Cable lifts

[auplater]

Originally posted by GVLEA
Andy you would love our house here in Vegas. I get Jacobs Ladder type sparks in my house touching anything remotely electrical. That is a tough job for a professional reviewer.

We have tried just about everything only to have it work so, so. I am always looking for answers to eliminate or certainly reduce the static electric shock around here. Our static would probably put a child to the ground :-).

Well, you know... as stated above...."What happens in Vegas,,,"


might try using a humidifier... works wonders during winter months here

[GVLEA]

Believe me we have tried the humidifier but the problem is that it cannot handle the size of the house or the room. Requires filling it every 20-30 minutes. Walking around barefoot keeps the shocks to a minimum but even that at times is not enough. Just a part of living in the desert SW .

[dmills]

Originally posted by HK26147
I have however, seen excess speaker wire coiled up, which for all practical purpose is an inductor and a invitation for a problem.
Or running speaker lines close and parallel to noise emitters instead of perpendicular. Wrapping speaker and/or light cable around metal truss also cause problems. [/B]

Sigh!

Speaker lines have equal and opposite currents flowing in the two conductors, there is no net magnetic field, so a coil does NOT act as an inductor as far as the signal driving the speaker is concerned. It may however act as a rather poor common mode choke.

The problem with coiling cables is that the inner layers are thermally insulated, not inductive heating (the heating is resistive not inductive, and the buried layer overheats).

Speaker lines can of course pick up RF and act as a route for it to get into the amplifier if the amp is poorly designed.

Lighting cables are similar with the exception of things like the single pole 4/0 used with Camlock and the like (Which are special cases). Dimmer output circuits can and do radiate, but if the audio system is set up correctly they will have no effect on the audio system.

Most hum and buzz is down to either unbalanced high impedance lines (Guitar cables) which are notoriously prone to pick up any hash going, or common impedance coupling where some clown of a 'designer' has dropped the ball when designing a balanced line input stage.

Regards, Dan.

[HK26147]

Speaker lines have equal and opposite currents flowing in the two conductors, there is no net magnetic field, so a coil does NOT act as an inductor as far as the signal driving the speaker is concerned. It may however act as a rather poor common mode choke.

I have seen the effects or coiling unshielded demonstrated on an oscilloscope, heard the consequence, and have also seen warnings issues against the practice in various light and sound manuals.
People I have worked with in stage/ sound and lighting are very careful about wiring practices in this regard

[dmills]

Odd..

I have ~20KW of Nexo Alpha E here, with some 20M long 8 core speakon leads, which are routinely coiled up and hung on the pin rail when the array is in the air (The amp racks are on the loading gallery), and I would have expected to notice something if there was any appreciable reactance there.

Operating impedance is ~2.7ohms for the mid/high packs which should be low enough to show up any problems, and the array does not sound in any way different with the cables coiled up on the gallery to what it does with the cables dropped over the side as a precursor to dropping the array.

You need to be a bit careful with using a scope to look at this sort of thing as unless the scope is set up with differential inputs, you can create a parallel earth path that introduces an effective common mode component into the measurement (And this can of course be effected by coiling the cable).

As to lighting, I have about 150KW worth in the roof (Mix of Par 64, Selecon Pacific and Selecon Rahma HP), not particularly big but I would have noticed if there was any worthwhile magnetic field being generated. As noted there are reasons to avoid coiling power cables due to the desire to avoid the derating needed for a group of cables, but I don't even see a theoretical way for inductive heating to occur in a standard power cable just because it is coiled (As noted, heavy singles are a little different).

I have heard about this from various people and read it in various books, but I think it is a myth based on a poor understanding of basic EM theory and completely misinterpreting the observed fact that a coiled up cable run near its free air rating will melt (Big surprise!).

Please enlighten me as to what you think the mechanism is?

Regards, Dan.

[HK26147]

I have heard about this from various people and read it in various books, but I think it is a myth...Please enlighten me as to what you think the mechanism is?

