MIDICraft's MIDINET CD-ROM/MIDICraft's MIDINET CD-ROM.iso/DOCS/HUMFIX.ZIP/GRNDLOOP.TXT

Text File | 1996-06-02 | 5.5 KB | 93 lines

Ground Loops - and how to deal with them
By Daniel Collins


The most common cause hum in any  audio  system  is a phenomenon known as a
ground loop.  A ground  loop  occurs  when  the  power  supplies of several
components are connected  to  grounds  of  different  potential.  Since all
solid state circuits operate on direct  current   and the power in our wall
outlets is alternating current,  the  power  supply  of  any such component
performs a function known as  rectifcation.  Rectifiers convert alternating
current into direct current in  different  ways (see illustration 1).  What
all rectifiers have in  common,  is  that  they  do  not produce a constant
direct current, but rather one  that fluctuates slightly.  This fluctuation
is in time with the frequency  of  the  alternating current source (60Hz in
North America, 50Hz in South America  and  Europe). In either case, this is
right at the low end of the  audible  sound spectrum.  It is the passing of
these "vibrating" voltages through various  audio amplification circuits on
their way to ground that causes the characteristic ground loop hum.

Illustration 1 - Rectifier output

Electricity, like water, follows the path of least resistance (ie. it flows
down hill).  This has the effect  of  having  all current in a circuit flow
towards earth ground (from this  point  on  I  will  use  the term earth to
indicate earth ground - the  green  wire  in  your  oulets  - as opposed to
circuit ground - the white wire  in  the outlet).  If everthing is designed
and works correctly, then all the components in your audio system will have
properly earthed power supplies  and  you  will  never  hear a ground loop.
However, in the real world, amplifiers tend  to be the only audio component
that are usually equipped with earthed power cords.  This situation creates
the ideal environment for ground  loops  to  occur.    Why, you may ask, if
this situation is so common, have  I never encountered this problem before?
In reality, you probably  have  and  do  not  know  it.  Many sound systems
operate with a ground loop, but the  level  of  the  hum is so low as to be
inaudible.  Only when the amplification factor  is  very large - such as in
the phongraph inputs - is the ratio  of  the  hum signal level to the audio
signal level high enough to be detected  (which is why turntable often have
"ground" screws on their backs.

This situation is altered  when  a  computer's  sound generation adapter is
attached to the sound system.  The voltages used in most audio circuits are
typically around 5 volts.  In a PC, there are also 12 volt components.  The
result is that the  hum  level  is  over  twice  as  loud  as between other
components of the stereo.   Added  to  this  problem  is  that  PCs are not
designed to be quality  audio  components.   Their  power  supplies and the
various circuits within the PC (which  includes  your sound card) all use a
common ground point, the chassis which is connected to earth.  This results
in all the power generated and  consumed  inside  your  PC ending up on the
common, or ground,  side  of  your  audio  outputs.   When  this  signal is
amplified, the result and be a very pronounced hum.

So, how do we deal with this  problem without rewiring our PC power supply?
Several different approaches are possible, depending on the severity of the
problem.

First and foremost, make sure that  both  the  PC  and the sound system are
connected to properly wired outlets,  preferably  on  the same circuit.  Do
not assume that your outlets are  correctly wired.  Many homes have various
wiring  errors,  most  commonly  the  neutral  and  earth  connections  are
reversed.  A small plug in  device,  available  at  most hardware stores or
electronics stores (like Radio Shack) will  visual confirm that the circuit
wiring is correct.  Many ground loops  will  be eliminated this way.  Since
all components share the same ground, all signals will flow to earth within
a single component.

Illustration 2  - If the earth on  the  computer  is not as thorough as the
amplifier's then a 50/60 Hz signal will  pass through the audio outs to the
amplifier.

Some cases of ground loops will not be  cured by the above fix, because the
internal connection to earth is  not  complete  in  one or more components.
So, although they all share the  same earth connection, the "downhill" path
still runs through  an  audio  connection.   In  these  cases,  the easiest
solution is to invest in a pair of "isolation transformers".  These devices
take advantage of the  fact  that  audio  signals  are alternating current.
When AC flows through a coil  of  wire,  it  produces a magnetic field that
fluctuates in synchronization with the AC signal.  In addition, a coil that
is placed in a fluctuating magnetic field produces an AC current that is in
synch with the field's flux.  So,  it  is  possible to pass an audio signal
from one device to another with no physical electrical connection.  With no
connection, no ground loop.

Illustration 3 - An isolation transformer

Finally, you may not have a very efficient earth connection.  Although this
not usually the cause of a ground loop it can be the cause of hum resulting
in the AC neutral (where a VERY  strong  60Hz signal resides) being used as
the circuit ground of your audio system.  This signal can feedback into the
audio system and cause various buzzes  and  hums.   While a volt meter will
indicate any voltage flow from earth to  neutral,  this is not a test to be
performed by individuals that are  not  experienced in dealing with 110/220
volt electrical circuits.