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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: Hi Tom posted by jneutron on August 10, 2006 at 07:00:30:
Hi jneutronA 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.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.
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.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.
When I have had ground hum problems it has usually been a result of several things.
In old Tube stuff, it was often a result of some power supply leakage current to ground within the electronics.
This current puts a small voltage on the chassis, with respect to quiet ground.
This current can be induced into a ground wire passing to close to the transformer winding (if not shielded).
Many old style transformers have a copper loop (shorted turn) on the outside of the transformer to shunt stray field induction.
Another situation is when separate power circuits are used, the return lines have Rdc and load current shows up as a voltage on the return line according to Ohms law.
Two different feeds with different stuff on each (in addition to the stereo) can easily produce ground signals.I tried to patent a "fix" for ground hum about 15 years ago but there were too many similar things cited.
I called it a humfree, this trick didn’t eliminate all hum but it reduced it by at least 30 dB which is normally enough to make it inaudible.
One can use a transformer to isolate but one is stuck with the response, level and distortion limitations of transformers.
This trick has none of those limits but also does not supply DC isolation.Take an offending amplifier and preamp, take the interconnect pair and wind both side by side onto a HIGH permeability core
with enough turns to have an inductive impedance of say 30 Ohms at 60Hz.
Now, any differential voltage sees the inductor instead to the DC resistance, the Current flow and voltage across Rdc is greatly reduced (a good thing) and the Voltage differential between each end is induced equally on both the ground and signal conductors, the hum is gone by common mode addition.
Try it.This is a common mode choke / transformer it is used for RF noise very often but with the right core and inductance, the trick works very well for this too. A ferrite core will not work, it is low permeability.
Unlike the isolation transformer, the signal response is DC to high RF, no distortion.So far as hifi, I would imagine you rarely encounter the magnitudes of properties you deal with at work.
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.
Best,Tom
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Follow Ups
- Re: Hi Tom - tomservo 09:26:22 08/10/06 (1)
- Re: Hi Tom - jneutron 10:05:42 08/10/06 (0)
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