I’ve carefully wired
up my gear using all balanced inputs and outputs, and proper balanced
cables, but I’m still getting occasional digital hash in the background.
What have I missed?
Even
with balanced cables you can sometimes experience ground loops, so
here’s the best place to break one without risking RF interference.
Jamie, via email
SOS
columnist Martin Walker replies:
Ground-loop problems can be absolutely
infuriating, and I wrote a step-by-step guide to tracking them down
back in SOS July 2005 (www.soundonsound.com/sos/jul05/articles/qa0705_1.htm).
In essence, you have to temporarily unplug all the cables between your
power amp and mixer. If the noises go away, you’ve found the location of
your problem. If not, plug them back in and try unplugging whatever
gear is plugged into the mixer — and so on down the chain.
The
majority of ground-loop problems occur with unbalanced connections, so
my next advice would have been to replace the offending unbalanced cable
with a balanced or pseudo-balanced version. However, as you’ve found,
sometimes such problems occur even in fully balanced setups where you
carefully connect balanced outputs of one device to balanced inputs of
another via ‘two-core plus screen’ balanced cables.
I
recently had just such a problem in my own studio and, to make it even
worse, it was an intermittent one, so whenever I got close to
discovering its cause, it mysteriously vanished again. Here’s what I
did to track it down, so others can try some similar detective work in
their own setups.
First of all, you’ve got to be
systematic, and note down everything you try, particularly with an
intermittent problem, so you don’t have to start from scratch every time
it occurs. In my case, I could hear the digital low-level hash through
my loudspeakers even with my power-amp level controls turned fully
down, and it also persisted when I turned off the D-A converter box
feeding my power amp. However, it completely disappeared as soon as I
disconnected both cables between the D-A output and power amp input.
These
quick tests confirmed that the noise wasn’t coming from the output of
the converter, or from the power amp itself, but instead from a ground
loop completed when the two were connected. However, just like you, I
was already using balanced cables. I double-checked the wiring of both
of my XLR balanced cables and there were no errors: the screen of the
cable was connected to pin 1 at each end, the red core connected to pin 2
at each end, and the blue (or black) core to pin 3 at each end. So far,
so good.
Next, I double-checked with a
multimeter that there was no electrical connection between the
metalwork of the two devices via my equipment rack (a common source of
ground-loop problems, and curable by bolting one of the devices to the
rack using insulated washers or ‘Humfrees’). Again, there was no
problem.
The best wiring for balanced audio
equipment is to tie the cable screen to the metal chassis (right where
it enters the chassis) at both ends of the cable, which guarantees the
best possible protection from RFI (Radio Frequency Interference).
However, this assumes that the interconnected equipment is internally
grounded properly, and this is where things can go awry. The cure is to
disconnect one end of the cable screen, and the best choice to minimise
the possibility of RFI is the input end (as shown in the diagram).
By
this time, my intermittent problem had disappeared again, so here’s
another tip. I carefully cut the screen wire of one of my two cables
just before it arrived at pin 1 of the XLR plug, but left the other
cable unmodified. Then, the next time the ground loop problem occurred a
few days later I quickly unplugged the unmodified cable, whereupon
the noise disappeared immediately. This proved that I’d correctly
tracked down the problem, and modifying the other cable in the same way
ensured that it never happened again.
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