AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
A question of balance – Part 1
L
ast month, we finished a
marathon, four-part project
to produce a high-quality
microphone preamplifier. I’ve used a
lot of different mic preamps, and for ‘a
couple of tenners’ this one is as good
as many commercial models costing
10 or 20 times as much. An important
side effect of this is that it really matters
how you connect your microphone to
the preamplifier – a few feet of bell wire
is definitely not the way to go, and so
in this and next month’s Audio Out
I am going to walk you through how
to build a balanced lead. Even if you
don’t need a microphone cable, the
principles discussed here are applicable
to a wide range of audio, and other highsensitivity analogue circuits.
This month, I’ll provide an overview of
the technology, especially the ubiquitous
XLR connector and I’ll also look at the
main cable options.
Why balanced?
It’s well known that balanced leads reduce noise pick-up in audio systems, but
what’s the secret behind their success?
The key point is that balanced inputs only
respond to differential signals; that is the
voltage difference between the two signal
leads. If the noise or unwanted voltage is
the same on both conductors – such as
with induced interference – the voltage
difference is zero and the interference is
rejected by the input difference amplifier.
Thus, the key advantage is the reduction
in magnetic interference, both received
and transmitted. The basic balanced line
set up is shown in Fig.1.
Most balanced audio interconnections
use 3-pin XLR connectors, which originally stood for ‘extra low resistance’.
They are always wired as male for outputs
and female for inputs, as shown in Fig.2.
(Tip-ring-sleeve (TRS) jack connectors are
also used, which are gauge A or B ‘stereo’
quarter-inch jacks. Occasionally on old
US equipment, screw terminal strips are
encountered. However, if you are building
from scratch then XLR is without a doubt
the preferred superior route.)
Fig.2. Panel/chassis-mount XLR connectors:
the outputs are always male (with pins) and
the inputs female (with holes).
Earth loops and safety
Unlike unbalanced connections, such as
phono, mono jack and BNC leads, the earth
Phantom power
Sometimes power is applied to a balanced
line, as shown in Fig.1. This is used to power microphones and other devices, such as
direct injection (DI) boxes. It is normally
supplied at +48V and rides on top of the
audio. Because it is equal on both signal
conductors, it is ignored by the system.
The phantom power return current goes
back through the screen. It is not ideal, but
it is a well-established standard.
Fig.3. On rare occasions, yellow phasereverse leads are encountered where hot
and cold conductors are flipped. In this
case it is a patch cord lead.
T r a n sm i t t i n g e q u i p m e n t
e g , m i cr o p h o n e o r p r e a m p l i f i e r
R e ce
i vi n g e q u i p m e n t
O u tp u t tra n s fo rm e r o r
p u sh - p u l l o u t p u t a m p l i f i e r
V +
M i cr o p h o n e
O p tio n a l
p h a n to m
p o w e r
D r i ve
a m p lifie r
M a l e ch a si
o u t p u t so cke
s
P in 3
P in 2
P in 1
C o ld –
H o t +
G ro u n d
2 x
D iffe r e n tia l a m p lifie r
o r i n p u t t r a n sf o r m e r
t
S cr e e n e d t w i st e d - p a i r ca
b le
M a le lin e p lu g
1
F e m a l e ch
i n p u t so cke
a si
t
C a se
C a se
O u tp u t
–
1
S cr e e n
F e m a le lin e p lu g
b le
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F ilte r
3
3
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lin k
+
2
2
S ig n a l
g ro u n d
O p tio n a l p h a n to m
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4 8 V
P S U
0 V
s
/m e ta lw o r k
P o si
lin k
b le
S ig n a l
g ro u n d
/m e ta lw o r k
Fig.1. The balanced line system – now standardised except for the ground-lift options, which are inserted in the area marked link.
Note also the optional phantom power connections.
40
Practical Electronics | September | 2021
n
Fig.4. An XLR connector earth pin (top right)
always connects first because it is a bit
longer than the others – what a good idea!
plays no part in signal transmission in balanced connections. In balanced leads, the
earth is just for screening; ie, preventing
radio frequency (RF) noise pick-up. It has
the added advantage that the loud earthloop hums that can occur in unbalanced
systems when more than a few units are
connected are avoided. Balanced leads
also avoid the worst-case audio scenario where incompetent people try to help
by disconnecting mains earth leads from
mains plugs. This can lead to metalwork
and microphone cases becoming live and
sadly quite a few musicians have died
through such stupidity.
