LINE LEVEL CABLE CONSTRUCTION AND MATERIAL PREFERENCES
by Jon M. Risch                  Last Updated 10-22-2002
[email protected]

Information compiled over the years while listening to cables.

-Unbalanced Connections-
Unbalanced connections involve only two separate conductors, where
one is considered "hot" and one is considered "ground"
(i.e. typical CD player to preamp, preamp to amp using RCA phono
style plugs).  If the "hot" conductor is earth grounded, the signal
is shorted out.  If the "ground" conductor is earth grounded, the
signal is theoretically unaffected (in practice, ground loops, RF
injection, and other undesirable effects can occur if this is done).
Classic construction is coaxial, an insulated wire surrounded by a
cylinder of conductive material, usually copper braid, or a foil
sheath (sometimes both), covered overall in an insulating jacket.

Preferred Construction for Unbalanced Connections:
After listening to cables with identical materials, and LCR
(Inductance, Capacitance and Resistance) parameters within an
acceptably similar range, but different physical geometry's,
it was determined that a twisted pair cable with overall shield
(a telescoping arrangement) was superior to a coaxial or triaxial
type.  A moderate to high amount of cabling is desirable (cabling
is how much twist a wire pair has per distance) depending on the
amount of hum rejection necessary and the shield type.  The
difference is fairly subtle between coaxial and twisted pairs,
especially when the twisted pair is shielded.

-Balanced Connections-
A balanced connection nominally involves two conductors, both of
which could be considered "hot", but with opposite polarities of
signal.  If either conductor is earth grounded, theoretically the
signal does not short out, but has been unbalanced by this earth
connection.  As with the unbalanced connection, earth grounding
either of the "hot" conductors has it's problems.
Since audio signals typically require some sort of shielding from
hum, a third conductor, in the form of a conductive cylinder
surrounding both "hot" conductors is classically used.
This most common construction is called a twisted pair.

Preferred Construction for Balanced Connections:
It was determined that a twisted pair with overall shield
(a telescoping arrangement) was superior to a triaxial type.  A
coaxial type is normally not used for balanced connections.

It was further determined that if it was possible to use the
twisted pair without a shield physically present, that this
resulted in higher performance.  In lieu of no shield, a shield
that was spaced away from the twisted pair of conductors performed
better than a shield in close or intimate contact with the twisted
pair.

If a shield was introduced between the pair of wires, the
performance suffered greatly, rendering this version inferior to
the coaxial style.  See Shields for telescopic ground explanation.

If a device is designed to be used with a balanced interconnect,
it should never be used with unbalanced interconnects unless
unavoidable.
Examples include: Phono cartridges (sometimes manufacturers ground
one side of a balanced cartridge at the cartridge body, or tie the
left and right channels together on one side and call it "ground".
If this can safely be undone, and the proper cables used, it will
result in much better signal transfer), pro microphones, and most
pro audio electronics.
If a preamp/power amp pair both have balanced outputs and inputs,
they would be the preferred hookup method over the unbalanced (RCA)
jacks.  Even when the signal passes through an extra stage of
electronics in order to implement the balanced signal, the benefits
of maintaining a balanced connection seem to outweigh all but the
most marginal added electronics.

-Shields-
In coaxial type cables, the shield also carries the audio signal.
This may explain the superiority of the twisted pair construction,
as when it is shielded, the shield doesn't have to carry signal
current.  It is much more difficult for a conductive cylinder sheath
to provide shielding action when carrying signal currents.  If the
ground at either end is less than perfect, any hum or interference
that the shield is intended to intercept gets mixed with the audio
signal.  This may occur at a very low level and be "inaudible" in
the sense that hum or RF are not heard outright, yet cause a subtle
blurring or graininess to be added to the signal.

With twisted pair or triaxial cable, the outer conductive sheath
known as the shield should only be hard grounded at one end, typically
the low impedance signal source end.  The other end can either be left
unconnected, or connected to ground via a 0.01 uF ceramic disc
capacitor to aid in RFI suppression.  This can be one of the basis
for "one way" or "unidirectional" cables.  This is known as a
telescoping or telescopic ground, due to the "nesting" of the inner
conductors within the shield from one end.

_________________________________________ Outer braid
_______________________________________________________ Inner braid

============================================================== Center wire
_______________________________________________________
_________________________________________
                                                                           |
                                                                           |
                                                                        -------
                                                                          ---
                                                                           -
Special Note:
Coaxial cables MUST have the shield connected at both ends, as it
supplies the ground return path for the signal.  This is why the shield
braid can and will affect the audio quality.

