Cable nonsense -- article #3
John Dunlavy
Thanks to all who responded to my original posting concerning
audiophile cables and their audible/measurable properties.
Since some of the responses seemed to convey a discordant position,
perhaps a more detailed exploration of the issues is justified. A good
beginning might be to examine the issues that separate those whose
opinions are based mainly (or entirely) on subjective grounds (perhaps
from poorly controlled listening evaluations) from those who favor an
objective approach based upon correlating relevant measurements with
the findings of "blind", "double-blind" or other types of
properly-controlled listening comparisons.
To begin, I would like to make clear that I do not believe that a set
of cable measurements, taken alone, can consistently and reliably
predict how one cable will sound when compared to another cable,
without considering relevant "system interface parameters". This is
because the interaction between the electrical properties of a cable
and the input/output impedances (and other properties) of typical
audio equipment/components being connected by the cables are an
integral part of the overall performance equation. Thus, a full and
accurate set of measurements is only relevant if interpreted in the
context of such system interactions.
Given such interpretation, measurements can provide an important, if
not indispensable, guide as to the potential performance of a given
cable within a given system. To say otherwise is to acknowledge an
incomplete grasp of present-day measurement technology and the ability
of credible engineering knowledge/expertise to fully define and
accurately assess all of the relevant properties that affect the
performance of cables within an audio system. Despite the
pontificating of some individuals to the contrary, well-known laws of
physics and principles of engineering are fully adequate to meet the
challenge. (A Nobel nomination awaits anyone who discovers and
adequately identifies a property that proves otherwise!) The notion
that "physics lies", expressed in a recent magazine editorial, is
absolute hogwash!
Most "seemingly" unexplainable, yet truly audible differences between
cables, can be explained if critically examined with respect to
equipment interface considerations. For example, a well-designed,
low-loss loudspeaker cable (with a relatively-low
characteristic-impedance of perhaps 6 to 8 Ohms) can cause many
expensive, well-regarded power-amps (with a slew-rate exceeding
stability limits created by an improperly designed inverse-feedback
loop) to oscillate at frequencies well above the audio range. This is
sometimes audible as a low-level, high-frequency "crackling noise"
(usually emitted by the tweeter as it's voice-coil is being
cooked). Such amplifier instabilities may also alter the "sound" of
the amplifier by creating an "edgy" quality on musical transients or
an exaggeration of high-frequency notes, etc.. But the amplifier, in
this case, is at fault - not the loudspeaker cable.
Unfortunately, this is the reason many audiophiles avoid using
high-performance cables. Yet, a simple "Zobel" network (typically a
6.8 Ohm resistor in series with a 4.7 uF capacitor) in parallel with
the loudspeaker end of the cable can almost always cure the
problem. (A multi-turn coil of 20 AWG wire wound around a 6.8 Ohm, 1
watt resistor, connected in series with the amplifier output
terminals, will usually accomplish the same thing!)
However, while low-loss, low-impedance loudspeaker cables are
technically the ideal choice, from a purely academic point-of view,
most loudspeaker cables are quite short with respect to a wavelength
within the audio spectrum, diminishing the effects of "standing-waves"
and "reflections" that would normally be of concern at frequencies
well above the audio spectrum. But low-impedance low-loss loudspeaker
cables, represent the technical and deserve serious consideration
where "ultimate accuracy" is the goal!
With respect to identifying the cause of audible differences between
some interconnect cables, excessive capacitance is usually the
villain. This is true because transistor output stages of pre-amps, CD
players, etc. are frequently "load-sensitive", especially with respect
to excessive capacitance. This is also true of some single-ended tube
types. Thus, an interconnect cable with a relatively high capacitance
(exceeding 20-30 pF per foot) can often cause some equipment to
exhibit non-linear properties at higher frequencies and/or higher
output levels, resulting in audible levels of distortion. But again,
the cable is not always to blame, although no good engineering reasons
exist for not designing an interconnect cable with a suitably low
capacitance, e.g., below 10-15 pF/ft. However, some of the most
expensive interconnect cables, with a high-tech appearance, exhibit
measured capacitance exceeding 75 pF/ft. while some of the least
expensive ones clock-in at only 12-15 pF/ft. (We believed the problem
sufficiently important to justify the development of an interconnect
cable with a capacitance of only about 8-10 pF/ft.)
Thus, I sincerely hope that the above explanations help to explain why
measurements alone may not always fully explain the differences heard
between cables - without taking into consideration the interactions
between cables and the proclivities exhibited by the output stages of
some amplifiers, etc.. However, accurate measurements, properly made
and interpreted, can almost always predict how a given cable will
react within a given system, taking into account all of the
"interface" considerations that must be evaluated. Therefore,
measurements can be an invaluable design tool when properly
interpreted by a competent engineer seeking optimum performance from a
cable or a system.
So what about subjective listening comparisons for evaluating
"audible" differences between cables? Well, I will once again state
my belief that the "placebo effect is alive and well" and that
listening comparisons are virtually useless unless significant
differences exist and/or proper controls are employed! I base this
belief on a considerable number of carefully conducted and critically
analyzed comparisons between different cables over the past 20-plus
years. Initially, I and my staff fully expected to observe audible
differences - which we did, in the absence of proper and sensible
controls. But in virtually every instance, when controls were
instituted, the differences thought to be easily heard and identified,
either totally disappeared or closely approached the level predicted
by "chance". Yes, we have frequently consulted psychologists and
other experts familiar with "audibility testing" in devising
procedures and controls for our comparison evaluations, etc. But the
results we have obtained have always been consistent: we have simply
not been able to identify any audible artifacts that could not be
explained by a critical examination of the equipment, components,
etc., coupled with an analysis of their interactions --- period!
Keep up the questions - we all have a lot to learn!
Best regards,
John Dunlavy