Avoiding Ground Loops - Alesis ADAT-LX20 Reference Manual

A
VOIDING
In today's studio, there are many opportunities for ground loop problems to occur.
These show up as hums, buzzes, or sometimes radio reception, and can occur if a
piece of equipment "sees" two or more different paths to ground, as shown below.
One path goes from device A to ground via the ground terminal of the three-
conductor AC power cord, but A also sees a path to ground through the shielded
cable and AC ground of device B. Because ground wires have a small amount of
resistance, small amounts of current can flow through ground and generate a voltage
along the cable shield. This signal may end up getting induced into the hot
conductor.
The loop can also act like an antenna into which hum is induced, or can even pick up
radio frequencies. Furthermore, many components in a circuit connect to ground. If
that ground is "dirty" and contains noise, it might get picked up by the circuit.
Ground loops cause the most problems with high-gain circuits, since massive
amplification of even a tiny bit of noise can give an audible signal.
Most ground loop problems can be solved by plugging all equipment into the same
grounded AC source. However, it is important to make sure that the AC source is not
overloaded and is properly rated to handle the gear plugged into it.
For really tough cases, you may need to break the connection that causes the loop
condition. If your circuits are balanced, one way to do this is to simply break the
shield of the shielded audio cable at some point, usually by disconnecting it from
ground at one jack. (The other end should remain connected so that the shielding
properties are retained, even if there is no direct path for ground.)
Please note that not all hums and buzzes are caused by ground loops; your cables
must be of high quality, particularly with -10 dBV setups.
ADAT LX20 Reference Manual
Chapter 1 - Setting Up and Making Connections
G L
ROUND OOPS
shielded
cable
Device A
path 1
path 2
Device B
To AC power service
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