|AUDIOPHILES RECENTLY BEGAN re-examining the
performance of every link in the audio playback chain, and before long their attention
turned to the lowly loudspeaker cable. In response to demand, a number of companies are
producing or distributing new and exotic cables claimed to improve audio power
transmission from amplifier to speaker. Pointing to lower resistance and inductance,
proponents of the newer cables insist they sound significantly better ("better than
an expander!"); however, the subject is controversial, and some hi-fi notables claim
performance increase is negligible and the higher capacitance of some new cables can cause
amplifier instability and damage.1-4
Neither view is completely
correct: the new cables are neither panacea nor placebo, but components whose
characteristics must be evaluated in the context of their usage. Hoping to shed some light
on the subject, I obtained samples of various cables, performed a number of tests, and
drew a few conclusions.
Almost everyone seems to agree that ideally the amplifier should be so intimately
coupled with the loudspeaker that the cable can cause no power loss or distortion. This
corresponds to a wire having no resistance, inductance, or capacitance, which in real life
translates to an infinitely short cable. I treat this premise as fundamental, because in
general it results in the best performance. (It may not do so in some specific situations;
for example, one could imagine a special case where some resistance or inductance might
improve the sound.)
Regardless of the cable type, the effects it introduces to a signal are proportional to
its length: the shorter the cable, the more intimate the connection between amplifier and
loudspeaker. Subtle differences between cable types become more dramatic with increasing
length and shrink toward zero as the cable gets shorter; thus the audiophile whose
amplifiers sit close to his speakers need be less concerned than he whose cables are 40
feet long. To this end, some manufacturers have installed amplifiers within their
loudspeakers, exchanging speaker cable problems for preamp ones; commercial sound
distribution systems have resorted to higher voltages, which improve transmission much
like the high voltage utility lines which carry power many miles.
Fig. 1 shows a fairly simple first order model of a
loudspeaker cable. The inductance, resistance and capacitance are approximated as
components in a circuit, sectioned off per unit of length. In this example, the values are
for simple 18 gauge "zip" cord and one foot lengths, so that L is the inductance
per unit foot, R is the resistance, and C is the capacitance, measured in Henries, Ohms,
and Farads respectively. As a practical matter, the values of these elements represent
tradeoffs against each other: for example, low inductance is easily achieved with high
capacitance and vice versa, and the ratios of these values give rise to the cable's
characteristic impedance, as I shall discuss later.
Researchers have chiefly concentrated on the cable's inductance and resistance, for
they impede the flow of electrons between the amplifier and the loudspeaker. Resistance
causes loss at all frequencies while inductance causes loss proportional to the frequency.
Capacitance has not usually been considered significant because its values do not impinge
upon the audio band. However, we will see later that it may sometimes important.
The new kinds of cable seek to reduce resistance and/or inductance and thus improve the
amplifier- speaker connection. They fall into two categories: multistrand twin lead of
various gauges (lamp or "zip" cord being an example) and low inductance - high
capacitance coaxial or interwoven types. Their measured performance also falls into two
categories, 0-100kHz effects and 100kHz - 40MHz effects, which for convenience I will
treat separately. My analysis was greatly simplified by the fact that within the two cable
categories performances were very similar; indeed, many of the cables were virtually
indentical at higher frequencies.
I tested five different types of twin lead cables: 18 and 24 gauge "zip" cord
and three specialty cables, "Monster" Lucas cable, and Fulton wire (gold). I
bought two samples of each of 18 and 24 gauge wire off reels at a local Radio Shack and a
hardware store. All the cables tested were 10 feet in length.
"Monster" cable, marketed by Audio Sales Associates in San Francisco,
California, is an approximately 111/2 gauge twin lead, similar in
construction to very large lamp cord with a thick clear plastic jacket, with large spade
lugs at each end for attachment to large screw terminals on "five-way" binding
posts as commonly supplied on loudspeakers and amplifiers. Lucas cable is approximately 14
gauge, jacketed in green plastic with a ribbon shape, and is marketed by S.O.T.A.,
Halifax, Canada. Fulton "gold" cable, available from Fulton Musical Industries
with dual banana and other connectors, is an extremely large gauge twin lead having by far
the lowest resistance of any cable tested; it is also useful for pulling up tree stumps or
jump starting locomotives. Fulton also make a "brown" version similar to Monster
The four samples of low inductance cable I tested boasted more exotic construction than
zip cord. Two colorful types, Polk Sound Wire and Audio Source Ultra High Definition Wire,
use large numbers of separately insulated strands closely interwoven in such a way that
the wires cross at an angle to each other instead of running parallel. This reduces the
magnetic induction between strands and lowers cable inductance, at the cost of higher
High definition cables, another variety of low inductance cable from Audio Source,
consists of eight twisted pairs of wire arranged into a flat ribbon. Mogami wire is a
large coaxial cable consisting of a grey plastic housing containing two concentric
"shells" of wire strands, the inner conductor enclosing a plastic core.