‘The pin-one problem’
It is still possible to get mild earth loops
on balanced systems. The balanced system only works perfectly if earths on
the balanced connectors are connected
to the metalwork or mains safety earth
in accordance with the Audio Engineering Society’s standard AES48. If they are
connected to signal earth, such as 0V on
PCBs, the system can still suffer from mild
earth loops, since earth currents flowing
can then cause voltage drops on the signal ground conductors, thereby impinging
on the signal. Audio engineers describe
this as ‘the pin one problem’ This is because all balanced XLR connectors have
always used pin 1 for ground. I spend a
lot of my professional life rewiring gear
to conform to the new standard. I’ve often
had to carve away ground planes on PCBs
to disconnect the signal ground from the
metalwork. Note that the RF filter ground
should also go to the metalwork ground –
not the signal ground. The only exception
for connecting pin one to signal ground is
if phantom power is used. Even this isn’t
essential if the phantom power supply 0V
goes to pin 1.
Ground lift
On Fig.1 there is a link indicated between
the metalwork (mains safety) ground and
the signal ground. This can consist of several options. Normally, there is simply no
Practical Electronics | September | 2021
link, in accordance with AES48. On some
equipment, such as DI boxes, there is a
ground-lift switch. Ground lift is employed
to interrupt the currents that circulate in
the ground wiring of any system. These
currents are the normal consequence of
induction from nearby magnetic field
sources, such as transformers.
Although ground-lift switches are usually
left open, the reason why they are provided is that in any complete audio system,
such as a studio, the signal ground and
mains ground should be joined at only one
place. This is normally the most sensitive
input point, such as the mixing desk in a
studio or the phono input socket on a HiFi system. Having a system with lots of
ground-lift switches is a recipe for hum
loops. I try not to use them because people
randomly flip them. Sometimes, the link
is simply a 10nF RF bypass capacitor or
a 0.5W 1k ground-lift resistor. The best
system is to use both the resistor and capacitor in parallel and do it on the input
socket. Possibly a better system (suggested
by a reader) is to use two high-current (at
least 2A) rectifier diodes in parallel backto-back which provides ±0.7V ground lift
and a high-current path for faults. Resistors can burn-up in these circumstances
whereas the diodes survive. Ground-lift
techniques have not yet been standardised.
One way to avoid these earthing ambiguities is to go completely digital with
optical fibre interconnects. However, this
introduces a whole new raft of digital set-up
problems, such as word clock synchronisation. We won’t be discussing that here.
Hi-Fi vs professional
Sadly, balanced leads are not universally
used in audio, since they are relatively expensive. They are almost essential when it
comes to microphone leads which transmit low-level signals in the order of a few
mV and can often be up to 10m long for
stage work. For short low-level leads, say
around a metre for vinyl record players,
unbalanced leads tend to be the norm. In
fact, balanced leads tend only to be used
by the professional audio community. They
are considered unnecessary and undesirable in the minimalist Hi-Fi world. This
is because the additional electronic noise
from op-amps and resistors on simple differential inputs is significant, while the
transformers and extra output amplifiers
required also add distortion. The RF interference performance of short balanced
Hi-Fi leads is not significantly better than
unbalanced ones unless transformers are
used and twice as many filtering components are needed.
Blowing hot and cold
Since balanced interconnects are differential, the two signal-carrying conductors
are designated ‘plus’ and ‘minus’ to signify
Fig.5. The original and still the best –
Cannon’s XLR connector. Note rubber
insert on the female connector.
their phase. In audio technician/roadie
parlance, they are designated ‘hot’ (positive going) and ‘cold’ (negative going)
respectively. XLR pin 2 is hot and pin 3
(the middle one) is cold. Originally, the
Americans had it the other way round.
