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DESIGNER CABLES: a critical look

Designer interconnects for audio have, in many ways, become something of a cult industry in recent years. Many golden ear audiophiles claim to hear audible 8/01/1998 8:00 AM Eastern

Designer interconnects for audio have, in many ways, become something of acult industry in recent years. Many golden ear audiophiles claim to hearaudible differences between ordinary cables and those of exoticconstruction and price. Sometimes the listeners can confirm the differencesin controlled double-blind listening tests, but more often, they cannot. Insome cases, big differences that were described in great detail werereported when listeners were led to believe they were comparing two cablesbut nothing was actually changed [1]. In other cases, listeners were unableto tell whether their own designer speaker cable or a piece of 16-gauge"zip cord" was in place while listening, without time limits, to programmaterial of their own choosing reproduced on their own (expensive) soundsystems [2]. Such double-blind tests are mainstays of real science and areused, for example, to separate the real from the imaginary effects of drugsin medical trials. When applied to audio, such tests prove that even highlytrained and experienced listeners can be strongly biased by their ownexpectations and beliefs.

Some audio experts believe audio is too important to be trusted totechnology, and consequently, they dismiss all scientific methods,including double-blind tests. This attitude, combined with the widespreadnotion that more expensive products must be better, has opened the door toa flood of marketing hype and misinformation. Promotional white papersabound with pseudo-science buzz words, theoretical explanations based onabsurd and fanciful physics, and new proprietary measurement techniquesreplete with previously unknown units of measure.

Simple interactions vs. solutions to nonexistent problems

When audible differences truly do exist, most are easily revealed witheither traditional frequency response or distortion tests and are theresult of simple interactions between the cable and the equipment's inputand output circuitry. At audio frequencies, the most significant parameterof unbalanced line level interconnect cable is its parallel capacitance,shown as a single equivalent capacitor C in Figure 1. For most ordinarycable, this is about 50 pF per foot. This capacitor and the line driver'soutput impedance Zo form a simple low pass filter (Zi has no significanteffect). Because output impedance Zo can be 1 kV or higher inconsumer/audiophile gear, long- and/or high-capacitance cables cansignificantly degrade high frequency response. The Zo of some vacuum tubeequipment is so high that it can drive only a few feet of typical cable.With a typical Zo of 1 kV and a 50 pF per foot (305 mm) cable, 20 kHzresponse will be -0.5 dB for 50 feet (15.2 m), -1.5 dB for 100 feet (30.4m), and -4 dB for 200 feet (60.8 m) of cable. Some designer cable has muchhigher capacitance and will produce these responses with much shortercables. For long cable runs, consider a low-capacitance cable, such asBelden #8241F. Although listed as a video cable, its 17 pF per footcapacitance allows about three times the lengths for the listed responses.It also has a heavy gauge, low-resistance shield, which minimizescommon-impedance coupling (more on this later).

At audio frequencies, speaker cables can be represented by seriesresistance R and inductance L, shown in Figure 2. The parallel capacitanceis normally insignificant. The Zo of a typical power amp is less than 0.1V, while speaker impedance Zs will vary with frequency but have a typicalminimum value of 5 V. The most significant effect of R and L is on thefrequency response, which is caused by the loudspeaker's impedancevariations. Generally, using a wire size that keeps R under 5% of Zs willhold response variations to less than 0.5 dB and have no significant effecton actual damping factor [3]. Depending upon cable length, the inductance Lof simple "zip cord" may introduce some measurable, but most likelyinaudible, response roll-off and phase shift at 20 kHz. However, inductancecan be made negligible by simply using 40 (or more) conductor flat computerribbon cable with alternate conductors paralleled at each end. In additionto being low in cost, this cable is ideal for routing under carpeting.

Some designer speaker cables have very high capacitance, creatinginstability problems for poorly designed power amps. Of course, there's anexpensive solution for this, too-cables with built-in LC or RC terminationnetworks. Properly designed power amps have these well-known "Zobel"networks (or their equivalents) inside where they belong in the first place.

Some cable manufacturers would have you believe that audio cables aretransmission lines, but in the engineering meaning, cables are nottransmission lines unless they are "electrically long" (about a quarterwavelength) at the highest frequency of interest. At 20 kHz, this is almosttwo miles (3.2 km)of cable. For audio cables less than 1,000 feet (305 m),transmission line effects are not an issue. But because of the much higherfrequencies involved, most video, RF and data cables are transmission linesand require termination. The response of an audio cable to nanosecondimpulses is irrelevant because audio signals contain no significant orintentional energy at those frequencies.