"Smog Lifters," another tested cable, is distributed by Disc Washer. It bears a
resemblance to Audio Source's high definition cable, with loosely woven braids of
Fig. 2 shows the relative values of resistance,
capacitance and inductance of each of these cables.
SERIES IMPEDANCE TEST
I tested a 10 foot sample of each cable type using the Fig.
3 setup. I drove the cable by a high source impedance and measured the voltage across
it, showing its series impedance. This voltage, referenced to a 0.1 ohm non-inductive
resistor and measured from DC to 100kHz, clearly shows the cable impedance's resistive and
inductive components (Fig. 4). For the twin lead types,
inductive and "skin effect" (an additional high frequency resistance effect)
components begin to show up at about 1kHz; they increase the impedance, causing high
frequency loss in addition to the cable's resistive losses. Interestingly, all the twin
lead types have similar cable inductance values, approximately 2uH per 10 feet, and in the
region just above the audio spectrum they are nearly identical. Below 20kHz they fan out
to their respective resistance values. The lightest wire, #24, clearly has the most loss,
while Fulton cable has the least.
The series impedance test differentiates the low inductance cables from twin lead as
they exhibit an order of magnitude less impedance at 100kHz. Of these, Mogami Wire had the
lowest series impedance, by virtue of its lower resistance; but each of these types has to
all intents and purposes inductance effects. The series impedance is a more or less linear
function of the length of the cable, so a one foot length will have one- tenth the series
impedance shown while 100 feet would have 10 times the amount.
IMPLICATIONS OF LOW FREQUENCY TEST
The existing literature covers the subject quite well. However, I think you will find
it useful if I briefly touch on the effects of this series impedance as it relates to
frequency response and damping factor 4, 5, and 6.
The performance context lies also in the amplifier's source impedance and the
loudspeaker's load impedance. The system's performance will depend on the complex sum of
the impedances involved:
Z source + Z connections
+ Z cable + Z load
The speaker has generally been designed to be driven by a voltage source, so our ideal
premise requires source and cable impedance to be very small compared to speaker
impedance. In this case, the speaker's design dominates the performance as intended so the
variations in the loudspeaker's impedance do not interact to produce frequency response
deviations. Fig. 5 gives an example of such a case,
where the aberration is relatively minor; however, Fig. 6
shows the effects with one particular loudspeaker (which really exists) where the
deviation is dramatic.
Just as the cables have an inductive element, so do the amplifiers which drive them. In
tube amplifiers the output transformers provide the inductance whereas in most solid state
designs the designer has deliberately provided inductance in the form of a coil for added
circuit frequency stability. The reason that damping factor (8ohm/output Z) has been
traditionally quoted at low frequencies is not only because much of our interest in
damping centers on the woofer, but also because this coil destroys the damping factor at
high frequencies. In fact, examples exist of solid state amplifiers which quote damping
factors of 500 or greater at 20Hz but which have damping factors on the order of 15 at
20kHz. More recently a few designs (Threshold, Audio Research, Yamaha) have dispensed with
output coils, giving them more constant damping factors across the audio band. The new IHF
test standards call for measurement of damping factor at all audio frequencies.
Again, this parameter must be evaluated in the context of the system. For example, most
loudspeakers have a considerable inductive component of their own, which may easily
provide the desired case of amp Z + Cable Z < speaker Z. Conversely, the source and
cable inductances become more important with loudspeakers whose load impedance is
resistive or capacitive, especially with some electrostatic designs. Our initial premise
does not always hold either, for I have seen "poor quality" cables used to
isolate an amplifier from a reactive loudspeaker and thus improve the performance. In
general, however, we are looking for amplifiers with low output impedance and cables
having low series impedance.
