On The Bench Issue 64
TUTORIAL
ON THE BENCH
Where the hell is that buzz coming from?
If your want your studio
wiring to be hum- and buzzfree, you’ve got to plan ahead.
Text: Rob Squire
Studio wiring is one of those tasks that should
be so straightforward and predictable that we
could happily and confidently leave it till the last
phase of a studio’s construction. Unfortunately, this
is not the case. Wiring a studio takes planning and
skill, yet it still remains one of the most neglected
and misunderstood aspects of studio construction.
Many new studio owners not only leave the wiring
till last, they also fail to consider the costs involved,
or appreciate the research, time and effort that
cabling requires.
Right now I’m mapping out a wiring scheme for a
client who has bought and refurbished a ‘vintage’
console, as well as added a multitrack tape machine
and ProTools rig. The focus to date has been on all
the big ticket items but now that all the fancy gear
has been levered into the control room, the client
seems somehow surprised that a random bunch of
dirty old looms that came with the console won’t
magically plug this new setup together.
ESTABLISHING THE GROUND RULES
Obviously, some setups are much more complex
than others, but are there some common rules that
will yield a reliable, clean and noise-free result for
any setup, large or small? The answer is yes… and
no.
I often see studio wiring as a combination of science
and art. The technician in me cries out that it should
be the preserve of science alone, however, hard
won experience has shown me that sometimes the
perfect solution for a clean wiring installation breaks
all the rules and you have to yield to what works.
The key to a successful wiring installation is to
start off on the right foot and from there on in, be
consistent. It’s more a marathon than a sprint and
time spent planning and getting the first steps in
place will pay dividends down the track. Thus, the
first step is actually nothing to do with the audio
wiring, it’s all about your mains wiring. So leave
the soldering iron turned off for the moment and
instead go and find yourself a good electrician!
BUZZES & HUMS
I had a call this week from a studio owner who runs
a well established facility, asking about getting the
studio re-wired. “Why the sudden desire to rip it
all out and start again?” I immediately asked. “The
wiring has been in place for years.”
“Well, we’re getting all these intermittent hums and
buzzes,” he said, “and it’s driving us and our clients
nuts. Nothing seems to make sense.”
Following some very specific questions, a picture
began to emerge. When asked: “So, when you plug
a guitar amp into the wall in booth A, and you don’t
have anything else connected – no DI linking, not
Fig. 1
AUDIO CABLE
GEAR B
GEAR A
EARTH LOOP
MAINS EARTH
DIST
BOARD
GROUND
STAKE
AT 64
“
What is important here is to be
absolutely consistent in the
choice of which end is
connected and which end is
disconnected, or left ‘floating’.
even a guitar plugged in – does it buzz more than
when it’s plugged into a socket in the kitchen?” The
answer was, “Yes!”
My advice to them at this point was that they
needed an electrician, not a technician. The studio
in question is in an old inner city building, and
the history of the original construction and studio
wiring is lost in myth and legend. Older inner city
buildings are likely candidates for low mains voltage,
as are residential buildings at the end of a supply
distribution run. If you notice incandescent lamps
dimming at various times of the day and find a
correlation with an increase in hums and buzzes,
then it’s time to either have an electrician (or your
power authority) come and monitor your mains
power voltage over, say, a 24-hour period.
GOOD POWER
In a studio you need good power. Good power is
about the consistency of the mains voltage supply,
but unfortunately this can fluctuate wildly in the
real world. Establishing that your studio is receiving
230Vac – or something pretty close – and that the
voltage doesn’t wobble all over the place as the
demand for power in your area goes up and down, is
an excellent starting point.
The voltage range that authorities can legally supply
may not be good enough for your studio, especially
if you have tube equipment or large, hungry console
power supplies. Inconsistent voltage causes all
manner of dramas. I’ve struck tape machines that
would throw up error messages and go nuts trying
to transport tape smoothly because the mains
voltage into the building was dropping to 210Vac,
which is at the lower end of what is legally allowed
to be supplied.
GROUND RULES
The earthing system of mains power outlets is also
a very important aspect of a studio’s wiring. This
is one of those jobs that is much easier to do when
the electrician first installs the mains wiring and
outlets or GPOs (General Purpose Outlets), than
it is to come back and correct down the track. The
electrical requirements of a studio are actually very
simple, however, they’re unusual compared to runof-the-mill domestic or industrial installations, so an
electrician who can work with you on this one and
understands your special requirements is essential.