Surprisingly, the European convention
was formally adopted by the US-based
AES. However, audio equipment has a very
long life and there is plenty of gear wired
with the old system. This often gives rise
to unwanted phase inversions. Indeed, it
is still so common that many engineers
carry with them a phase reversing lead
whereby pins 2 and 3 are flipped on one
connector. Phase is one of the few areas
where two wrongs make a right. These
leads are often colour coded yellow, especially in patch cords for patch bays in
studios. (Note that with jack connectors
(see Fig.3) tip is always plus.)
Commercial XLR connectors
There is a high level of standardisation
with XLR connectors. Outputs are always
male (with pins), and inputs are always
female (with sockets). This means leads
can be plugged into one another for extension purposes. One clever feature is
that the earth pin always connects first
due to the pin 1 socket being slightly
longer than the other two, as shown in
Fig.4. This prevents those horrid bangs
and buzzes one gets with jack and phono
leads when they are plugged in and out.
There are six different types of three-pin
audio XLR hardware: cable plug (male)
and cable socket (female); male and female
chassis-mount equivalents; and male and
female PCB-mount versions.
The original XLR connector was designed by Cannon, later becoming ITT in
the US. These are still the most reliable
XLRs and are recognisable from their use
of resilient rubber for the female connector – see Fig.5. They are no longer made,
so I hold on to any that I have. Many are
over 50 years old and still work perfectly.
Their only problem is that they have three
non-standard (US thread) screws which
can get lost. Fortunately, I have bags of
old spare bits and pieces from my time
in the very early 1980s when I worked at
Future Film Developments, purveyor of
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Fig.6. The Switchcraft XLR connector for
those who like a solid 1970s American
aesthetic. This is a right-angle version
that does not stick out so much.
A unique aspect of the Neutrik design is
that the case is slid on after it has been
soldered. These are shown in Fig.7. The
chassis connectors were also changed
(Fig.8) so that male and female types had
the same mounting hole. The mounting
holes were also tapped M3. This format
was unique to Neutrik for a while, then
others followed suit.
XLRs became cheap when the Chinese
entered the market, and XLR connectors
became a commodity component. Unfortunately, tolerances deteriorated, resulting in
some tight-fitting connectors that were difficult to unplug. Some of the plastics used
were brittle and cable clamps snapped.
To be fair, some commodity XLRs were
excellent, but they have to be discovered
and stock bought in before the company
accountants change things for the worse.
Combined XLR / jack sockets
Fig.7. With Neutrik XLR plugs the case/
shell is put on after soldering. Only the
cable clamp bush has to go on the lead
first. The clamp assembly clips over the
cable via a slot. There are no screws, so
it’s quick to assemble.
parts to the BBC. This was my first proper
student sales technician job and where I
learnt to be a wireman.
Next on the scene were the Switchcraft
Q-G XLRs (Fig.6) which are the best looking, with their bright plated cases and green
inserts (the bit of plastic that holds the
pins). They have a reverse threaded screw
to hold the insert in, it is under the cover
and so can’t fall out. It has to be turned
clockwise to tighten it and it moves up to
press on the inside of the case. Q-G stands
for quick ground and they are still available from Action Hardware.
XLR connectors were horribly expensive
until the Swiss company Neutrik entered
the market. These had inferior plastic cable clamps, but only one insert retaining
screw. Later models had no screws at all.
Fig.8. Nowadays, both male and female
chassis connectors have the same sized
mounting format. It’s possible to buy
ready-punched panels and all sorts of
other connectors (eg, USB) to fit the
same-sized holes.
42
Fig.9. Combined TRS jack and XLR
socket – these are useful for musicians
who forget to bring the correct leads.
Neutrik invented a combined XLR and
TRS Jack socket shown in Fig.9. Musicians
love these because they allow a standard
mono jack lead from guitars or semi-pro
equipment to be plugged in keeping the
show on the road. When using these, pin
1 must go to signal ground.
Cables
The main requirements for a balanced cable are screening and to have two inner
cores. These cores need to be twisted together to minimise magnetic field pick up
and radiation. There are several different
types of screen available:
Foil – 100% screen coverage, but it
breaks when moved frequently and is
only suitable for fixed installations in
equipment, studios and patch bays.
The foil is connected by a drain wire
and is thus very easy to solder up. A
Belden foil cable is shown in Fig.10.