Mutual Inductance:
Beside the various handbooks on wiring practices that stress that crosstalk between cables carrying different types and levels of signals can be minimized by isolating the cables into separate groups for video, pulse, audio,control, data, and power; It is further recommended that audio be sub divided into low ( < -20dbm), middle ( -20 to +20dbm ) and high level (> +20dbm ). Straight, direct wiring practices reduce the inductance. In large installations running speaker lines in conduit may become necessary.
Calculations exist for self inductance of a single conductor, and inductance for any number of various parallel conductor combinations ( same circuit or separate circuit relationships ).
Including formulas derived on the assumption that, for an increasing current in one conductor, the direction of the induced EMF in the other conductor is the same as the direction of current in the first conductor. This is a transmission line, the basic geometry for energy transport, with distributed capacitance and inductance. The motion of electrical energy requires the presence of an electric field and a magnetic field.

The opinion about coiling wire is split among the home audio/ theater crowd. One interesting thing I noticed is the disclaimer that has been placed under the opinion; In the home context with cable lengths normal found therein ( ie less than 40 - 50' ).
This is not situation we are dealing with here. Home systems have a small fraction of the cabling and circuits involved compared to live work. Live work has multiple video, pulse, audio,control, data, and power circuits, in most cases hurriedly setup.
A properly implemented permanent setup in a theater is not likely to have the same common errors made by traveling bands. This includes coiling the excess cable and hanging it placing it next to dimmer pacs and on or worse wrapped around metal truss or other offenders to create mutual coupling. I have also seen it coiled up and laying on fluorescent ballasts with bad results.
Recommended practice for excess cable is to orient as as figure 8 as opposed to a coil for reduced inductance.
Syd

[auplater]

So now this thread has morphed into the "Idiot's Guide to Band Groupie/Road Crew Set-ups".

Maybe we should also consider trans-Atlantic telecommunication issues before we decide to hang our stereo cables across the room, eh?

You could shield your rig with a Faraday cage. maybe that would help... hmmn a new concept for high end audio.... Faradaic compensatory anti-chronoelectrophoretic cable stilts....

John L.

[HK26147]

So now this thread has morphed into the "Idiot's Guide to Band Groupie/Road Crew Set-ups".

That diversion took place back near post 20, but point taken.
BTW: I would prefer Practical or Handymans guide.
Darwin has a way of eliminating Idiots in Pro work via high voltage and heights

[dmills]

I am well aware of the issues with live work as I am the crew chief at a medium sized arts centre (Day job).

This is getting into serious thread drift!

Transmission line issues do not apply unless you are the phone company (or running something a great deal faster then analogue audio, but that is a whole other can of worms), as to mutual inductance, sure if it were not for the equal and opposite current flowing in he return leg of the speaker circuit it might be an issue, but the presence of that current renders it largely irrelevant.

I am aware of the separation requirements between the various classifications of signal, and by and large agree with them, especially where unknown gear is concerned (There is a lot of pin one problem infested crap out there, including some by folks that should know better).
See rather scary work by Neil Muncy on this subject.

The only major issue with speaker lines that I am aware of is that if the amp is poorly designed you can get RF picked up by the speaker line coupled back into the amplifier (A partial cure is to wind the speaker lines around a ferrite core so as to form a common mode choke (Most RF pickup is common mode). Coiling the stuff (provided it does not overheat due to resistive heating) is a non issue (figure of eight is often easier with the very heavy stuff because it avoids kinks).

Mutual coupling is a far bigger issue with the low level signals that make up the vast majority of the on stage wiring then it is with speaker circuits that are low impedance and very high level, fortunately almost all signal wiring is twisted pair (This is far more important then the screening), which provides substantial immunity.
In extreme cases starquad is actually better, but is normally not required.

One to watch is the output side of the inverter drives used for variable speed motors on truck and revolve winches, they can radiate like nothing else and I always wire these in SY cable (screened mains power cable), it is the only case where I do anything special WRT power cables.

[dmills]

Darwin has a way of eliminating Idiots in Pro work via high voltage and heights

Indeed!
The paperwork when one removes themselves from the gene pool gets tedious however.

Regards, Dan.