Would you change your amp selection knowing...?

Unsound, thank you kindly for the gracious response.  Cycles2, separating the two conductors as you describe will result in capacitance being very low, although not zero.  However, it will cause inductance to be considerably higher than if the conductors were in close proximity.  And in a speaker cable inductance is much more likely to be a significant factor than capacitance, aside from the situation that has been discussed in which cable capacitance is ultra-high and the amplifier is sensitive to that.

High inductance is particularly likely to have significant sonic consequences if speaker impedance at high frequencies is low (such as in the case of most electrostatic speakers) and cable length is long.  That is because the impedance presented by an inductance is directly proportional to frequency, and cable inductance is directly proportional to length.

Regards,
-- Al
 

@unsound
I found a link to the paper from which the excerpt shown at the link you provided was taken:

http://sound.whsites.net/cable-z.htm

My conclusion reading all of this is that I wouldn’t worry about his allegation that the resistor value chosen by Goertz is not ideal. As he even said, based on his simulations: (Comments shown in brackets are mine):

Even 100nF in series with 10 ohms restores the amplifier phase margin to normal.... 4.7 ohms is preferable, but the phase margin is barely affected. The speaker end response has a small ’lump’ with 10 ohms [between about 5 and 10 MHz!], and phase goes ’wobbly’ at above 20MHz. This is probably not a concern, and you will almost certainly get away with it.

I don’t, however, see any reason to doubt his statement that:

It is very evident that this particular cable [Goertz MI 1] should never be used without a Zobel at the speaker end....

Best regards,
-- Al

Hi Ian,

Thanks for the clarifications.

While Zo (denoting characteristic impedance) = ( L / C )^0.5 is an equation that is widely used in various EE applications, I don’t recall ever seeing a **simple** derivation of it. The Wikipedia writeup I referred to on Characteristic Impedance, in conjunction with the Telegrapher’s Equations writeup it links to, provides a derivation, although it is rather complex. Another derivation is shown at this link in the first answer to someone’s question.

Note that in both derivations the bottom line equation which includes series resistance R and shunt conductance G reduces to Zo = ( L / C )^0.5 when R and G are zero, and therefore to a close approximation of that equation when R and G are small enough to be negligible (on the same per unit length basis that is used for L and C). And under those circumstances Zo becomes essentially independent of frequency. Keep in mind also, as you probably realize, that characteristic impedance is essentially independent of length.

Best regards,
-- Al

@ieales

Hi Ian,

I’m afraid I have to question or disagree with several things in your analysis:

1)I’ll start with the least significant of the issues that I see. What length are you assuming in your calculation that resulted in 0.05 ohms at 1 kHz? Plugging the numbers for the particular cable into your methodology I find that the result at 1 kHz is almost completely dominated by resistance, with the result therefore being not much different than the cable’s resistance spec of 0.00098 ohms per foot (x2 conductors, presumably, although that isn’t made clear in the table).

2)Your equation "Z=1/(1/(ZL+ZR)+1/ZC) * Length" would reflect the parallel combination of (ZL + ZR) and (ZC), yet as you correctly state L and R are in series, while C is in parallel.

3)Related to that, specifically to the fact that L and R are in series, I don’t see the basis for your statement that "this impedance is in parallel with the amp and speaker." Certainly the amp is not being loaded with 0.05 ohms!

4)Most significantly, I believe you are conflating "impedance," derived as a combination of the individual impedances of R, L, and C at a given frequency, with "characteristic impedance," which is not the same thing.

I recognize that the two terms are sometimes used interchangeably, but that is incorrect and potentially confusing. (Even the heading in the Goertz table that I referred to makes that mistake, although the writeup above the table makes clear that they are referring to characteristic impedance). For example, a 75 ohm coaxial cable has a "characteristic impedance" of 75 ohms, but at most frequencies certainly does not have a 75 ohm "impedance" based on any series and/or parallel combination of the individual impedances of R, L, and C at each frequency.

"Characteristic impedance" is essentially independent of frequency, assuming, as I alluded to earlier, that conductor resistance per unit length and dielectric conductance per unit length would not affect a calculation based on the square root of (L/C) significantly. See the Wikipedia writeup on "Characteristic Impedance," which is consistent with my understanding of the matter.

Best regards,
-- Al

I’d like to see how Goertz calculates a Z of 4 or 8Ω at audio frequencies from their geometry.

Hi Ian,

The table near the bottom of the following page of their website indicates R, L, C, and Z for their various speaker cables:

http://www.bridgeportmagnetics.com/contents/en-us/d62_MI_AG_Speaker_Cables.html

As I’m sure you are aware, characteristic impedance can be calculated to a close approximation as the square root of (L/C), using those parameters on a per unit length basis and provided that conductor resistance per unit length and dielectric conductance per unit length are insignificant. The L and C values shown in the table appear to be consistent with the indicated characteristic impedances, which range from "~1.7" to "~4" ohms.

Best regards,
--Al

Kalali, that’s an excellent question, and I’ve wondered the same thing myself. While I’ve seen a number of reports over the years of solid state amplifiers self-destructing as a result of having to drive cables having ultra-high capacitance, I don’t think I’ve ever seen a report of an amp being damaged from having to drive an electrostatic speaker.

But while I’m not sure how to explain that, if I were to hazard a guess I’m thinking it may be related to the presence of the step-up transformer that I believe is used at the input of nearly all ESLs. Perhaps the bandwidth limitations and/or other characteristics of the transformer cause the amp to see a load impedance that is much less capacitive at ultrasonic and RF frequencies than it is at audible frequencies, and in comparison with the impedance of a highly capacitive cable at ultrasonic and RF frequencies.

And my suspicion is that the destructive oscillations which have been reported to result from the use of high capacitance cables typically occur at ultrasonic or RF frequencies, not at audible frequencies.

Also, I believe that the few ESL designs which don’t have a step-up transformer at their input, such as some older Acoustat models, have a built-in amplifier to step up the input voltage. In those cases presumably the built-in amp provides a relatively non-capacitive input impedance.

Best regards,
-- Al

Hi Steve (Williewonka),

I suspect that **many** high end solid state amplifiers are prone to adverse effects from speaker cables having ultra-high capacitance, at least if a Zobel network is not used with the cable. (My understanding, btw, is that if requested Goertz will supply such networks for use with their ultra-high capacitance cables).

In extreme cases those effects may include destructive oscillations, as you indicated and as several members here have reported experiencing. In less extreme cases, though, there may be subtle but significant adverse effects on sonics, including things like overshoot, ringing, or low level ultra-sonic or RF oscillations that are not directly perceivable as such. Whether or not such effects occur will depend in part on how much feedback the amp uses, and on its gain, bandwidth, and output impedance.

I’m somewhat surprised to see Ayre on your list of susceptible amps, btw, since most or all of their amps are zero feedback designs. In general I would expect lack of feedback to minimize or eliminate such sensitivity.

To answer your question, though, the possibility of this issue, whether mentioned by the manufacturer or not, would have no influence whatsoever on my selection of an amplifier. It would certainly influence my selection of a cable, though. A cable having extreme and /or unusual parameters would be a non-starter for me. And if as is often the case basic parameters such as inductance, capacitance, and resistance are not specified for a particular cable, and if the manufacturer can’t or won’t supply that information, and if a rough idea of these parameters can’t be inferred from the cable’s description, I would look elsewhere.

Best regards,
-- Al