Newsgroups: rec.audio


"Ground loops" refer to the problem that arises from the fact that "ground"
really isn't, always.  Clear enough?  No?  Well, perhaps some further
explanation is in order:

Nearly all consumer audio equipment is designed so that signals are passed from
one stage to the next in "single-ended" fashion (this includes both the
connection from one piece of gear to the next, as well as the connections
between stages within a given box); this means that the signal is passed on
only one conductor, referenced to "ground," which is assumed to be at the
same potential (the "reference") at all points.  (Note that it is incorrect
to say that ground is at "zero volts" - a single point cannot be spoken of
as possessing a voltage, as voltage (in circuit theory) ALWAYS refers to the
potential *difference* between exactly *two* points.  We get into the bad
habit of saying "this node is at X volts" precisely BECAUSE we are assuming
that the reference node is understood.)

However, it is absolutely impossible to have "all points" of a "ground"
at the same potential, given that the "ground" is carrying current (and it
must at least be carrying the signal return current) and has a non-zero
resistance (the resistance can be Damned Low (tm), but won't be zero until
somebody starts building superconducting chassis).  The "real" situation is
at a minimum this:


             ______________________________
            |                              |
            |                              |
            |                              /
            +                              \
                                           /  load
      signal source                        \
                                           /
            -                              |
            |                              |
            ----------/\/\/\/\/\-----------
                          Rg

(making suitable allowances, of course, for the lousy ASCII graphics! :-))

Rg represents the resistance in the ground.  Now, what will happen if this
ground is also used by other sources and loads?  The answer should be obvious -
these signals will cause a voltage to appear across the resistance in the
"ground," and so will result in some small signals appearing across all other
loads connected to the ground.  These signals may come from "legitimate"
sources elsewhere in the system (such as, say, the other channel of audio),
or may represent noise sources (such as the infamous 60 Hz hum).  But the 
problem is the same in any case; the "ground loop" results in unwanted
signals appearing on what is SUPPOSED to be a stable "reference point."

To avoid this problem, the preferred practice is to tie all "grounds" (meaning
the "reference" end of all sources and loads, or at least all stages) together
at a single point (as opposed to, say, just connecting the "ground" of the
stage at hand to a convenient spot on the chassis).  This way, even though 
there will be resistance in the leads running from the individual circuits to 
this common point, the signals cannot interact.  Another way to avoid "ground 
loop" problems is to use differential drive, in which each signal is carried 
on *two* wires - in other words, each signal includes its own "reference" 
connection.  However, this makes for more complicated (read: expen$ive) 
circuits, and so is not commonly used in consumer equipment.
   


Bob Myers  KC0EW   Hewlett-Packard Co.      |Opinions expressed here are not
                   User Interface Tech. Div.|those of my employer or any other
myers@fc.hp.com    Fort Collins, Colorado   |sentient life-form on this planet.
Newsgroups: rec.audio