AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
A question of balance – Part 2
L
ast month, we looked at
the fundamental ideas behind
balanced microphone preamplifier
cables, including XLR connectors and
suitable cable varieties. This month we’ll
go through the procedure of actually
making a cable.
Making your own – why do it?
You might wonder why bother going to
the trouble of making your own – surely
just buying one would be simpler. First,
you gain the satisfaction, knowledge and
self-sufficiency that comes from making
(and repairing) things yourself. Second,
you can do a really good long-lasting job;
for example, using proper leaded 60/40
solder that doesn’t crack. In the UK, all
commercial cables must be made with that
awful lead-free, tin-based solder. Last, although ready-made Star Quad microphone
cables are available, they cost around £25
– you can make one for a lot less.
Strip tease
The conductor wires are best stripped
with a proper multi-toothed stripper,
such as the AB Mk 1 that can get to the
short leads without pulling too much.
Fig.18 shows the tool in action.
The thick outer sheath or jacket of the
cable can usually be ‘nibbled’ around
with side cutters, as shown in Fig.19.
The trick here is not to go in too far. If the
braid or inner conductors get nicked you
Fig.18. Stripping the signal conductor
wires needs a decent wire stripper that
can deal with the short length available.
I’ve used this AB Mk 1 since 1983. They
are still available from Grove Sales.
46
have to snip it off and start again. You
cut around 80% of the thickness down
and bend it until the final uncut lower
portion breaks. Pulling the sheath away
outwards from the cable while cutting
also helps. These tricks require practice,
and it gives an unsightly, ragged edge.
Single-sided razor blades and scalpels
can be used, which – with practice – can
give good results, although I don’t like
this approach, electronics dribbled with
blood is unsavoury.
Professional stripping tools are available for coaxial leads (Fig.20) that do a
better job, but some don’t work. All engineers have a drawer full of tools bought
and then abandoned. My drawer seems
to have more useless wire cutters/strippers/benders than any other tools. There
is a low-cost blue plastic squeeze blade
tool that can give good results (shown in
Fig.21) for occasional use. I use the MK02
swivel blade tool from Ripley-Tools.com
– shown in Fig.22. You set the depth of
the blade and press it into the insulation.
Then you spin it round to do the cut. The
button is then pressed to rotate the blade
90 degrees to do a slit along the sheath
as you pull it off. It’s worth getting one
if you do lots of leads.
Fig.20. A selection of jacket-stripping tools.
Fig.21. A surprisingly clean jacket cut done
with the cheap blue blade tool in Fig.20.
Teething troubles
Do not use your teeth for stripping! I’ve
had to spend £3000 on bridge work and
I’ve a friend who had an electric shock
so bad he was flung into a filing cabinet
that still bares a massive dent. I fantasise
that if I ever need dentures, I’ll have an
Fig.19. Side cutters can be used to
‘nibble’ the outer jacket to strip it – fine
for small batches of cables.
Fig.22. The rotatable blade of the MK02
tool can just be seen. The circular bush
enables the depth of cut to be set and
the black button is the blade rotate.
Practical Electronics | October | 2021
n
Fig.23. The wireman’s mantra: slide on all required bits onto the cable before soldering.
Tinning
It’s always a good idea to twist and tin the
conductors before soldering. High-quality
cables have tinned conductors and look
silvery. Cheaper cables are untinned and
have the characteristic pink bare copper
colour. It is essential to tin these cables
first. Twisting and tinning before soldering
avoids random whiskers of wire that can
cause intermittent crackling during the first
and last time the Fat Lady sings. Always
cut the conductors to length after tinning,
to allow for insulation shrink-back.
Fig.24. Twist and tin the wires, then cut
to length. A properly prepared cable
termination using Star Quad.
Fig.25. Solder the hot conductor wire
by heating a bucket filled with solder.
Then put in the cold conductor and
solder. Finally, solder the shorter screen
by pushing it in leaving a ‘loop’ in the
conductor wires.
insulated multi-gauge titanium stripping
set, along the lines of ‘Jaws’ in James
Bond films.
