Speaker Cable Face Off 1
Speaker cables have become one of the least quantified and objectively analyzed audio products in the audio marketplace. Many cable vendors and forum cult hobbyists insist on the abandonment of measurements and engineering truths in favor of subjective listening tests in hopes that the listener will think they hear a difference, even if none exists, to justify their unproven claims and sometimes extravagant prices. Let's refer to these people as "cable soothsayers" since they seem pre-dispositioned against applicable engineering truths and proven sciences despite reliance at times on the names of these truths (IE. Skin effect, diode rectification, etc) as a mechanism for the alleged problem(s) and their corresponding solution(s). The belief set forth by these "cable soothsayers" that "you can't measure it" leaves open the avenue of convincing prospective buyers of the differences in product by pseudo-science, clever marketing ploys, and sometimes just blind faith. Audioholics.com on the other hand is not a faith based website, nor are we cable "cable soothsayers". We believe and cherish objective measurements and feel they are the first step in developing and/or analyzing a high fidelity product. If a product measures well, and the measurements are applicable to its corresponding fidelity, it will usually sound good, however the converse is usually not true unless one seeks inaccurate audio reproduction of music. With that in mind we have analyzed six different brands/models of speaker cables ranging from as little as $0.46 / ft to $55 / ft. Don't let price fool you however, as in many cases, cables especially, price doesn't always dictate quality.
The Original Monster and Monster Navajo, and Sound King are typical 12AWG stranded Zip Cords, while the Cobalt Cable is 10 AWG stranded. The Stealth Fine Ribbon is a flat ribbon silver cable with unpaired conductors individually encased in a quasi air dielectric with equivalent of 17-18 AWG (Note the manufacturer claims an equivalent of 14.5 AWG per cable pair, but we measured between 17-18 AWG as can be seen in the Rdc and Rs measurements in Table 1. I could only assume the manufacturer estimated their gauge spec based on a bi-wired configuration. Since their website doesn't explicitly state this, it is open for interpretation. The Stealth Premier II is basically a Litz construction with individually paralleled insulated strands for each unpaired conductor to yield an equivalence of about 7 AWG (Note: The manufacturer claims 4 AWG per pair which is about what we measured considering the inaccuracy of measuring small cable resistance of cables so low in DCR and associated length). According to the manufacturer, the Stealth Premier II has recently been revamped with more conductors per cable resulting in a slight lowering in gauge to 6AWG per conductor, or 3AWG per pair.
Cable Measurements Comparison
Tabulated below are the cable metrics of six different speaker cables.
Table 1: Measured RLC Parameters of Various Speaker Cables
Cable Metric Definitions
- Rdc - Commonly referred to DCR which is the series resistance of a cable at zero frequency.
- Rac - The resistive portion of the cables series resistance as a function of frequency due to skin effect.
- Rs - Total Series Resistance (mohms) measured tip to tip at one end of the cable while the other end is shorted. Note: Rs = Rac + Rdc (minus instrumentation inaccuracies identified below)
- Ls - Series Inductance (uH) measured tip to tip at one end of the cable while the other end is shorted.
- Cp - Parallel Capacitance (pF) measured tip to tip at one end of the cable while the other end is open circuited.
Cable Measurement Test Set-Up Notes
All of the above measurements were completed on a fully calibrated and certified, WAYNE KERR Model# 3260B Magnetics Analyzer. The 3260B was calibrated for two different bandwidths for greater accuracy (100KHz or less - for audio frequencies, All Frequencies - for greater than 100KHz measurements). The measurements and calibration process was repeated twice for consistency. All cable lengths were between 6 to 12 feet and divided by their length for a normalized per foot measurement. The Monster Cable and Sound King cables were unterminated (bare wire), while the Stealth cables were terminated with spades and the Cobalt cables were terminated with banana connectors. Special care was taken to ensure solid contact was made with these cables when all Rs, and Ls measurements were taken.
At low frequencies the results illustrate Rs being lower than Rdc, which is inaccurate, as Rs tends towards Rdc as frequency approaches zero or DC. The LCR measurement derives Rs from signal phase and amplitude, while a DC meter measures exactly what it is looking for, thus this discrepancy is likely due to a meter resolution issue, as the meter in AC mode does not sport the high accuracy it would in DC mode. The cable should ideally be modeled as multiple parallel resistors, and those resistors treated as a lumped element in series with an ideal inductor. Each resistor is a frequency dependent element, and the inner ones fall out as the frequency increases. It is important to note the difference in measuring techniques, and caution the reader not to attempt to derive any relationships with the two numbers, as the absolute accuracy between the methods has not been established. However, the rising trend of Rs vs frequency is indicative of increased Rac due to skin effect and should also be noted.
As we can see by the graph above, one of the most expensive cables (Stealth Fine Ribbon) in this comparison has the highest measured inductance. In fact, the manufacturer of this cable ships these cables and the Premier II two as separate loose conductors. The measurements above are based on keeping the two conductors as closely together as possible, using electrical tape every foot to ensure no open loop areas. When the conductors are left dangling on the floor like the manufacturer recommends, inductance measurements of these cables more than doubled! See tables 3 and 4 for the resultant impact this had on frequency and phase response.
The Cobalt Cables measured the lowest inductance out of all the cables in this comparison, despite the commonly held myth that the lower the gauge, the higher the inductance. Conductor spacing plays a significant role in the overall loop inductance as clearly evident in this example.
Notice how the inductance of these cables began to decrease slightly between 50 KHz to 100 KHz and continued to decrease linearly with rising frequency. This is a clear result of skin effect pushing the current density profile to the outer surface of the conductors and thus minimizing internal inductance.
The Original Monster Cable seemed to prematurely lower in inductance at around 20-30 KHz. "This premature breakpoint is most likely due to inter-strand copper chloride build up from to the PVC insulation, causing a non skin-effect based current profile variation as a result of magnetic field variation within the conductor bundle." (John Escallier)
Unfortunately my test sample had clear evidence of copper oxide and copper chloride corrosion as indicated by its browning green conductor color. Since most PVC insulation's outgas chlorides with time, and the time any cable spends on the shelf prior to use is uncontrolled, it was decided to use the existing, old sample. Characterizing the shift in electrical parameters being discussed here as a result of inter-strand chemistry changes falls outside the scope of this article, and will be considered for further evaluation. In any event, this did not appear to cause any significant measurable effects within the audio band, and is unlikely to result in any deleterious audible effects.
The bottom line about internal inductance vs frequency is "at infinite frequency, the internal portion of the wire has no field, hence, no energy stored, and no inductance." (John Escallier) For more information on cable inductance relationships, review our article on Calculating Cable Inductance of Twin Feeder Speaker Cables.