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current moving cables

Helen Bach

if it ain't Baroque ...
I was watching a programme with Guy Martin, who trying to get a battery powered car (VW Beetle) do go rather quickly! As a safety check, the team decided to deliberately set fire to a car (NOT his VW) by push high current into the battery/circuits.

As he turned the on switch, the cables (several of them) jumped. Just wondering if other cables do the same, in an audio situation!
 
I was watching a programme with Guy Martin, who trying to get a battery powered car (VW Beetle) do go rather quickly! As a safety check, the team decided to deliberately set fire to a car (NOT his VW) by push high current into the battery/circuits.

As he turned the on switch, the cables (several of them) jumped. Just wondering if other cables do the same, in an audio situation!
Yes they do.

I'm not instantly sure it's exactly the same physics, but in a zipwire loudspeaker cable the current flowing out in one wire and the return current in the other parallel wire give rise to Lorentz forces that alternately pull the conductors apart and pull them together as the signal goes positive and negative. The dielectric between the conductors is not infinitely stiff so there is a small deformation of the cable in time with the signal.

This is very small, but it does modulate the cable's inductance and capacitance a very small amount, causing second harmonic distortion. But the amount is so small you will never hear it (even with the very lowest distortion loudspeaker drivers you can find).
 
Just wondering if other cables do the same, in an audio situation!
There's clearly a gap in the cable market here, we need a solution for 1 V / x mA interconnects to prevent them from jumping which will have a negative impact on audio quality. Or maybe they jump in time to the beat?
 
IIRC, the force in a pair of parallel conductors carrying the same current in the same direction is attractive, so where this current is opposite directions it is repulsive. However the elasticity of the insulator will of course pull them back together so that as @John Phillips said you get second order distortion as the currents are always in opposition.
Just checked and it is 0.2 micro Newtons per meter length at 1A and 1m separation. This is used as the SI definition of current. At 2mm separation this would be 0.1 mN per amp per meter length. Not much. I wonder if the effect described is more due to a reaction to a very high current in the stray fields we get from ring main circuits?
 
I've seen cables move in industry
We had a look at a three phase electric arc furnace when we were apprentices
Very impressive
 
Yes they do.

I'm not instantly sure it's exactly the same physics, but in a zipwire loudspeaker cable the current flowing out in one wire and the return current in the other parallel wire give rise to Lorentz forces that alternately pull the conductors apart and pull them together as the signal goes positive and negative. The dielectric between the conductors is not infinitely stiff so there is a small deformation of the cable in time with the signal.

This is very small, but it does modulate the cable's inductance and capacitance a very small amount, causing second harmonic distortion. But the amount is so small you will never hear it (even with the very lowest distortion loudspeaker drivers you can find).

Oh great, thanks for that, now Audio-bloody-quest will be marketing anti-jump cable technology.

:D
 
Yes but only when no ones in the room.... this is why it's never been reported on... ever....

I've seen it in industry with like 1000A and cables jumping a couple of inches
I've seen the cable between the power unit and flash head on studio photo gear twitch. The current would have been in the vicinity of 1 kA (for a millisecond or so).
 
Not in audio, but if you're trying to blow a 5kA fuse they do:

Watched a 130ka ( 130.000mp fuse ) under 300ka 300,000 amp fault conditions almost blow up a building once in a test Lab
cables/busbars where huge & in slow motion flexed like a snake then turned into gas
found out later the source generator size of a block of flats some idiot had connected the wrong output breakers should have been 690v not 13,800volts
this is what 65ka looks like

65kA Arc Fault test - YouTube
 
Oh great, thanks for that, now Audio-bloody-quest will be marketing anti-jump cable technology.

:D
Yes - that line of thought had occurred to me too :D.

Actually I saw the idea already mooted a very long time ago. I did the sums to see if the effect could reach audibility (I still have the notes). It's real but nowhere near audibility even with the lowest impedance, lowest efficiency 'speakers and the softest PVC insulation on the loudspeaker cables. I know that truth won't stop the determined marketing, though :(.
 
Yes - that line of thought had occurred to me too :D.

Actually I saw the idea already mooted a very long time ago. I did the sums to see if the effect could reach audibility (I still have the notes). It's real but nowhere near audibility even with the lowest impedance, lowest efficiency 'speakers and the softest PVC insulation on the loudspeaker cables. I know that truth won't stop the determined marketing, though :(.
I calculated this through many years ago as a possible justification for biwiring.
It turns out that the effects and energy stored in the cable are insignificant.

Watch car jump leads leap when you engage the starter
 
This is very small, but it does modulate the cable's inductance and capacitance a very small amount, causing second harmonic distortion.

Why would a change in passive L and C cause 2nd harmonic distortion? It wouldn't suddenly introduce a non-linearity into the cable.
 
Yes, the cable's behaviour would nominally become non-linear in a complex way. Consider the simpler example of fitting a varactor at the output end of a cable that has a non-zero series impedance. The nonlinearity of the shunt C alters the voltage at the speaker terminals and the current drawn by the speaker.

BTW the biggest arc I've experienced was when someone was working on a 1 MV Marx Bank switch. (Think that's the correct spelling.) This was for a high power pulsed radar. The Marx Bank was in a tank of insulating oil. But they built and tested it in a lab where the ceiling wasn't high enough. So when the switched the bank to series it arced into the floor beams in the ceiling. It also generated smaller arcs between the taps of the sink next door. 8-]

They moved it to a bigger room before trying it again...
 
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Largest fuse I’ve seen blow was a big spanner accidentally dropped across terminals of a fork lift battery pack - a big whump as handle went cherry red and exploded, it caused the whole forklift to shake.
 
"The change in L and C would vary with the signal, I think?"

Yes, by small amounts but that wouldn't/shouldn't introduce a non-linearity, just a change in frequncy response. Generation of harmonic distortion requires a non-linearity, such as ferrite in an inductor, or an active device.
 
Yes, the cable's behaviour would nominally become non-linear in a complex way. Consider the simpler example of fitting a varactor at the output end of a cable that has a non-zero series impedance. The nonlinearity of the shunt C alters the voltage at the speaker terminals and the current drawn by the speaker.

? Well adding varactors will cause non-linearities because they are non-linear devices, but nobody's going to add a varactor to an audio cable.
 
? Well adding varactors will cause non-linearities because they are non-linear devices, but nobody's going to add a varactor to an audio cable.

If the force alters the distance between two conductors it will alter the capacitance between them. If the series impedance (mostly resistance but also inductance) to the charged parts is non-zero the result becomes a current-dependent potential divider. Akin to a varactor fed though a resistor/inductor. Nonlinear components can then arise.

Similar argument applied for variable inductors.

Although for a cable where both vary you might hit a special case where the two effect cancel.

In practice, though we're talking MOOM (Mountains Out Of Molehills) as the effect may be real, but far too small to care about in practice.
 


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