VERY HIGH FREQUENCY TESTS
Ordinarily a discussion of loudspeaker cables would stop here, at 100kHz, where we
could safely say that the performance is becoming negligible, if for no other reason than
that we cannot hear this frequency (a concept disputed by some audiophiles). However, the
advent of wide bandwidth power amplifiers has demonstrated other new effects; several
amplifier designs (stable with reactive loads such as capacitors) oscillate into low
inductance cables with a variety of results. Threshold Stax, and Electro- Research designs
behave violently, while others acquire oscillation-caused colorations, usually either a
hard, etched, high end or warmth and thickness in the vocal range (due to low order
intermodulation sidebands and harmonics).
Clearly things are happening above 100kHz; to display them I performed two tests on the
cable samples. In the first I swept the frequency output of a 1/4ohm resistive source from
100kHz to 40MHz while measuring the voltage at the other end of the cable with various
load impedances. In the second test, I used the same source to send a 5µS pulse down the
cable for viewing at the other end. (see Fig. 7).
Fig. 8 displays a condensation of over 50 photos
showing the test system's response to each variety of cable and load. The unterminated
"no load" series shows the equivalent performance of loudspeakers with a
substantial series inductive component, while effects of resistive (8ohm) and capacitive
(.047uF) loading appear in the following two vertical columns. In the frequency response,
we see large peaks occurring in the 1-10MHz region. Remembering that we sent 50mV down the
cable from the oscillator, it is surprising to discover amplitudes as high as 5V on the
other endÄan apparent gain of 1001 As shown by the ringing in the pulse waveforms, this
highly resonant condition occurs with every variety of cable, but at different frequencies
and "Q" factors depending on the source impedance, the load impedance, and the
type and length of cable.
The explanation for this resonance is reasonably simple if we consider that it takes a
certain amount of time for the signal to travel down the cable. A wave's velocity
proportional to its "characteristic impedance," Z01 a value expressed in ohms
and determined by the inductance and capacitance:
When a wave travelling down a length of cable reaches the end of the cable, it will do
one of three things depending on the impedance of the load. If there is a high impedance
load, so that ZL, > Zc, the load will reflect energy positively back down the cable to
reappear at the source (Fig. 9). If the load impedance
is less than the characteristic impedance of the cable, the wave is reflected back
negatively; and if ZL = Zo, then the wave is fully absorbed and none is reflected.
This mismatch of load impedance to cable impedance causes the resonance observed in Fig. 8, which we see diminish in Polk and Mogami cables
when they are loaded with 8 ohms, a value near their characteristic impedance. By
contrast, twin lead conductors have a higher characteristic impedance and perform better
at multimegaHertz frequencies with about so ohms load impedance. The effect of twin lead
cable on a 5uS pulse with an 8 ohm load shows the effect of a load impedance lower than
Zo, where the cable inductance cable rolls off the edges of the pulse, but where the 8 ohm
resistance is sufficient to damp the ringing which occurs with Z. = 0 or ZL = infinity .
Not so for capacitive loads as shown in the fourth column where another resonance
altogether has developed due to the inductance of the cable and the capacitance of the
Note that these effects exist with all cables. The fact that only the newer, low
inductance cables appear to affect amplifier stability brings us to a point which
justifies our examination of a cable's performance in regions which are simply not
audible. The lower resonant frequencies of the cables having low Zo enter into the output
bandwidth of the amplifier as it approaches its unity loop gain, and by altering its phase
response cause oscillation at the resonant frequency.
Earlier amplifiers as well as some currently available, having slower output stages
(less than 1MHz), did not interact with these resonances because they occurred above the
cutoff frequency of the active devices by an order of magnitude. However, as the newer
cables decreased impedances and as amplifier output stages increased in bandwidth beyond
5MHz, the two effects met and resulted in various forms of sonic problems, fuse blowing,
and worse. At Threshold we first ran across the problem with the mating of our 400A and
Polk sound cables which caused fuse blowing (due to oscillatory cross-current conduction)
with great regularity.
After a period of confusion, Matt Polk and I realized independently that the lack of a
characteristic termination was causing the problem. Polk developed and patented a
"damper" consisting of a .047uF capacitor and 6 ohm resistor in series placed
across the loudspeaker, while I used the same network but with .1uF and 5 ohms. The
results of this network are seen in Fig. 8 where the
resonance in the pulsed waveform is damped out, restoring stability to an otherwise
oscillating amplifier. Since Polk's commercial introduction of the damper circuit we have
found it cures oscillation problems caused by the other exotic low inductance cables. It
is necessary whenever a reasonably long length (>3 feet) of low inductance cable is
mated with any wide bandwidth amplifier. It interacts unfavorably with twin lead
conductors (Fig. 8) which require higher impedance values (say, .01uF, 60(2); however,
twin lead's higher characteristic impedance and resonant frequencies are in any case
unlikely to induce oscillation in amplifiers now available.
