Why is high capacitance good?

Thanks for the info Al.

Hi Pani,

What would be best, of course, is an analysis reflecting the particular component impedances, cable lengths, and cable parameters that are being considered. But that is probably impractical to cover here in a generally applicable way. And if you were to present your question about how low is low to ten different engineers, probably no two sets of numbers would be the same. But as a rough general ballpark I would say the following:

For interconnects, where capacitance is particularly important:

Low capacitance: Below 25 pf/ft (picofarads per foot)
Medium capacitance: 25 to 75 pf/ft
High capacitance: Above 75 pf/ft

For speaker cables, where resistance and inductance are particularly important:

Low resistance: Below 12 gauge (lower gauge = thicker)
Medium resistance: 12 to 16 gauge
High resistance: Above 16 gauge

See a wire gauge table, such as this one, to convert gauge to resistance, if necessary for analysis purposes.

Low inductance: Below 100 nH/ft-run (nanoHenries per foot-run, representing two feet of wire for the round-trip through both conductors)
Medium inductance: 100 to 500 nH/ft-run
High inductance: Above 500 nH/ft-run

Best regards,
-- Al

Thanks for the world of info you have provided Al, very rarely do I come across such meticulously written technical posts.

Just one last question, could you please suggest what range of figures of Capacitance, Inductance and Resistance do you consider "low" for speaker cables and interconnects ?
I dont want to ask the manufacturer of my cables to answer these questions because he will try to always respond in a positive manner :).

Thanks, Pani!

As you probably realize, there is no one right answer to your questions, because there are many system-dependent variables involved. And as you've probably seen, there have been lots of inconclusive debates on those questions, here and elsewhere. But following are some general guidelines, as I see it.

First, keep in mind that overall system synergy can sometimes be best served by cabling that functions in a non-neutral manner. But assuming that neutrality is the goal:

1)The likelihood that long interconnects will function in a neutral manner is increased if:

-- The interface is balanced (xlr's).
-- Cable capacitance is low.
-- The output impedance of the component driving the cable is low, particularly at high frequencies. (Having low output impedance at low and mid frequencies, as well as at high frequencies, can be important in relation to the input impedance of the destination component, but that is a separate issue).
-- The resistance of the cable shield (or other signal return connection) is low, particularly for rca cables. That is something that is not usually specified, but figures to have some degree of correlation with the overall quality of the cable.

2)The likelihood that long speaker cables will function in a neutral manner is increased if:

-- Resistance is low.
-- Inductance is low.
-- Speaker impedance is high.
-- The speaker is capable of sounding good with an amplifier that does not have a high damping factor. A speaker which is tube amp friendly will generally meet that criterion, irrespective of whether it is actually being used with a tube amp.

Best regards,
-- Al

Also, how long an IC is okay to use in a decent hi-rez setup ?

Al, I have heard that it is always best to keep the speaker cables as short as possible even if it means one has to use longer interconnects to get the power amp closer to the speaker. What is your take on this ? I always thought since IC carries low level signal it should be kept short (especially RCA based IC), but the general thought is opposite. I am sure there must be a scientific explanation for this as well. Please shed some light.

Superb post Al.
Learnt a lot.

Hi capacitance cables may be used to 'tune' a too bright system.

That is true in the case of interconnects, especially if the output impedance of the component driving the cable is high. It is not true, though, for speaker cables, at least to the extent that cable effects are explainable by generally recognized science (and assuming that the cable does not include a network box of unknown design).

The reason that interconnect cable capacitance can roll off high frequencies is that it interacts with the output impedance of the component driving the cable to form an RC low pass filter (the "R" term corresponding to the output impedance, and the "C" to the total capacitance of the cable). The 3-db bandwidth of the filter (the frequency at which the signal is attenuated by 3db relative to its amplitude at low frequencies) equals 1/(2 x pi x R x C).

In the case of a power amplifier, output impedance is near zero (a small fraction of an ohm for solid state amps, and at most a few ohms for nearly all tube amps), so that effect will not be significant for audible frequencies.

High inductance in a speaker cable, on the other hand, can reduce brightness to some degree, by introducing an impedance that rises at high frequencies and that is in series with the speaker impedance. The inductance of even an average cable can have that effect, if cable length is long and speaker impedance at high frequencies is low.

Best regards,
-- Al

Hi capacitance cables may be used to 'tune' a too bright system. Too bright a system may be fixed in other ways.

