire back to
the insulation so it won't short to any of the pins in the
encounter.
Also, if you just have to cut a microphone cable, mark it as
ground lifted immediately or you'll shoot yourself in the foot
at some future gig, especially if you try to use that mic cable
with a phantom powered microphone.
In some applications, particularly those sometimes encountered
with electronically balanced inputs at the end of a balanced
signal line; lifting pin #1 from ground will eliminate the
ground loop hum but you may then experience ‘RFI' (radio frequency
interference) in the system.
The solution is this case is to add a small capacitor (such
as a .001 mFd) from the shield or ground wire to pin #1. Adding
the capacitor allows the balanced line to be lifted (open)
from ground at audio frequencies, while acting as a closed
circuit for radio frequencies.
In order to explain in any further detail would require a thorough
discussion of electrical power distribution. The foundation
of the above tip is: Not allowing the audio signal path to
go ground in more than one place.
If you have more than one signal path, then each path should
only have its signal path's shield connected to ground (pin
# 1) at one end, usually at the termination or receive end.
The real cause of the ground loops has to do with the way that
our electrical power is distributed. In 1996, a new article
was written into the NEC code that covers balanced power distribution.
If you want more information on this, see my article on balanced
power distribution.
Good Luck! |
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