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Old 29th November 2006, 04:03 PM   #1
AndrewT is offline AndrewT  Scotland
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Lightbulb power amp output currents (to speakers)?

When wiring up a mains distribution board it is imperative that the incomers all come in through the same hole in the metal enclosure. Never feeding one of the poles in through one hole and another pole/neutral/earth in/out through another hole.
The three core we use on the consumer side virtually guarantees compliance with the rule. But the double insulated single cores coming from the meter can be wired wrongly.

In a power amp the input(flow) and output(return) to the speakers are almost always fed in and out through different holes.
Could this be a problem for amps.
Whereas the coax/twisted pairs used on the inputs are almost always fed through a single hole (the input current and return current cancel).

Could there be a message here?

Could a field be set up by the speaker currents using the case as a coupling device?
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Old 29th November 2006, 04:11 PM   #2
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" ... Could a field be set up by the speaker currents using the case as a coupling device? ..."

This is always a concern = that's why a set of quality binding posts are usually used to penetrate the chassis, usually with a reasonable spacing between + and - connections.

Twisting of the speaker pairs is usually OK over short runs, but can very modestly affect sound quality over very long runs. Advise using larger speaker leads (of lower resistance) to reduce these kinds of "problems".

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Old 29th November 2006, 04:17 PM   #3
AndrewT is offline AndrewT  Scotland
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I don't think we are considering a resistance issue here.
Quote:
that's why a set of quality binding posts are usually used to penetrate the chassis
seems to get closer to the problem.

Do we need to fit the PAIR through ONE hole?
Or is the separate hole issue of no consequence?
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Old 29th November 2006, 04:31 PM   #4
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I think that aspect of the regs is purely a safety issue, it keeps everything organised and prevents mistakes with live incomers.
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Old 29th November 2006, 04:34 PM   #5
AndrewT is offline AndrewT  Scotland
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Pinky,
no.
It's something to do with current amplifiers or induced currents or somesuch (you'll gather I am beyond my comfortable zone) due to the LOOP caused by taking the in and out around the metal forming the link between the two holes.

It is to prevent that mechanism that the ONE HOLE rule is there.

I suppose the analogy may be the detection of unbalanced currents in the L & N to check for earth leakage and shut off the breaker (RCCB). Similarly the use of a current transformer to detect current flow by using an isolated detector coil.

What if the metal was not ferro magnetic but instead aluminium or copper?
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Old 29th November 2006, 06:29 PM   #6
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See http://www.oselectronics.com/ose_p111.htm = middle of page ... but one supplier example of dual binding posts w/ pics = 3/4", center to center ~~= 1" wide x 5/8" tall hole ... they all seem to work in relatively standard chassis holes. Lab equipment uses these to offset your very concerns.

The technical difficulties regarding passing speaker wiring through a metal chassis hole you refer to are trivial compared to wire resistance. After all the total wire "impedence" is a derivation of the sum total of stray capacitance (surface area of the wires compared to the surface area of the metal chassis and each other = very few picoFarads) and inductance (the effect of the parallel wires on each other and to the inductive interefrence or inductive "load" caused by the chassis treated as a conductive plane = very few picoHenrys) ... AND the resistance of the wires = a few milliOhms or more per foot. All in all far and away the largest impedence component is the wire resistance, being very much greater in effect than either the stray capacitance or the back EMF caused by the inductance ...

Restated: the wire resistance has much more effect on the quality of audio signals (small or large signals) than all the other effects of passing your speaker wires through a chassis hole.

In still more words: the wire resistance is so much more important to audio quality that capacitance and inductance of the wires and the chassis can be almost totally ignored ... So ... I refer the gentleman to the arguments given just moments ago.

Consult Maxwell's equations for greater details.

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Old 29th November 2006, 06:39 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Could a field be set up by the speaker currents using the case as a coupling device?
and the effect on the input stage.
Not the signal sent to the speakers (although ultimately affected, if the input is mutilated by some kind of feedback mechanism using the case as a transmitting aerial device).
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