Lapped – 77% coverage and screen
can open up in places over time. This
is shown in Fig.11. It’s easy to twist
and solder though. There is also a
double-layer version called Reussen
screening, with the laps wound in opposite directions.
Conductive plastic – here the screen is a
soft semi-conductive carbon-loaded vinyl
sheath which gives low movement noise
by dissipating static charges. Unfortunately, the resistance is high and a simple
drain wire is not good enough. However,
combined with a lapped copper screen
this can give excellent performance
and easy preparation. Fig.12 shows a
semi-conductive cable for guitars.
Braided – This is the best giving greater than 94% coverage while being very
long lasting with repeated flexing. It
is fiddly to unpick and twist into a
solderable length. Fig.13 shows how
a pointed tool is needed to carefully
‘comb’ it out, rotating as you go round.
Fig.10. Foil-screened cable used in fixed
studio wiring is quick to prepare.
A bradawl will work, but I find the best
tool is a dental probe. Some technicians
just push the braid back, punch a hole
in it and pull the conductors through.
However, with dense braids, such as
those used by Canare (Fig.14) such an
approach, along with cutting the outer
insulating sheath too deep, is a recipe
for broken strands, which can lead to
braid damage – see Fig.15.
Special cables
Most audio signal conductors use polyethylene conductor insulation for its low
capacitance properties. Unfortunately, this
easily chars at a low temperature and has
bad insulation meltback when soldered.
Irradiated polyethylene is much tougher.
In fact, low capacitance is not vital for microphone and line level cables, since the
source impedance is quite low. Therefore,
PVC, which gives higher capacitance is
fine. However, for high-impedance sources, such as magnetic cartridges in record
decks and electric guitars, low capacitance
cable insulation is essential.
Fig.11. Lap-screened cable is easy to
unwind and solder up.
Practical Electronics | September | 2021
Fig.12. Some screened cables have a semiconducting plastic sheath (always black)
which reduces handling noise. This cable
only has one core for unbalanced use.
Fig.13. Braided screened cable is
the best but requires labour-intensive
combing out to enable it to be twisted
and soldered properly.
Tinsel
This is a special type of wire with string
on the inside to take the strain, while the
copper conductor is wound around it. This
gives long life with repeated flexing, such
as in headphone, patch and boom microphone leads. It is a pig to solder because
the string burns, contaminating the joint.
and gives a ten-fold (20dB) improvement
in interference rejection. This is the result
of implementing a special twisting of two
parallel pairs that put the magnetic centres of both signal conductors bang in the
middle of the cable. On a normal twisted
pair, the centre jiggles about a bit. Comparatively, the residual differential voltage
induced by magnetic interference is down
by a factor of ten in Star Quad cables and
thus there is less for the differential input
amplifier to reject. On top of its superior
interference rejection, it is also best cable physically, having a supple ‘jelly’ feel
which never gets tangled. Originally made
by the Japanese company Canare, it is the
best microphone cable available – Canford
Audio still sell it. Cheaper alternatives are
Van Damme XKE and Mogami. The cheapest StarQuad type of cable I have found
is Pro Power STAR00100 with a lapped
screen (order No CB13301 from: https://
cpc.farnell.com).
When Future Film was the exclusive
distributor for Canare cable, we had a special demonstration machine built into a
briefcase consisting of a triac-based light
dimmer and a sensitive audio amp. The
lamp cable ran parallel to the XLR cable
under test. The buzz was always noticeably
less with the Star Quad. When purchasing cable, be careful: ‘Star Quad’ is now a
generic term to cover this particular fourcore conductor geometry, as shown in
Fig.16. Along with the extra two cores to
prepare, terminating Star Quad is labour
intensive (see Fig.17). Note also that it has
50% higher capacitance than normal microphone cable.
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This cable takes the core twisting and
magnetic-field cancellation to new levels
PoScope Mega1+
PoScope Mega50
Fig.14. Stripped cable exposing the
screen braid.
Fig.15. Damaged braid due to careless
stripping.
Practical Electronics | September | 2021
Fig.16. Star Quad cable has four cores
and has better rejection of magnetic fields.
Fig.17. Correct termination of Star Quad
cable onto XLR. The same-coloured
cores are paralleled. Note: blue is the
‘cold’ conductor.
- up to 50MS/s
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