So what are these special requirements? Well,
normal mains wiring simply loops the active,
neutral and earth wires from one GPO to another.
A single cable containing these three wires is used
and GPOs are typically strung in a daisy chain, one
after another, until the maximum for that feed from
a single circuit breaker is reached. Unfortunately,
for us audio types, this standard daisy-chaining
approach has the potential to create an earth
loop, whereby connecting various pieces of audio
equipment together, fed from different mains GPOs
and also interconnected to each other with audio
cables, enables small leakage currents in the mains
earth cables to flow through the audio earths. This
superimposes hum and buzz on your audio signal –
bad news indeed.
The solution is not to connect the earth wires from
one GPO to another, but rather, for each individual
GPO to have its own earth wire that travels directly
back to the mains distribution board. Once there,
it joins with all the other earth wires coming from
each GPO, and it’s at this point that the connection
to physical earth, i.e. the ground beneath our feet,
is made. This is a good example of what is referred
to as ‘star earthing’, where each individual point
at which you can access a mains earth – namely a
power point – is at the end of its own ‘arm’. That arm
only connects then to all the other ‘arms’ at a single
point.
Now before you rush out and wildly start ripping
out GPOs with the view to star earthing them
all, you need to understand that this is a job for a
”
licensed electrician only. That earth exists to provide
electrical safety, not to keep your studio hum free,
and the importance of a correctly connected and
low-resistance mains earth is paramount.
WHAT ABOUT THE OTHER EARTH?
In a studio environment there is, of course, that
other earth that we’re all more familiar with, that
is always a topic of discussion. This is the signal
earth, the one connected to Pin 1 on an XLR, or the
sleeve of a TRS plug. This signal earth is also the
connection made to the shielding of a cable that
carries audio signals. On this type of audio cable,
the shield protects the internal signal wires from
electrical interference, including the radiated 50Hz
field from mains cabling and radio frequency (RF)
signals of TV, radio and mobile phones etc.
All off-the-shelf cables connect this earth at both
ends of the cable, i.e., an XLR male-to-female mic
cable has Pin 1 of both XLRs connected to the
shield of the cable. In fixed studio wiring, however,
this isn’t always the most desirable scheme! From
examining the earth loop created in Fig. 1 (left) by
the combination of signal earths and mains earths,
we can see that one way to break this loop is to
disconnect the signal earth wire at one end. It might
come as a surprise to some, but this is common
practice in many studios with installed audio
wiring. Disconnecting the signal earth (which is
also the cable shield) at one end (as shown in Fig.2
- previous page), eradicates the link responsible for
these dreaded hums and buzzes.
What is most important here is to be absolutely
consistent in the choice of which end is connected
and which end is disconnected, or left ‘floating’. The
only cable that must have the earth connected at
both ends is a mic cable, or any loom that may be
used to connect microphones. The reason for this
is that microphones are generally ‘floating’ devices
already, and while the body of the microphone will
Fig. 2
MALE XLR
FEMALE XLR
EARTH LIFTED AUDIO CABLE
EARTH
EARTH
-
-
+
+
ALL CONNECTED AT MALE END
DISCONNECT EARTH AT FEMALE END
AT 65
!
LIFTING THE RIGHT EARTH
This article talks quite a bit about lifting or floating grounds
to avoid creating earth loops. It is worthwhile to ensure that
we are all very clear on what this doesn’t mean so as to avoid
creating a potentially dangerous situation.
I’ve lost count of the number of times I’ve seen the mains or
electrical ground lifted on a piece of audio equipment and I’m
sure most folks reading this article have seen it also. It takes
many forms; the earth pin on a mains plug cut off or even just
folded back, the green or green and yellow wire removed from
inside a mains plug and that old favorite the double adapter
that has been disassembled, the whole earth contacts
removed and then put back together again. These are all
examples of ways of lifting the mains earth and in the right
circumstance can solve an earth loop hum problem. However,
this is NOT the right way to do it. It is both dangerous and
illegal. Recalling the article last issue on test and tagging, any
item dealt with in this way would fail the electrical safety test.