With this much information, we might think we have the subject nailed down. However, we
could easily install the finest amplifiers, cables, and terminating impedances and achieve
100 times the distortion of the amplifier alone. Loose, dirty, or oxidized connections
can, while measuring well with an ohmmeter, cause high amounts of harmonic and
intermodulation distortion. When high distortion occurs during an amplifier checkout at
Threshold one of the first things we do is replace or tighten the cable from the amplifier
to the load; we have thus cured many "defective" amplifiers.
Copper and aluminum oxidize quickly and oils from our fingers find their way to the
conductor surfaces, causing poor contact; so on more than one occasion the dramatic
improvement provided by an exotic cable has merely demonstrated the extremely poor quality
of the previous cable's long neglected connections. Wire connections can age, and anyone
wishing to accurately evaluate the newer cable's improved quality should first renew the
contacts on his current set. Banana plugs and five-way binding posts make excellent
connectors as long as they are kept clean; however, while the connector's plated surface
resists corrosion, the wire to the connector interface can become bad and should be
periodically checked, especially if it is subject to motion.
At this point many audiophiles are wondering, "Where are the listening
tests?" I have listened to these cables on a variety of amplifiers (mostly my own)
and loudspeakers, including Magneplanar Tympani 1 D's, MG II A's, modified Dayton Wright
XG 8 MK III's (as shown in Fig. 6) Cabasses; I have also
heard some examples on Dahlquist and Snell loudspeakers.
Frankly, I found it difficult to assess the results except at the extremes of
performance. For 10 foot lengths with properly terminated cables and speakers with
inductive high frequency characteristics, the differences between low inductance cable and
twin conductor are extremely subtle and subject to question. With a low output inductance
amplifier and a Heil tweeter (whose impedance is a nearly perfect 6ohm resistive) the
difference was discernible as a slightly but not unpleasant softening of the highest
frequencies. Fulton or Monster cables were a clear improvement over 24 or even 18 gauge,
though a little less subtle than I would have expected, leading me to believe that the
effort associated with heavier cables pays off in bass response and in apparent midrange
definition, especially at crossover frequencies. The worst case load, the modified Dayton
Wright electrostatics, presented some interesting paradoxes: the extremely low impedance
involved showed the greatest differences between all the types of cables. However, the
best sound cables were not necessarily electrically the best because several amplifiers
preferred the highest resistance cable. In one case, I had to use 24 gauge cable to
prevent tripping the amplifier's protection circuitry.
Who am I to dispute the feelings of audiophiles who, evaluating any cable in the
context of program source, amplifier, speaker, and listening room, decide they can hear
the difference? A few guidelines have emerged here, but the final judgment belongs to the
user. All the special cables mentioned worked well on the test bench and, given the
assumption that series impedance should be minimized, all of them work better than 16
gauge wire. If, like many audiophiles, you have spent a small (or large) fortune on your
hi-fi system, money spent for high quality cables and connectors is a reasonable
a. Fulton Speaker Cable. Fulton Musical Industries, 4428 Zane Ave. North, Minneapolis,
b. Mogami Speaker Cable. At press time we are unable to locate a manufacturer or
distributor for Mogami.
c. Polk Autio Sound Cable. Polk Audio, 1205 South Carey Street, Baltimore, MD 21230.
d. Ultra High Definition Speaker Cable. Audio Source, 1185 Chess Drive, Foster City, CA
2. Hiraga, Jean, "Can We Hear Connecting Wires?", Hi Fi News and Record
Review, August 1977.
3. Retsoff, Alexander, "Retsoff's Remedies ," Stereo, Fall, 1978.
4. Klipsch, Paul, "Speaker Wire," Dope From Hope, Sept. 13, 1978.
5. Pass, Nelson, "Loudspeaker Damping," Audio Magazine,
6. Gailus, Mark, "Speaker Wire: What Size is Sufficient?", Audio Forum,
7. Greinier, R. A., "Another Look At Speaker Cables," Speaker, The Boston
Audio Society, Dec. 1978.
8. Gross, T. A. G., "Multistrand Litz Wire Adds "Skin" To Cut AC Losses
in Switching Power Supplies," Electronic Design, Feb. 1, 1979.