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FAQ:
rec.audio.* Wire 2/99 (part 8 of 13)
15.0 Wire
More than any other topic, speaker cables
and equipment interconnects seem to use up rec.audio.* bandwidth echoing
the same theoretical arguments, testimonials, and opinions. Controversy
can be stimulating, educational, and also amusing. Please try to keep
postings aimed at one of those three goals, and avoid the insults and
emotion. Also, try to avoid echoing a common position or principle, as
described below.
15.1 Do speaker cables matter?
To avoid confusion and repetition,
here is some terminology, thanks to Steve Lampen of Belden Wire &
Cable Co. A wire is a single conductor made up of one or more conducting
elements, but all configured (as in a stranded design) to act as a
single conductor. Mostly, this is coated or covered by plastic, rubber,
enamel or similar insulators. Groups of wires are called cables. So zip
cord is a cable, because it contains more than one insulated conducting
element. Coaxial cable is also cable. Cables can introduce noise into
the signal, act as a filter (and thus change the frequency response of
the system), attenuate the signal (change the amplitude), and provide
nonlinearities from oxidized or otherwise poor connections.
Nonlinearities can distort the signal which add harmonics.
Nonlinearities can also rectify or demodulate higher frequency
signals into audible signals.
It is quite scientifically conceivable that
some cables do cause a difference in sound, because of the differences in
DC resistance, interconductor capacitance, and connector attachment alone.
The effects of exotic conductor weaving and materials are not so well
established. In general, these effects (once we eliminate DC resistance),
seem to be small. However, if your system is at least fairly good, then
some folks have observed (although not in an experimental, double-blind
sense) significant differences in system performance with different
cables. The effects are said to be quite system specific; the only real
guideline is to try them and see which ones seem to sound better in your
system.
Roughly speaking, the price ranges for
speaker cables is low (under $1/ft), medium (under $6-8/ft), and high (up
to $100/ft and more). Try to arrange it so you can trial such cables; at
several hundred dollars per set, experiments can be expensive.
In any system or experiment, it is
essential that the differences between cables be separated from the
differences between connectors.
You should have an EXTREMELY solid
connection between cable and speaker. Speakers operate at very low
impedances, so that bad connections will create significant artifacts or
signal losses at any power level. For example, if the connection has a
linear resistance of just 1 ohm, the speaker damping factor may be
changed, and the bass may suffer. If the connection contains imperfect
metal oxides, then a slightly rectifying junction will block the signal,
producing compression, distortion, and other non-linear effects.
There are many magazine articles on cables
with various perspectives which are worth reading, including:
"Speaker Cables: Testing for
Audibility"
Fred E. Davis
Audio, July 1993, pgs. 34-43
15.2 What speaker cables are available
and how good are they?
There is a wide range of speaker wire
available, ranging from 30ga zip cord (~$.10/ft) to exotic wires costing
over $300/ft. The material used ranges from copper to oxygen-free copper (OFC)
to silver. (There are a bunch of others as well.)
Oxygen-free copper is probably NOT any
different from common copper in sound. If you hear a difference between
two cables, it is not a difference between oxygen-free and common copper.
Resistance may be significant for speaker
cables. The higher the resistance, the more the cable will affect the
sound, all else equal. The resistance characteristic of metals is called
resistivity. The resistivity of copper is 1.7 microohm-cm. Silver is very
slightly lower, 1.6. Gold is a bit higher, 2.4.
Silver and gold are different from copper
in other ways than resistivity. Gold does not oxidize in normal
environments, so gold contacts will not need periodic cleaning and will
not create rectifying junctions. Silver will oxidize, but the oxide of
silver is conductive, so oxidized silver will still make good contact.
Copper oxide is a bad conductor. Oxidized copper contacts may insulate,
may conduct, or may rectify. Copper is a bad material for cable terminals,
but this may or may not mean anything for the conductor itself.
15.3 What can I use for budget speaker
cables?
First, a few words on terminology. Wire is
sized by AWG or BS gauge number. Larger numbers represent smaller wire.
AWG 40 (also called 40 gauge) is as fine as human hair.
AWG 12 is 2mm or .081" diameter. If
you reduce the AWG number by 3 (such as from 29 to 26) then the wire
cross-sectional area increases by a factor of 2 and the series
resistance drops by a factor of 2.
Some wire is classified as solid, because
it contains one strand per conductor. Other wire is called stranded,
because it consists of many strands per conductor. Stranded wire is far
more flexible than solid wire. Most wire is made from drawn copper. Some
wire is sold that is claimed to be made with a process that produces
oxygen-free copper. Oxygen-free copper has a different metallurgical
structure than common copper and may or may not conduct current better.
Some critical listeners have reported
excellent sound from large diameter solid copper wire, such as home wiring
"Romex 12-2". At least one expert has said that common 18-gauge
solid copper hook-up wire sold by Radio Shack also works very well. Also
recommended on a budget is Sound King wire, a 12 gauge oxygen free copper
stranded cable. This is available from MCM Electronics for $.39/ft.