-Interconnect Cable Materials-
================================

-Preferred Conductor Materials-
In descending order of preference:
Bare copper
Enameled copper
Tinned copper
Silver plated copper

NOT RECOMMENDED FOR SERIOUS AUDIO USE AT ALL:
Cadmium copper, beryllium copper and other copper alloys.
Nickel plated copper
Silver plated copper clad steel
Copper clad steel
Tinned steel
Bare steel
 

-Insulator Materials-

-Inner Conductor Insulation-
In descending order of preference:
Foamed Teflon (TFE)
Solid Teflon (TFE)
Foamed FEP Teflon
Solid FEP Teflon \
Foamed Polypropylene /  These two are real close
Solid Polypropylene
Foamed Polyethylene

NOT RECOMMENDED FOR SERIOUS AUDIO USE AT ALL:
Solid Polyethylene
Rubber
PVC (Polyvinylchoride) \       These two actually attack most conductors
Polyurethane                  /      over a period of time, the severity
                                                depending on the exact formulation.

-Filler or molded insulation-
Fillers with lots of air are best.
SEE "Inner Conductor Insulation" for order of plastic style fillers
Cotton
Rayon
Nylon
The above fibers would all be placed just below foamed
polyethylene, but above solid polyethylene.

The quality of the cable dielectric can be related to a
capacitor, so work or information on capacitor quality
can be applied to audio cables.  See the site:
http://www.capacitors.com/pickcap/pickcap.htm
This is a reprint of an excellent article on capacitors
by Walt Jung which explains and gives measurement data
about various capacitor types.

Also, for information on how dielectric absorption can affect
signal pulses, see:
http://www.nsc.com/rap/Application/0,1570,28,00.html

-Shield Materials-
In descending order of preference for coaxial cables:
Bare copper
Tinned copper
Silver plated copper
Foil

If the shield is of the telescoping type, and is not carrying current,
then the material it is made of is not that critical to the sound of the
interconnect.  When constructing twisted pairs, often it is convenient
to use a larger braid that is on a cheap cable, these are often tinned.
This will not adversely impact on the sound of the twisted pair cable
as long as the braid is adequately spaced away from the twisted
pair.

CONSTRUCTIONS AND MATERIALS NOT RECOMMENDED FOR SERIOUS AUDIO USE AT ALL
(in coaxial cables):

Double copper braids
Foil/braid combinations
Cadmium copper, beryllium copper and other copper alloys.
Nickel plated copper
Silver plated copper clad steel
Copper clad steel
Tinned steel
Bare steel

-Jacket Materials-
In descending order of preference:
Solid Teflon (TFE)
Solid FEP Teflon
Fluorocopolymer
Silicone Rubber

Borderline:
EPDM
Flamarrest \
Rubber     These three are real close
Neoprene /

NOT RECOMMENDED FOR SERIOUS AUDIO USE AT ALL:
PVC (Polyvinylchoride)
Polyurethane

It would seem that jacket materials could have
no possible affect on the audio, due to the shield
isolating the signal from the insulators.  However,
for coaxial cables (or even twisted pairs with braided
or served shields), the shield coverage for a braided
copper shield maxs out around 95% and often runs
87%-90%; with served copper shields, the shield
coverage often runs only 85%.  And due to the
inherently unbalanced construction of a coaxial cable,
and the lack of 100% shielding, this means that the
electric field leaks through the shield and interacts
with the surrounding environment, where the jacket
is the nearest thing.

SPECIFIC RECOMMENDATIONS

Some specific recommendations now follow.  If some of
the recommendations seem contradictory, it is because
of conflicting priorities, i.e. bare copper and foamed
polypropylene vs. tinned copper and teflon.  It must also
be kept in mind that the specific recommendations are
based on the overall performance of the cable as it is
configured, and how it ranked in actual listening tests
vs. the other cables.

I have looked into the current availability (through Newark) of my
favorite cables and have found to my dismay that some of them have
been bumped from distributors stock by the rise in usage of LAN
and other computer cables.  The cable manufacturers can still
provide most of them, but only if you purchase a minimum quantity,
usually a 1,000 foot spool or more, and are willing to wait till
the next time it is convenient for them to run it!
So, with that in mind, I will pass along my recommendations with
a number of alternatives, in descending order of preference.
If the preferred cable is available and in stock at a local
distributor, I would recommend getting it ASAP!

