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Author Topic: feeder cable under load induction hum  (Read 3662 times)

Don Lind

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feeder cable under load induction hum
« on: January 02, 2015, 08:36:30 pm »

I've always heard that feeder should not be wrapped in a coil when it going to be under load.
This I assume is because it will introduce hum in the sound system as well as heat.
Is this true?

I've understand figure 8 will negate that. Will the over under wrap also negate that?
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Mac Kerr

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Re: feeder cable under load induction hum
« Reply #1 on: January 02, 2015, 09:00:48 pm »

I've always heard that feeder should not be wrapped in a coil when it going to be under load.
This I assume is because it will introduce hum in the sound system as well as heat.
Is this true?

I've understand figure 8 will negate that. Will the over under wrap also negate that?

I don't know that figure 8 eliminates the magnetic field that causes hum, but over under does not. Over under is still a coil, it just has a twist introduced in the cable to counter the twist caused by coiling.

Mac
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Stephen Swaffer

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Re: feeder cable under load induction hum
« Reply #2 on: January 02, 2015, 09:11:13 pm »

IMO, the only consideration is heat generation.  Consider that the NEC requires a derate for more than 3 current carrying conductors in a conduit, and a 50% derate for more than 9 conductors a coil with only 5 wraps gets you up to 10-and SO is rated to be in the open not in conduit.  Where is the heat supposed to go?

Two parallel conductors 180 out of phase (hot/neutral) should be a minimal inductance/hum issue if I am thinking correctly?
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Steve Swaffer

Jason Glass

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Re: feeder cable under load induction hum
« Reply #3 on: January 02, 2015, 09:52:53 pm »

Two parallel conductors 180 out of phase (hot/neutral) should be a minimal inductance/hum issue if I am thinking correctly?

USA 3 is 120 out.  You can hear it when you get a guitar with single-coil pickups too close to feeder coils and crank up the amp gain.
« Last Edit: January 02, 2015, 10:00:41 pm by Jason Glass »
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Jeff Carter

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Re: feeder cable under load induction hum
« Reply #4 on: January 02, 2015, 10:17:33 pm »

USA 3 is 120 out.  You can hear it when you get a guitar with single-coil pickups too close to feeder coils and crank up the amp gain.

I don't doubt it, but that would be a higher-order effect due to the feeder cables not all being *exactly* the same distance to the pickup. If the total current in all conductors of the feeder doesn't add up to zero net current, you've got a fault to ground (or I've got a fault in my understanding of electricity).

I don't see how 1-phase vs 3-phase makes any difference in that regard.
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Jason Glass

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Re: feeder cable under load induction hum
« Reply #5 on: January 02, 2015, 10:30:07 pm »

I don't doubt it, but that would be a higher-order effect due to the feeder cables not all being *exactly* the same distance to the pickup. If the total current in all conductors of the feeder doesn't add up to zero net current, you've got a fault to ground (or I've got a fault in my understanding of electricity).

I don't see how 1-phase vs 3-phase makes any difference in that regard.

Good point.  Now that you mention it, it usually sounds like a sawtooth, not a sine, sometimes at higher octaves than 60Hz (through guitar pickups).

Aside from many years of dealing with single-coils on the road while touring, my home practice room is just above the main breaker box in my basement.  The company service runs down that wall on the outside, through a metallic conduit, and when I switch to my single-coil neck or middle pickups and move to within a few feet of it, the hum is unbearable.  I can clearly hear it get stronger or weaker as I turn toward or away from the wall.

BTW, this is with a guitar in which I've lined the pickup cavities with Mu-metal connected firmly to the connector shield.

Jay Barracato

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Re: feeder cable under load induction hum
« Reply #6 on: January 03, 2015, 02:59:59 am »

I would be more worried about heat also.

That was my unlikely introduction to a festival crew one time. "Hi I am Jay the band tech for ________. Did you know you Ac feed is smoking?"

Sent from my DROID RAZR HD using Tapatalk

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Jay Barracato

Mike Sokol

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Re: feeder cable under load induction hum
« Reply #7 on: January 03, 2015, 07:46:33 am »

I've always heard that feeder should not be wrapped in a coil when it going to be under load.
This I assume is because it will introduce hum in the sound system as well as heat.
Is this true?

I've understand figure 8 will negate that. Will the over under wrap also negate that?

I ran the numbers on this once, and even if there was a single-wire/open-air coil it would generate almost zero heat due to induction. You would have to wrap a single cam-lock feed around a big chunk of iron to get any significant induction heating effects. I don't think that's going to happen. 

However, wire resistance under load will contribute to heating, and a tightly coiled wire won't be able to dissipate this heat. So it can get hot enough to melt insulation. That's why we unwrap power cables rather them leaving them coiled up in a box. They need air flow to dissipate the heat from the normal wire resistance, not impedance.

Note that in a standard 3-wire or 4-wire setup the return current on the neutral line will cancel out the magnetic field to begin with, so there will be almost zero induction effect for normal wiring. But resistance heating is still there, so it's still a good idea to pull the feeder out of the box so it gets airflow.
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Art Welter

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Re: feeder cable under load induction hum
« Reply #8 on: January 13, 2015, 11:57:25 am »

Good point.  Now that you mention it, it usually sounds like a sawtooth, not a sine, sometimes at higher octaves than 60Hz (through guitar pickups).

BTW, this is with a guitar in which I've lined the pickup cavities with Mu-metal connected firmly to the connector shield.
Guitar amp speakers usually don't have much response below 80 Hz (low E on a six string)so the harmonics of 60 Hz (120, 180, 240, 300 etc.) come through louder than the fundamental.

This fix takes time, but will almost eliminate the single coil "noise antenna" problem:

https://sites.google.com/site/stringsandfrets/Home/noise-reduction-for-sc-pickups

I'm going to do the mod on my single coil guitars, eventually...Until then, I 'll just point the guitar in the direction that the noise is least :^).

Art
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Re: feeder cable under load induction hum
« Reply #8 on: January 13, 2015, 11:57:25 am »


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