Scientifically, thinner wire has more
resistance than fatter wire, so fatter wire will have less
resistance-related effects. Resistance effects can be eliminated by using
at least 12 gauge wire, particularly for long runs. Of course, shorter
runs are always preferred, because they come much closer to the ideal
zero-length wire, with no resistance, no capacitance, no inductance, and
no change in signal.
15.4 What can I use for budget speaker
connectors?
The worst connectors are push-down, or
spring terminals. Screw terminals with solid copper wire are much better.
Gold-plated binding posts and gold spade lugs are inexpensive by
audiophile standards and are extremely stable. Binding posts with spade
lugs can be tightened to get a very good mechanical joint, and may offer
the lowest electrical resistance of any connector.
Gold plated banana plugs and jacks are very
good speaker terminals. Good ones are more expensive than gold spade lugs,
however, they also provide a bigger area of contact, and are more
convenient when you must frequently reconfigure the system. Banana plugs
should be periodically monitored for corrosion and loss of spring tension.
Monster offers a banana-plug connector with an expanding center pin that
forms an even better connection than common gold banana plugs. At
approximately $25 per pair, the Monster banana plugs aren't a budget
connector.
All else equal, connectors with gold
surfaces are better than connectors with any other surface. This is for
two reasons. First, gold is extremely inert, meaning that unless gold is
exposed to very harsh chemicals or harsh vapors, it will not corrode or
oxidize. It will remain a pure, low-resistance conductor. Second, gold is
quite soft, so that if a gold-plated connector is squeezed between two
metal surfaces, it will deform slightly to fill scratches and voids,
giving a very broad, low-resistance contact area.
Corrosion of connectors is often a problem.
Gold-plated terminals and connectors somewhat avoid this problem; problems
with other connectors can be mitigated by unplugging and replugging the
connector on a regular basis, cleaning the contact areas with a pencil
eraser, or by using a contact enhancer such as Cramolin or Tweek. When you
use a contact enhancer, be very sure to follow the directions, and avoid
spreading enhancer about your equipment.
15.5 What about interconnects, such as
the cable between tuner and amp?
Line-level interconnects conduct smaller
signals than speaker cables; the typical signal ranges from -2V to +2V
(the CD output standard) with currents in the microamps (the corresponding
values for speaker cables attached to a largish power amp might be -70V to
+70V and currents of many amps). Line-level interconnects can be divided
into single-ended (or unbalanced), and balanced interconnects. Home audio
is almost always single-ended interconnects.
Single-ended interconnects almost always
use a form of the RCA connector (or phono plug). RCA plugs form fair to
poor connections that degrade with time as corrosion works into the
metal-metal contact and as the spring tension of the connectors relax.
Gold-plating reduces the effect of corrosion and locking RCA connectors
solve most of the mechanical problems. However, these premium phono
connectors are rare and expensive. For example, a gold-plated Vampire
locking RCA plug costs approximately $23/pair. If RCA connectors weren't a
de facto standard, we'd recommend against them.
Unbalanced interconnect wires vary in
geometry, material and price. Cheaper wires have a single conductor
(normally stranded) and a shield and cost $.20-$2/ft. Medium (complexity
and price) wires have two conductors (often arranged as a twisted pair)
surrounded by a shield and cost from about $3-$20/ft. Exotic wires have
all sorts of geometries and materials (such as stranded silver conductors,
or ribbon cable braided around a core, or in one extreme case, a tube
filled with mercury!). Prices may be as high as $200-$300/ft.
Balanced interconnects have three
conductors: two for the signal one for ground, and additionally a shield.
The standard connector for balanced cable is the ITT/Cannon XLR connector,
which is quite good mechanically (they lock). Equivalent connectors are
also available from Switchcraft, Neutrik, and other vendors. If you have
to run cables longer than 12 feet or 4 meters, the greater noise immunity
of balanced interconnects is often a good idea. For this reason, balanced
connectors are standard equipment in professional installations such as
studios, public address systems, and broadcast stations. There is not much
variation in balanced cables. The three brands mentioned above are known
to be rugged, high quality and moderately priced. Slightly weaker imported
connectors are available, but they aren't dramatically cheaper. Better
connectors are also easier to assemble and have a more durable cord
strain-relief.
For most systems, the most important aspect
of a cable are the mechanical reliability of the connectors; in
particular, the joint between connector and wire, and the joint between
connector and socket. Typically, interconnect cables are short. It is
worth getting just the right length; cables often come in .5 meter
increments. With quite good systems, some people observe differences in
sound between various interconnects. This is quite system-specific and the
same advice as given above applies: try several brands. Most good dealers
will loan interconnects for home evaluation.
In cables where the shield does not carry
the signal or ground, the shield is normally only connected to ground at
one end. In systems where there are significant differences between ground
levels on various components, it may make a difference which way such
cables are connected. Typically, the end where the shield is grounded
should be at the source of the signal. Often, such cable has arrows on it
pointing in the direction of the signal flow. In any case, try both
orientations.