Slide it on
There is nothing more annoying when
making cables than doing a beautiful soldering job and then finding that you’ve
forgotten to put the case, cable clamp
or insulation sleeve onto the cable first
(Fig.23). I still do it occasionally.
Anti-tension ‘loop’
Fig.26. How a correctly soldered insert
should look using standard lap-screened
microphone cable.
Practical Electronics | October | 2021
Lead length
For XLR connectors, the conductor leads
should be about 13mm long, including
3mm of tinned portion. The bare screen
lead needs to be shorter, at 10mm long.
It is always better to twist and tin a longer portion before cutting to the 3mm
required. This is so you can hold it with
your fingers and to obtain a proper circular cross section. Twisted wires usually
go a bit splayed at the end of the twist,
which can inhibit insertion (Fig.24).
For mains power leads, the earth lead
must be made the longest so that it is the
last to come off if the lead is strained. Obviously, this is necessary for safety. However,
with audio leads the opposite applies. If
the earth lead comes off first, all you get
is extra noise. This is preferable to loss of
signal from broken signal conductors in a
live situation; and it’s easier to trace. Also,
the braided or lapped shield conductor is
thicker and stronger than the signal conductors. Because the signal conductor wires
are more flexible, these are soldered first
and the screen soldered last with a pushing
action, as shown in Fig.25. The finished
result is shown in Fig.26.
Fig.27. Clean and tin oxidised buckets.
Use 3% activated rosin 60/40 tin/lead
solder for top results. No-clean fluxes and
European tin-based unleaded solders
are no good. Best of all is Multicore low
melting point (LMP) 2% silver-loaded
62/36/2 solder.
Fig.28. How not to do it. Everything that
could go wrong has gone wrong: insulation
melt back, non-wetted joints, grey
unleaded solder ball-up and shorts due to
loose strands. Poor student – grade F.
in flying solder. Fig.28 shows the result
of not tinning things properly and using
an iron that’s got insufficient heat capacity. The wires are just held on by flux.
One of the thing I like about XLRs is that
the pin numbers are always marked. If you
can’t see them, you need a magnifying glass.
Some XLRs have an extra tag for earthing
the metal case to provide RF screening.
This can be connected to pin 1, as shown
in Fig.29. In rare cases where no connection is wanted between metal work and pin
1 (which incorrectly may be signal earth
Solder buckets
On some XLR connectors the silver-plated solder buckets can become oxidised if
stored near contaminants such as diesel
fumes or other sulphurous smoke. Use a
small brass brush or needle file to clean
them. I fill them with solder and knock
it out while still molten to tin them
(Fig.27). Use a big iron, such as a 60W
Weller; a little 15W Antex iron used for
PCBs won’t cut it for high thermal-capacity XLR pins. Wear eye protection,
since it’s mostly desoldering that results
Fig.29. It’s not normally needed, but
sometimes the case of an XLR can be
earthed to pin 1 via a tag.
47
Fig.30. Silicone sleeving used to prevent shorts from loose strands.
This is shown before cable clamping. If you solder well, it’s not
necessary. Sometimes it’s used to hide bad joints underneath.
Fig.31. Tighten up the cable clamp first before pushing the
XLR insert in fully (see screw hole). This ensures the signal
wires are not tensioned and pulling against the solder.
on some systems) the link may be left off.
The AES recommends leaving it off since
they worry about the metal case touching
and earthing something it shouldn’t. On
my workshop test leads I prevent this by
making the link with a 10nF capacitor,
which provides an RF path.
Sheathing
Fig.32. If you’re using connectors with
many screws it can be worth heatshrinking the case to hold them in. It can
also prevent the case causing shorts if
dropped inside equipment.
Fig.35. Old speaker magnets are a good
way of holding leads while tinning and
soldering. Also, I often use the tester
shown in Fig.33 as a way of holding the
inserts while soldering.
