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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: Re: Hi Tom posted by tomservo on August 10, 2006 at 09:26:22:
ts: ""
A coax conducting a signal has essentially no external magnetic field and as you point out, if you were to include a coax cable intercepting a changing magnetic field, a voltage is produced equally on both conductors (see trick below)
As you point out induction done at dimensions which result in a sharp gradient over one portion, do generate local current which in the idea case is shunted by the eddy current’s field in the shield.
So far as an ideal power cord, a Triaxial coax would be about it, with RG-8 or equiv. with a ground wire might be next.""Good, somebody said triax..center conductor hot, first braid neutral, outer braid safety ground. RG-8 with ground wire will suffer intercept, but not transmit.
Issues with triax: center wire can no longer be rated as per guage, it has to be derated based on insulation thermal conductivity. I used #10 awg for a 15 amp run hot, with about #12 equiv for neutral, and #14 as outer ground...cable rated for 15 amps max.
In addition, insulation has to be more robust, as bending with a triax will induce more severe normal forces internally. Given the propensity for diyers to use cold flowing teflon, that cannot be a good thing. Which explains my trepidation at providing the design.""
ts: ""
The issue in hifi power cords though is when would one run into this situation at a level where it is a problem and if such a problem exists in a real item, how large are the "other" problems which go along with those stray magnetic fields, for example, how much eddy current and field are in the chassis around the transformer etc.""Agreed. But as you know, there are no test standards for these kinds of effects. Additionally, there are no limits recommended, simply because there is little discussion of this.
ts: ""
A twisted pair has no net far field magnetic radiation, near field it is as you describe, a twisted linear dipole, of dimensions related to the conductor spacing. Normally, this field has the highest magnitude between the conductors and dimensions rapidly with distance. As one is dealing with one turn, a permeability of one and limited current (in hifi) the B field is not significant like in a transformer.""Also agree. However, with any flux linkage, the loop voltage will be proportional to the freq (well, ok, rate of change of integrated flux), therefore loop current. Confounding measurement is the fact that high power levels are what cause the problem, and then ya tryin ta pick up milliamps in a loop without altering the loop..
If any coupling exists at all, moving the other ground farther away only picks up more, not less..distance doesn't help.
ts: ""
I have no experience with Romex, everything around here has to be in steel conduit and no more than 4 feet of armored BX can be used.""No romex? Some people have all the luck. Mine of course, is at home. At work, 500mcm is normal in trays. But for the most part, the electricians do that stuff. Then, we hafta figger out where the noise comes from (no triax there). Sometimes, it's easy, with 150 volt spikes on the 120 lines in the office..
Your trick works great for one amp, but won't for two. Monoblocks are out..
ts: ""
So far as hifi, I would imagine you rarely encounter the magnitudes of properties you deal with at work.""You've never seen my stereo...:-)
What suprised me many years ago was this: Running a 6Ka test widgit through a 250 uOhm shunt resistor, I found an error signal from the CVR. It looked like my diode was taking about 2 milliseconds to shut off, it duplicated a TRR waveform. Turned out the mag field collapse in the shunt was the error source, the two wires connected to it formed the pickup loop. Moving it away just increased the trapped flux. By running it coaxially down the shunt, in the geometric center of the fins, I zero'd the trapped flux. Error dropped to below 10 mV. At 20Khz, 1 ampere, I saw the same error component of about 1.5 volts. Audio load resistors suffer the same thing.
But in home stereo, if you have hum caused by a poweramp haversine, and drop to just inaudible, what about the loop pickup when the amp is not idling? Loop pickup is proportional to the freq.
ts: ""
I had to deal with 200 amp single turn em levitator coils running at 12MHz and it is another world once one is that many octaves higher up.""Definitely agree. When the FCC locked ya up, how long were ya in for??
I'm larnin bout hydrogen thyratrons at the moment, need 12 kV at 2500 amps in 200 nSec for 600 nSec duration..man, I never knew this stuff existed. Weird, but cool.
Cheers, John
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Follow Ups
- Re: Hi Tom - jneutron 10:05:42 08/10/06 (0)
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