There are many objective reasons why cables
might cause differences in sound by interacting with the drivers in the
signal sources as well as by providing non-linear effects in the RCA
connector. Most of these effects are again related to interconductor
capacitance and resistance, and the quality of the shielding provided by
the "shield" conductor. In balanced cables the quality of the
"twisted pair" inside the shield is also important. One might
note that a shield protects from only capacitively coupled interference,
and not from any magnetic field interference. The twisted pair in a
balanced line provide some magnetic rejection, as does steel conduit.
However, steel conduit has other characteristics which make it undesirable
for audio in general.
15.6 What about Phono Interconnects:
Phono interconnects are part of the link
between a cartridge on a turntable and a preamp (or head amp or receiver).
They are a special case of line-level interconnects because the signal is
much lower, typically 1 to 50 millivolts. They are also intended to
operate into a higher impedance, typically 47K ohms, and form part of the
capacitive load for the cartridge.
The low signal levels mean that the
shielding of the cable, and the presence of a separate drain/shield are
more important, as is a good ground. A separate solid ground should come
along with the cable as a separate lead co-routed with the cable.
In addition, the low signal levels make a
good solid connection to and through the connectors MUCH more important,
because of the greater sensitivity to low-level nonlinearities.
Wire capacitance is often ignored in
line-level interconnects; however, in a phono interconnect, it may
constitute half of the total capacitive load of the cartridge. Obviously,
then, two cables with significantly different capacitances should sound
differently. In this sense, the "right" cable for one cartridge
may be too low or high in capacitance for another cartridge.
For low-impedance cartridges (most moving
coil cartridges), the wire must have low resistance to prevent cartridge
unloading and frequency-dependent signal loss. In addition, as the signal
levels are quite low, shielding is important.
Unfortunately, copper shields do not block
stray magnetic fields, so in the case of phono cables, careful routing may
be even more effective at reducing hum than special wire.
15.7 Is there really a difference in
digital interconnects?
There are now three kinds of digital
interconnects that connect transports to D/A converters: coax, plastic
fiber (Toslink) and glass fiber (AT&T ST). In theory, these should
sound EXACTLY the same (bits are bits). However, this assumes good circuit
design (in particular, the clock recovery circuits of the DAC, and careful
consideration of electronic noise) which may be compromised because of
cost considerations or ignorance. Note: different signaling schemes are
used on plastic and glass fiber.
In any case, some people claim to hear a
difference; of those who do, most seem to prefer the glass fiber. However,
the technology of fast digital data transmission in consumer electronics
is evolving very quickly now. Any specific recommendation should be
treated with suspicion until the industry matures.
15.8 Can I make very good interconnects
myself?
Yes. You will need to be the judge of
whether or not they are as good as $100 interconnects, but it is easy to
make interconnects that are better than the $2.00 set which comes with new
equipment.
There are two necessary ingredients:
two-conductor shielded cable and RCA connectors. There is a lot of debate
over what is the best cable, but in general, the lower the capacitance per
foot, the better. Choice of insulation is harder. There may be an
advantage to polypropylene or teflon over polyester or rubber, but even
that is debatable. If you are buying wire from an electronics distributor,
some have successfully used Belden 1192A microphone cable. It is rubber
insulated, so very flexible. Another recommended cable is Belden 8451.
This is a polypropylene cable with foil shield. Finally, consider Belden
89182. This is foamed teflon insulated, so very low capacitance, and foil
shielded. If you plan to make a long cable, this low capacitance cable may
be the best choice.
There is also a variety of RCA connectors
available. A good connector would be gold plated and machined to tight
tolerances. A poorer connector will not fit as well, will make poorer
contact as the connecting surface oxides, and will lose its springiness
with use.
When wiring the cable to the connector, use
one wire for signal, (the tip of the RCA connector) and one wire for
ground (the shell or outer conductor of the RCA connector).
Some cables use a foil shield which is
difficult to solder. These cables typically have a drain wire parallel to
the foil which can be used for soldering. Others use a braided shield.
Regardless of which type of wire you have,
connect the shield or the shield drain wire to ground on only ONE SIDE.
This will stop noise picked up by the shield from causing ground noise.
It can be a touchy job soldering RCA
connectors. Before you use your new cables, check with an ohmmeter or a
continuity tester to make sure that you have not accidentally sorted the
signal and ground leads together, either with a stray drop of solder or a
loose wire strand.
15.9 Is there a standard for wiring
balanced XLR-3 cables?
Yes. Connect pin 1 to ground/green, pin 2
to white, and pin 3 to black. Herb Hamilton suggests that you remember
"George Washington Bridge" and then use the first letter of each
word (GWB) to help you remember Green=1, White=2, and Black=3. This same
wiring convention works for balanced line level signals and balanced
microphone cables. |
