how much is too much resistance for a line level cable?

[MCH]

Hi, I got hold of a spool of UL 2547 2 conductors 24 awg shielded cable that i like very much because is just 3 mm total diameter and would like to use to in line level balanced connections.
However, i measure a resistance of 135 mOhm/meter, that is a bit too high for this type of cable. (I read ca. 90-95 mOhm in catalogs), what is not surprising due to the dubious origin of the cable, and makes me think that perhaps the conductor is actually aluminium (numbers match roughly). But is still less resistance than a real 26 awg pure copper cable.

Well, anyways, my question is, is this cable fit for purpose? would there be a maximum length or doesn't really matter? thanks.

 

[somebodyelse]

Line level output impedance is often of the order of 100R, and input impedance usually >=10kR so even at 135mR/m you'd need a long run (in domestic terms) for it to make a difference. How long are you looking at?

 

[AnalogSteph]

Yeah, resistance does not tend to be a dominant concern in this application. Capacitance and shielding or rather the lack thereof tend to be a greater interest.

 

[MCH]

Line level output impedance is often of the order of 100R, and input impedance usually >=10kR so even at 135mR/m you'd need a long run (in domestic terms) for it to make a difference. How long are you looking at?

thanks. Intended use is no more than, say, 5-7 meters

 

[Speedskater]

For RCA line level interconnects central conductors, resistance doesn't matter. The wire could be extremely small, but breakage would be a problem.
But for the shield, resistance does matter. Heavy braided shields will reduce power line related Common Impedance Coupling Noise currents (hum & buzz).

 

[MCH]

Yeah, resistance does not tend to be a dominant concern in this application. Capacitance and shielding or rather the lack thereof tend to be a greater interest.

shielding looks fine to me, much thicker than the cables and looks decently spreaded. resistance of the shielding is 50 mOhm/m, to give you an idea. Does one measure or calculate the capacitance? my LCR tells me 3.8 nF at 1kHz conductor to conductor for a 20 meter spool, but i wouldn't trust that measurement too much...

 

[MCH]

For RCA line level interconnects central conductors, resistance doesn't matter. The wire could be extremely small, but breakage would be a problem.
But for the shield, resistance does matter. Heavy braided shields will reduce power line related Common Impedance Coupling Noise currents (hum & buzz).

thanks. these would not be for RCA though, they are 2 conductor + shield and intended use is balanced interconnects. Shield resistance is 50 mOhm/m and looks decently spread, but i believe it is straight, not "woven" (not sure it is the right terminology, but i guess you know what i mean)

 

[AnalogSteph]

my LCR tells me 3.8 nF at 1kHz conductor to conductor for a 20 meter spool, but i wouldn't trust that measurement too much...

I mean, 190 pF/m still sounds realistic enough though a bit high even for TP... I've found mutual capacitance specs for CAT5e cable on the order of 40-60 pF/m, which in turn strikes me as quite low but obviously still possible. I'm used to (audio) TP rather being a bit higher than coax, maybe 100-150ish pF/m.

Either way, odds are it'll work fine.

 

[MCH]

I mean, 190 pF/m still sounds realistic enough though a bit high even for TP... I've found mutual capacitance specs for CAT5e cable on the order of 40-60 pF/m, which in turn strikes me as quite low but obviously still possible. I'm used to (audio) TP rather being a bit higher than coax, maybe 100-150ish pF/m.

Either way, odds are it'll work fine.

Yeah, assuming PVC insulation (will confirm soon) and measuring carefully all the diameters I calculated less than half of what I have measured. I don't really trust my measurement, basically because I don't know how to do it properly.

 

[AnalogSteph]

I don't really trust my measurement, basically because I don't know how to do it properly.

It shouldn't differ from measuring any old capacitor, really. At 1 kHz even a 20m cable is firmly in lumped element terrain. Attach probes to hot and cold wires (assuming the other end is open and not shorted to shield either) and let it do its thing. Got any competing opinions from a multimeter by any chance?