I think that's a bad approach. I personally try to find low capacitance cables, when such spec's are known.
Many cable companies don't advertise such things, do they?

Hi Keith,

RF transmission line effects, including characteristic impedance and vswr/reflection effects due to impedance mismatches, become significant when the length of the cable becomes a "significant" fraction of the wavelength of the signal (which is inversely proportional to frequency).

What is "significant" is a matter of degree, of course, and depends on the particular application, and for audio it would seem appropriate to define tolerances more tightly than for most other applications.

But to provide some perspective, the wavelength of a 20,000Hz signal propagating through a wire is in the rough vicinity of 6 miles. The wavelength of a 20Hz signal propagating through a wire is in the rough vicinity of 6,000 miles. The length of a typical speaker cable would certainly seem to be utterly insignificant in relation to those numbers.

The long-wave radio band is commonly thought of as beginning at around 150kHz, so it might be reasonable to consider some point between say 50 and 150kHz as a conservatively drawn point of demarcation between rf and ultrasonic frequencies. Long-wave transmitting antennas, btw, are HUGE, due to the long wavelengths they are transmitting.

DIGITAL audio signals, btw, have frequency components that extend well into the rf range, and rf transmission line issues are therefore very much applicable to connections between transports or other digital sources and dacs.

Best regards,
-- Al

Al- thanks for the detailed (and fascinating) response. How high are radio frequencies that you mention? Some of these particular cables may actually attenuate high frequencies (MIT with Wilson seems to be a popular combination because of it) so perhaps you are right.

KeithR

To get rid of any negative effects of capacitance, inductance etc - Spectron Audio use "Remote Sense" speaker cables, see - http://spectronaudio.com/cables.htm.

Basically they take full advantage of their ultrafast feedback loop and include speaker cable into it - making amp/speaker the single amplification system.

Does anybody else use the same concept?

Thank you

In itself, high capacitance in a speaker cable is not good. And with some amplifier designs, especially marginal ones, it can lead to poor sound, oscillations, or even damage, unless a Zobel Network is used to compensate.

However, high capacitance can be a side effect of designing the cable to have ultra-low inductance. Low inductance being likely to improve the neutrality of the cable, everything else being equal, although lowering it beyond a certain point will be unnecessary overkill.

High capacitance can also be the intended result of designing the cable to have extremely low characteristic impedance, characteristic impedance being approximately equal to the (square root of (inductance per unit length divided by capacitance per unit length)).

Some speaker cables are marketed based on the theory that their characteristic impedance should approximately match the speaker's impedance. Although I don't question that those cables can and do provide excellent performance in many systems, imho that theory makes little sense, and imho that theory is not what accounts for their performance. I say that for two reasons:

1)For any remotely reasonable cable lengths, characteristic impedance is a concept that is only applicable at frequencies in the radio frequency range, or that at least approach being in the rf range. At those frequencies, the cable's characteristic impedance should closely match the impedance presented by the load, or some of the incoming energy will not be absorbed by the load and will be reflected back toward the source, creating what are known as vswr (voltage standing wave ratio) problems.

2)At the high frequencies at which characteristic impedance may begin to become significant, speaker impedance will be much different than it is in the audio range.

I calculate based on the published capacitance and inductance numbers for the particular cable you referred to that its characteristic impedance is around 5 ohms, right in the vicinity of typical speaker impedances at audio frequencies.

For more conventionally designed cables that figure would typically be in the rough ballpark of 75 or 100 ohms. Which, interestingly, is probably a much better match to the impedance of typical dynamic speakers at frequencies that are in the lower part of the rf range, where the effects of characteristic impedance may start to become significant. Tweeter voice-coil inductance will cause the speaker impedance to rise substantially at those frequencies.

That said, based on user comments many of those cables do give excellent results in many systems. And the combination of inductance which is low to the point of overkill, with capacitance that is unusually high, would certainly seem likely to make them sound DIFFERENT than more conventionally designed cables.

Best regards,
-- Al

Well, you certainly don't want higher capacitance with longer cable runs. Higher cable capacitance will make the load values on either end more critical to proper matching. This is why folks recommend as low a capacitance as possible with long cable runs.

I find that the cables I've tried with high capacitance usually sound warm, bloated and slow. However, I'm sure that there are systems where these cables work.

Just a guess.

Do you mean that higher capacitance equates to lower resistance?

If so.........wrong.

.

Higher capacitance/lower resistance.