General Information About Noises
Hum, buzz, and other noises in audio systems are nearly always caused by so-called "ground loops." A ground loop creates
a voltage difference between the ground references (chassis) at each piece of equipment. This voltage can couple into the
signal path, especially in unbalanced signal interconnections. Since this voltage is ultimately coupled from the power line,
its usually heard as 50 or 60 Hz hum if the equipment has grounded (3-prong) AC plugs. For equipment with
ungrounded (2-prong) AC plugs, its usually heard as a harmonic-rich buzz because the coupling mechanism favors high
frequencies.
ground isolators can eliminate
hum
and
buzz
as well as the intermittent
pops
and
ticks
frequently caused by on/off operation of switches and relays or appliances which contain them.
isolators cannot eliminate
hiss,
which sounds like a waterfall or an FM radio tuned between stations. A
predictable amount of hiss (random or white noise) is
unavoidable
in any electronic system. Excess hiss sometimes is
the result of improper system gain settings. For example, if the output level control on a CD player were set too low, the
downstream preamp would have to operate at very high gain to compensate. Such high gain operation will generally
produce more noise at the system output.
Buzz which occurs
only
when an AM receiver is the signal source, and is worse when the receiver is tuned to weaker
stations, is probably caused by radio frequency interference or RFI. Power line operated devices such as fluorescent lights,
electric motors, computers, and TV receivers produce high frequency interference can be conducted through or radiated
by the power line and picked up by the antenna of AM receivers. These problems can often be eliminated by installing
power line interference filters as close as possible to the offending source. A portable battery-operated AM radio tuned to
a very weak station can be useful in locating the source.
Location, Location, Location
ground isolators use
transformers
to magnetically couple the signal while breaking the electrical paths between
input and output connections. Once these connections are broken, the ground loop current, and the resulting noise, is
eliminated. Ground isolators are
not
filters which attempt to remove noise from the signal after the fact they eliminate
the cause of the noise coupling.
For this reason, an
ground isolator MUST be inserted into the signal
path where the noise coupling
actually occurs
.
As a general rule, noise coupling most frequently occurs at interconnections using the longest cables. Problems are most
severe, even with short cables, when the cable connects two pieces of equipment which both have grounded (3-prong)
power connections or there is another ground connection (CATV, DSS, grounded rack, etc.) somewhere in the system.
NEVER defeat the safety grounding provided by a 3-prong power plug, whether with a 3 to 2 ground
adapter or by removing the plugs grounding pin.
This practice is not only illegal, it can result in shock trauma or
electrocution of persons touching equipment anywhere in an interconnected system.
Detailed technical papers on the
subject of system grounding and its effects on signal interfaces are available from Jensen Transformers
.
The following method (as opposed to trial and error) will save time, labor, and frustration when selecting
an effective location for the
isolator.
Existing system switches and gain controls can provide very
important clues about just where noise is entering the signal path.
If interrupting the signal path stops the noise,
the noise must be entering the signal path at a point upstream of the interruption.
For example, if the noise is present only when a specific signal source is selected, the noise entry point is likely upstream,
i.e. somewhere between that source and the device which contains the selector. Likewise, if the noise is present with any
input selected, the noise entry point is likely downstream, i.e. somewhere between the device which contains the selector
and the speaker (or other monitoring point). Generally, working backward (from the speaker toward the signal source)
through the system is the fastest way to determine which parts of the system are noise-free.
Obviously, the signal path can also be interrupted by selectively disconnecting signal cables between pieces of equipment.
Important note:
if a "ground loop" does exist between two pieces of equipment, it will not be broken until all cables
which directly connect them are disconnected
.
If the above tests are inconclusive, there may be a defect with one or more pieces of equipment. An insidious design defect
called the pin 1 problem is very common. Since it effectively turns the equipments input and/or output shield
connections into audio inputs, it can be revealed through a simple test: with the signal path interrupted (see note in
previous paragraph), temporarily connect only the shield contact (pin 1 for XLRs) of the cable to the shield contact of the
suspected input or output using a jumper wire. If this connection causes hum or buzz, you have a pin 1 problem. See
the Setting the Grounding Option Switches for proper settings to overcome this problem.