Then we have oxygen-free, high-conductivity, and linear-crystal copper andcombinations thereof. To the best of my knowledge, oxygen-free copper wasoriginally developed for the Navy to reduce flexure (work hardening)failures of cables. It had no special electrical properties until thedesigner cable people developed their speculative theories about thedifficulty of current flow in ordinary copper. I have never seen anyscientific evidence that the kind of metal used in audio cables makes anydifference beyond the expected (and usually negligible) differences inresistance. Long flex life, however, is a good thing, especially for suchapplications as headphone or patchbay cords.

Skin effect is another problem exaggerated by hype. As frequency increases,current flow in a wire tends to concentrate toward its outer surface,causing an increase in AC resistance. At 20 kHz, for example, skin depth is0.018 inch (0.46 mm), where 63% of current flow is between the surface andthis depth [4]. For line-level audio interconnects, the effect isabsolutely inconsequential even if center conductor resistance doubles ortriples, because it accounts for less than 0.01% of total circuitresistance anyway. For loudspeaker cable, there may be some measurableeffects because total circuit impedance is lower, but we're still talkingabout increases in resistances that, if the wire gauge is properly chosenfor reasonable losses, are negligible in the first place.

Noise coupling in unbalanced interconnect cables

The biggest real problem with unbalanced interconnects is that they fallvictim to common-impedance coupling. This coupling causes more than justaudible hum and buzz; the coupling of ultrasonic and RF interference causessubtle degradations of audio quality. This degradation mechanism has beenlargely ignored by the industry for many years. It's the "dirty littlesecret" of consumer (and "semi-pro") audio. This may also explain why thedesigner cable fad has relatively few followers in professional audio whereinterconnects are balanced.

Briefly, noise currents are coupled from the power line by each piece ofequipment and then flow through any wires that connect the two pieces ofequipment. As shown in Figure 3, the ground conductor or shield of anunbalanced interconnect becomes the path for the noise current. As thenoise current flows, it causes a noise voltage to be developed across thelength of the cable which adds directly to the signal at the receive end.The coupling is worsened by longer cables or those with higher shieldconductor resistance.

There's a widespread misconception that hum, buzz and other interferencearrives through the air and that it is picked up as a result of inadequatecable shielding. This idea results in a variety of cables with 100%coverage (foil) or double- and triple-shielded cables touted to solve noiseproblems. For the vast majority of systems, however, the most effective wayto reduce noise coupling is simply to choose cables with the lowest shieldresistance and keep them as short as possible. Although a designer cablemight slightly alter the nature of the coupling and its subsequent audioeffects, it cannot eliminate it. Power conditioning products, despite theirclaims, cannot overcome this inherent weakness of unbalanced signalinterconnects either. Only complete disconnection from the power line (aswith battery power) could eliminate the coupling. Where cables must belong, consider an audio transformer-based ground isolator, which caneffectively stop the interference/noise current coupling by preventingcurrent flow in the shield.

Ultrasonic noise coupling, bandwidth and spectral contamination

The audio signal degradation caused by ultrasonic and RF interferencecoupling is easier to explain than measure. Typically, it reveals itselfonly with rather sophisticated tests such as spectral contamination, whichwas proposed by the late Deane Jensen in a 1988 paper [5]. Basically, thistest applies a large number of simultaneous tones to a device under test.Any non-linearity in the device under test will create complexintermodulation products at new frequencies, collectively called spectralcontamination. Because the new frequencies are usually not harmonicallyrelated and appear only when audible signals are also present, they behavemore like distortions than noise. Generally, listeners describe the audioas "veiled," "grainy" or "lacking detail and ambience." I think suchdistortion tests may have the ability to correlate laboratory measurementsto real listener experiences.

Systems designed to produce subjectively pleasing and distinctivecoloration appropriately belong in the recording studio where, like musicalinstruments, they can be manipulated by artists and producers to generatethe desired effects. I believe, however, that the ultimate goal of a musicreproduction system is as much transparency and neutrality as science willallow. Further, real science requires skepticism, especially if anobservation suggests violation of well established physical law. Remembercold fusion?


References:

[1] John Dunlavy, "Cable Nonsense," letter, 5 Nov 1996 (available atwww.solutions.com/verber/cables.html).

[2] Tom Nousaine, "Wired Wisdom: The Great Chicago Cable Caper," test data(available at www.oakland.edu/~djcarlst/abx.htm)

[3] Richard Clark, "Amplifier Damping Factor," Autosound 2000 Tech Briefs,Nov 1994, pp 469-470.

[4] Ralph Morrison, "Solving Interference Problems in Electronics," JohnWiley & Sons, 1995, pp. 61-62.

[5] Deane Jensen & Gary Sokolich, "Spectral Contamination Measurement," AESConvention Nov 1988, Reprint #2725.



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