A lot of audio technicians like to slide
silicone rubber sleeves over the pins to
prevent shorts, as shown in Fig.30. A bit
of lubricant such as Hellerine makes this
easier. Larger sleeves are often placed on
the end of the cable sheath to cover loose
strands from the braid and to aid clamping thin cables. If the preparation is done
properly there should be no loose strands
and sleeving should not be necessary.
in the cable. The wires should never be
tight inside the case or they will break.
Locking and heat-shrinking
To avoid losing screws in Cannon XLRs
I usually heat shrink them as shown in
Fig.32. A dab of nail varnish is recommended to lock screws.
Cable clamping
Before putting in the insert retaining
screw, tighten up the cable clamp where
the cable enters. The insert should be a
few mm away from the screw hole before it is pushed in to meet up (Fig.31).
The idea is to have a little extra slack in
the system – ie, a strain-relief section
3 R ed L E D s
2
XLRs are latching connectors, meaning
they lock into the socket with a click. To
unlock them and withdraw the connector, the latch has to be depressed. This is
3
9 V P P 3 battery
–
P in N o.
3 x m om entary pushbuttons
+
1
Latches
P in N o.
1
2
3
3 x
1kΩ
M ale line plug
2
1
3
3
1
Fig.33. If you’re making lots of leads then
a cable tester is a sensible investment.
This BSS unit is designed for XLR and
TRS jack leads.
48
M ale ch assis
output socke t
C able under test
2
F em ale ch assis
input socke t
Fig.36. It’s easy to make your own XLR cable tester – this circuit works well.
Practical Electronics | October | 2021
Fig.36. Don’t wind cables up like this, it strains the conductors.
Fig.37. Knotting cable for storage just damages it even more.
Fig.38. Roadies wind a cable up like this, with a twist of the wrist
while feeding it into the coil to prevent kinks.
Fig.39. Velcro ties are a good way to prevent the cable
unwinding. A cheaper option is releasable nylon cable ties.
there is equal capacitance at both ends
then the break would be in the middle,
but I’ve never known this happen.
present on the female connectors only.
Latching is a mixed blessing; on the one
hand connectors don’t fall out, on the other, equipment can be pulled off stands
and benches if a lead is tripped over.
These latches are removable – but don’t
do it on the microphone end if you have
a singer who swings the mic by its lead!
Winding up
Testing
Always test your cables for continuity and
shorts. A simple cable tester by Brooke Siren Systems is shown in Fig.33. Two LEDs
illuminating at the same time indicates a
short. If the wrong led illuminates for the
respective switch it suggests a couple of
wires are crossed, such as a phase flip. It’s
worth making one of these (see Fig.34).
They are also handy for holding XLR inserts
when soldering; alternatively, I sometimes
use old speaker magnets for holding cables while soldering, as shown in Fig.35.
Cables usually break at the connector
ends – the points of worst mechanical
stress. If you have a capacitance meter
you can detect which end the cable is broken, a technique familiar to most telecoms
wiremen. You measure the capacitance
across pins 2 and 3. The fault is located
at the end with the least capacitance. If
Practical Electronics | October | 2021
Fig.40. All large installations need careful
labelling. These clip-on labels are useful.
Fig.41. This pile of home-made mic cables
has been earning its keep since 1984.
Many audio cables are prematurely aged
by improper coiling. The wrong way is to
tightly coil it against your elbow, as shown
in Fig.36. Worse, some people then go and
knot it together to stop it uncoiling (Fig.37).
The correct ‘roadie’ way of doing things
is to hang the coil loosely in one hand
and gently feed it in with an untwisting
action, as shown in Fig.38. Finally, a Velcro tie should be used to hold it together
(Fig.39). In complex set-ups it’s necessary
to label cables. There are many systems.
Fig.40 shows a clip-on type. Don’t use PVC
insulation tape, since the glue draws the
plasticiser out of PVC jackets and makes
a sticky mess. Fig.41 shows a box of mic
cables typical of a small band.
Last, here are a couple of useful links on
all the permutations of balancing. These
two Rane company notes provide lots of
different lead permutations and a good
summary of AES48 (only accessible free
if you are a member of the AES):
www.ranecommercial.com/legacy/note110.
html and www.ranecommercial.com/
legacy/note151.html
49