The only earth wire that should ever be lifted is the signal
earth or the shielding wire within an audio cable. The mains
or electrical earth must always remain intact to provide the
required level of electrical safety.
connect to this signal earth, it normally doesn’t
connect to any other earth. This prevents it from
becoming a part of any loop and it will always
be at the end of a star’s arm – i.e., only have one
ground connection. While dynamic microphones
will still work with the earth lifted from Pin 1,
condenser microphones won’t work at all if the
shield is disconnected, and as much as anything
this is a good reason for always maintaining a
microphone’s connection to Pin 1. Condenser
microphones use this earth connection as a part of
the powering circuit for delivering phantom power
and disconnecting the earth simply means the mic
will not receive any, and therefore not function.
This core requirement of microphones suggests
the rule for determining which end of an audio
cable should consistently be floated and which end
connects to earth, Pin 1 or the sleeve. If we leave all
the earths connected on any cable that connects to a
device’s input and float the earths on all cable plugs
that connect to a device’s output, then we ensure
that the earthing will always be correct, including
for microphones. To reiterate, the male XLR we’re
wiring up to the input of a mic preamp has the earth
connected, while the female XLR we’re wiring up to
the preamp’s output has the earth floating (or lifted).
Stage boxes and individual mic cables should always
have the earth connected at both ends.
From looking again at Fig. 1 you can see that,
regardless of the configuration of the mains earth
wiring, lifting the signal earth or shield wire at one
connector results in the earth loop being broken.
The result? A hum-free connection. What you must
ensure is that only one end is floated. If both ends
are lifted then the protective shielding of the cable
is lost.
When combined with fully balanced audio signals,
the combination of star earthing of mains GPOs and
floating one end of each audio cable interconnect
will get you as close to a guaranteed hum- and buzzfree studio as possible.
AT 66
THEN THERE’S THE REAL WORLD
However, all these good plans often come unstuck
in the real world, especially once unbalanced
equipment is introduced and other quirks are found
in equipment that just won’t play ball.
The general approach with unbalanced equipment
is to treat it, as far as possible, as you would a piece
of balanced equipment. To start with, always use
balanced cable for interconnects, with the shield
wire still connected at one end only. However, with
an unbalanced connector, what would normally be
the cold (or negative) wire will now need to connect
to the sleeve (or ground) contact of the tip and
sleeve or RCA plug.
In an ideal world, all manufacturers would stick
to a universal standard when it came to connector
wiring and the presentation of balanced and
unbalanced signals. However, while the situation is
certainly improving, you still come across occasional
curve balls to test your patience from time to
time. One of these – that is thankfully not all that
common – is the presentation of unbalanced signals
on XLRs wired with Pin 3 hot, rather than Pin 2. If
presented to a fully balanced input, the worst that
will happen here is that the phase of your signal
will be inverted, but if this sort of output hits an
unbalanced input, it will require a custom wiring job
for any signal to emerge unscathed.
Another more common situation with unbalanced
units is the use of 1/4-inch tip & sleeve sockets,
where the ring position in the socket goes nowhere.
A typical TRS (tip, ring & sleeve) plug is wired tip
(hot), ring (cold) and sleeve (ground). With the
ring connection missing in a mating socket, this
results in the cold connection flapping in the breeze
and the result can be unpredictable. You might lose
some level (say 6dB) or, if the unit at the other end
of this connection is transformer based, you’ll lose
lots and lots of signal. In fact, a very low-level and
thin-sounding signal has all the hallmarks of faulty
interconnection of transformer-based equipment
where one connection – either the hot, or more
usually, the cold – has not been made.
Many older outboard effects units often had simple
tip & sleeve sockets with unbalanced signal inputs
and outputs, and from looking at the socket from
the outside you won’t know if the ring connection
is absent. These are the ones that will require some
research or experimentation to discover if you need
to wire a TRS plug as a simple tip and sleeve, with
the cold (or negative) wire connected to the sleeve.
And the art in all of this? Neatness. Keep your
wiring neat and tidy. It helps when you come
back a couple of years later to add a new piece of
equipment or even when showing a potential client
around the studio: they’ll think; “Well at least the
wiring looks good!”
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