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Old 2nd November 2016, 05:00 PM   #1
AndrewT is offline AndrewT  Scotland
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Default Speaker Terminal holes thro' metal panels

There is a general rule that an AC current carrying wire should not pass through a metal panel, but instead both the Flow and Return wires should pass through the same hole.

Many conventional Amplifier enclosures have a pair of adjacent holes through the rear panel, into which are fixed the speaker terminals.

I have seen a very few amplifiers that link the two terminal holes with a pierced gap (slot), so that there is one hole that is spectacle shaped.

I am wondering if the spectacle linked holes are a way to meet that "same hole" requirement.

Why is there that general requirement to pass both Flow and Return through the same hole?
Is there a need to cut a slot between the terminal holes so that they become linked?
What is the effect if that slot is not cut?
What difference would the material of the panel make? Is a plastic panel different from an aluminium panel. Is aluminium different from steel?
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Old 2nd November 2016, 05:19 PM   #2
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Neutrik Speakon connectors. Problem solved.
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Old 2nd November 2016, 06:54 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post

Why is there that general requirement to pass both Flow and Return through the same hole?
It only applies to steel: you can view the surrounding steel like a sort of large, flat toroid.
Steel has poor magnetic properties, and in a gapless magnetic circuit, these properties are exacerbated.
The result will be a few µH's of non-linear inductance, which can slightly affect the frequency response, and more severely the THD if you are in the ppm range.
I think I have posted somewhere the result of such a measurement (but I don't remember where).

When flow and return pass in the same hole, they form a compensated system, and the net result is nil.
For non-magnetic materials, there are no effects (at least at the normal audio level)
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Old 3rd November 2016, 08:38 AM   #4
AndrewT is offline AndrewT  Scotland
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Are you saying the steel panel hole with a current passing through behaves something like a current transformer?
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Old 3rd November 2016, 10:01 AM   #5
DF96 is offline DF96  England
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Quote:
Originally Posted by AndrewT
There is a general rule that an AC current carrying wire should not pass through a metal panel, but instead both the Flow and Return wires should pass through the same hole.
No, there is no such rule. There is a rule for steel panels, or other ferromagnetic materials, for the reasons given by Elvee. Note that when flow and return go through the same hole the net effect is not zero (except for a coaxial cable or a correctly used quad cable), but it is much smaller than when they go through separate holes.

If one pair of currents pass through two holes then you have an inductor. If two pairs do it then you have a transformer.
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Old 3rd November 2016, 10:07 AM   #6
AndrewT is offline AndrewT  Scotland
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Originally Posted by billshurv View Post
Neutrik Speakon connectors. Problem solved.
Agreed.
But that does not absolve all the manufacturers of steel panel enclosures that have pairs of separated holes for the speaker terminals.
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Old 3rd November 2016, 12:33 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
Are you saying the steel panel hole with a current passing through behaves something like a current transformer?
The steel panel acts as the core of a transformer (or inductor), and the wire passing through as a single turn
Quote:
Originally Posted by AndrewT View Post
Agreed.
But that does not absolve all the manufacturers of steel panel enclosures that have pairs of separated holes for the speaker terminals.
The effect is so small that they probably think it can be neglected.
In the tube era, the effect would have been totally swamped by other effects, it's only now that we have sub-ppm capabilities that it becomes noticeable
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