As for a line level interconnect, I recommend Belden #89259
as my first choice.  It is of a coaxial construction, with bare
copper braid and shield coverage of 95%, an insulator of foamed
Teflon around a 22 Ga. stranded bare copper wire, and an outer
jacket of black tint solid Teflon.  Capacitance is a low 17.3 pF/ft,
which is almost half the capacitance of common interconnect cables.
It is superior sonically to many of the expensive/high-end cables
sold for hundreds of dollars, and can be terminated with your choice
of RCA plug, anything from the nickel plated Switchcraft RCA to the
Cardas/Kimber/Tiffany RCA's.  It is good enough to be worth using
the finest plugs.  Available in 100 ($100) and 1000 ($950) foot
spools.

2nd choice:  Belden #82259 Same as above except with a Flamarrest
jacket.  May be more flexible than the above, have not handled a
sample.  Available only in 1000 foot spools ($750).

3rd Choice:  Belden 1506A, a foamed teflon coax with a solid center
wire of 20 gauge copper, but has a tinned copper braid with foil
shield.  While this is one of the best commercial option for use as a
digital interconnect, it is also a decent analog interconnect.  Not quite as
clear as the 89259 for analog use.

These are head and shoulders above most of the overpriced
interconnects out there.  As for most other commercial coaxial
alternatives, their performance is a big step down from these.

It must be kept in mind that these have such excellent performance
due to the top rated materials; it is a shame that this combination
of materials is not available in a twinaxial or twisted pair cable.

---------------------------Update------------------------
Since first posting my DIY interconnect cable recipe (I also have a
speaker cable recipe note posted here as well) I have discovered
a way to improve upon the raw 89259.
It is labor intensive, and more expensive (about 3X) but the results
come close to matching almost anything out there at any price.

You need to start with twice as much 89259 as for the stock cable,
e.g., if your need is a 10 foot pair of interconnects, instead of
two 10 foot pieces of coax, you need four eleven foot pieces.  You
also need two 11 foot sections of another coaxial cable with a
larger diameter than the #89259, to use the braided shield from, the
shield can be either tinned or bare copper, but must be a copper
braided type. Either separate braided shield such as Belden #8668 for
a small cable bundle (no extra insulation) or Belden #8661 (for a
shield used with extra spacer insulation) or a full coaxial cable
such as Belden #9259, can be stripped of it's jacket and the braid
loosened off the center insulation/wire.  Either the braid or the
9259 cable cost about $40 for a 100 foot spool.
By the foot amounts of cheap coax for use of the braided shield
can be gotten at either local RS stores, or from their website.

Now, you disassemble the 89259, removing the jacket, and cutting or
sliding the coaxial shield off of the center insulated conductor.
When you are done, you will have cut jackets (discard), cut or
slipped off braid (may find useful) and a piece of foamed FEP
teflon insulated bare stranded copper wire.

Take the pieces of insulated center conductor, and twist (as
tightly as you can) them together for the length of the wires.
Try to twist each side equally, or one side will come up short.
More initial length is needed to account for twisting.  You have
now created a twisted pair!  It should be tightly held in the tight
twist, do not let it unravel or become loose if at all possible.
Some polyolefin heatshrink spaced every 6-8 inches will help here,
either SPC Technologies type PHD-032 1/2" adhesive-lined polyolefin
shrink tubing, or 3-M EPS-200 1/2" adhesive lined, flexible polyolefin
heat shrink tubing.  Available from Newark, Digi-Key, and Allied.
Thin wall polyolefin 3M type VFP-876, 1/2" diameter
(50 feet for about $19) can also be used for this purpose, or for
an overall outer covering.

Shield it with the larger shield braid from the other cable
mentioned, and ground the shield at one end only, insulating the
other end from the RCA plug.  If you really want the best,
find some "filler" material to pad the sides and keep the larger
shield spaced away from the twisted pair. Cotton string works OK,
but a higher quality insulator, such as the center insulator guts
(preferably foamed teflon) from another cable, would be even better.

The end result is slightly stronger bass, and a slight improvement
in the smoothness in the highs.  Exactly where Mr. Green in RAHE
found very minor weaknesses in the raw coaxial 89259 compared
to $1000 interconnects.

FLASH!     6-4-98

The latest DIY interconnect info:
For awhile there, I thought that the use of commercial cables for
DIY interconnects had reached a final plateau.  Twist together two
inner cores from Belden 89259 tightly, and space a shield off, and
that was the best that could be done without getting custom
materials.