 

[MCH]

It shouldn't differ from measuring any old capacitor, really. At 1 kHz even a 20m cable is firmly in lumped element terrain. Attach probes to hot and cold wires (assuming the other end is open) and let it do its thing. Got any competing opinions from a multimeter by any chance?

Will try again. I used a cheap LCR tweezer that i trust for sub Ohm R measurements (checked with known values) but when it comes to capacitance.... well, i don't have anything to compare against. My even cheaper DMM doesn't go to down pico farads unfortunately. Will report back, thanks.

 

[AnalogSteph]

I used a cheap LCR tweezer that i trust for sub Ohm R measurements (checked with known values) but when it comes to capacitance.... well, i don't have anything to compare against.

So I guess you don't have some small ceramic disc caps or film caps in the 1-10 nF range floating about?

You can still try another frequency if the meter supports that. I don't think anything of a few nF should hit self-resonance any time soon.

 

[MCH]

So I guess you don't have some small ceramic disc caps or film caps in the 1-10 nF range floating about?

You can still try another frequency if the meter supports that. I don't think anything of a few nF should hit self-resonance any time soon.

Dumb question. Does it matter if the wire is in a coil? Because it is 20m of cable in the form of a relatively small coil (many turns). Remember the cable is shielded.

 

[DVDdoug]

Dumb question. Does it matter if the wire is in a coil? Because it is 20m of cable in the form of a relatively small coil (many turns). Remember the cable is shielded.

No.

In any case, it's not worth worrying about. I wouldn't expect it to make ANY audible difference in a controlled-blind listening test. Slightly moving your speakers, or changing your listening position, or opening/closing a door, window, or drapes, or somebody walking around in the room will have a bigger effect! Of course, a lot of other things can make even bigger changes to the sound.

I was once doing some experiments with high frequency tones and an SPL meter on a microphone stand. When I moved my head slightly the sound level changed drastically
as the left and right and direct and reflected waves went in-and-out of phase. That was expected but what surprised me was that moving around behind the SPL meter changed the readings by several dB!

Here's another quote from Floyd Toole:

I have dedicated my work to solving the "10dB" problems first, the 5dB problems next so on.

Capacitance can make a difference with a phono cartridge or guitar because they are high impedance and inductive.

 

[MCH]

No.

In any case, it's not worth worrying about. I wouldn't expect it to make ANY audible difference in a controlled-blind listening test. Slightly moving your speakers, or changing your listening position, or opening/closing a door, window, or drapes, or somebody walking around in the room will have a bigger effect! Of course, a lot of other things can make even bigger changes to the sound.

I was once doing some experiments with high frequency tones and an SPL meter on a microphone stand. When I moved my head slightly the sound level changed drastically
as the left and right and direct and reflected waves went in-and-out of phase. That was expected but what surprised me was that moving around behind the SPL meter changed the readings by several dB!

Here's another quote from Floyd Toole:


Capacitance can make a difference with a phono cartridge or guitar because they are high impedance and inductive.

Thanks, nah at this point is only curiosity. My concern about resistance got clarified in post #2.

 

[Speedskater]

Coiling a speaker or interconnect cable will make no difference.
Coiling a single wire can.
* * * * * * * * * *
For modern interconnect systems (low output resistance and high input resistance), total capacitance of reasonable length cables doesn't matter.
If the cables are one thousand feet long, then we can talk about capacitance.

Note that phono cartridge, some microphone and musical instrument cables are a different ball game.

 

[babadono]

oh.....this is for a balanced connection? It'll be fine. More important is what is the shield connecting to inside the pieces of equipment.

 

[Speedskater]

Actually from Audio Engineering Society standard AES 48, all cable shields should be attached to the metal chassis at their connector before proceeding inside the unit.

 

[babadono]

Actually from Audio Engineering Society standard AES 48, all cable shields should be attached to the metal chassis at their connector before proceeding inside the unit.

Yes but how many are?