When I did my cable tests for materials and construction, I didn't
notice any reliable difference between stranded and solid wires in
a cable.  Neither coax or twisted pairs seemed to sound better with
one or the other type of center wire/twisted pairs.

Lately, I have been listening to some cables again .
I have been experimenting with the
use of solid conductors of various types for interconnects, and
the results were a bit of a surprise.

Two solid bare 18 gauge copper conductor foamed teflon cores
sounded pretty good, they seemed to have a slightly smoother and
more cohesive sound.  There was a fly in the ointment though.
There also was a region in the lower midrange which stood out,
almost a discontinuity to contrast with the smooth detailed upper
and lower ranges.  Imagine a mild honk in the sound toned down
about ten times.  Just barely there, but noticable by contrast
with the rest of the audio band.

Compared to the stranded 89259 cores, they sounded a minute smidgen
cleaner in the extreme highs, just a last vestige of tizzyness that
was only noticable by it's removal.  The shades of nuance are very
subtle in these differences, and not at all obvious and up front.
If they didn't have the lower midrange blip, then they would be
superior overall.  As is, they represent one of those trade-offs
so common in audio, they do some things slightly better, but fall
down a little elsewhere.

If I had left it at that, I wouldn't be posting this now.  Ever the
question, what if?  What if I used a different gauge solid wire,
with a different diameter foamed teflon core?  The previous pair had
18 gauge with 0.170" diameter cores.  Why not mix and match?
I twisted up a pair with one 18 gauge/.170" core, and one 20 gauge/
.135" core.  How would this sound?

Long story short:  most of the lower midrange micro-honk was gone,
and the rest of the range seemed even a bit clearer than the dual
18 gauge.  After some prolonged listening, a definite step closer
to neutrality than a twisted pair made from stranded wires.  Still
some trade-offs, but now the overall balance favored the solid but
different gauge twisted pair.

What if?  What if I mixed a stranded core with a solid core?
I mated a core from 89259 with a core from 89248 (18 gauge/.170"),
and built them up.  I paid particular attention on all these
prototypes to twisting them very tightly and firmly, about one full
revolution every 1 1/2 to 2", letting the cores rotate as needed to
avoid stressing the wires or the cores any more than they would
already be stressed mechanically.  With dissimilar size cores and
a different stiffness of wire, the smaller less stiff wire/core is
held out straight, and the larger stiffer core revolved around it.
The end result has them just about evenly twisted around each
other.

Who cares about the twisting, what does it sound like?!?
The pairing of the stranded with the solid wires seemed to provide
the best of both.  A very controlled and focused sound from top
to bottom, a deep and pinpoint soundstage, and a complete freedom
from any hint of false bloom and enhancement of ambience.

Construction details
As noted above, one core was from Belden 89248, the other from
Belden 89259.  Quartered pieces of empty core foam were used to
space the outer shield braid away from the crease of the twisted
pair, this in order to keep the braid away from the EM fields
that exist between the two conductors.
If no spacers are used, the shield ends up very close, even pushed
down in the crease, and the sound gets slightly dulled and squashed.
I believe this is due to the electric field that is directly between
the two wires being partiually shorted out.  The extra core
pieces get lashed hard against the twisted pairs, and follow the
twist, by being heatshrunk into a bundle before the shield is
slipped over the entire assembly.

I now recommend using teflon plumbers pipe thread tape, and to
mummy wrap the twisted pairs with crease spacers, before any
HS or shielding is done.   I have received e-mail from
some intrepid DIYers that used teflon tubing as a shield spacer.
They found halving and/or quartering the empty core too difficult.

Use good RCA plugs, the largest barrel you can find, or you
will have to shave some of the insulation thickness from the
core's foam in order to get them to fit through.  If this
becomes necessary, be sure not to shave too thin, keep as much
insulation between the wire and the barrel as possible, if the
need for such shaving is determined ahead of time, shave it
from the inside, or from between the two twisted cores, rather
than on the outside.

For those that want to try some solid wire cables, or mix and
match with some 89259, the solid wire cores can be gotten from
Radio Shack by the foot.  See the end ofthe next section for more info.

You can also try a twisted pair using a core form 89259, and a core
from 1506A, which is a 20 ga. solid wire with foamed teflon core.
It is a toss-up as to which one sounds "the best" between this and the
89259/89248.

End of Part 1, Cont'd in Part 2

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