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Radiating high frequency noise

There is a specific mechanism that I have been tracking down that is similar to what Al mentions.

Many components that have traditional linerar power supplies (not switchers) generate a fair amount of high frequency noise in their power transformer. When the diodes in the PS switch off they generate a pulse of noise, this by itself can be dealt with fairly easily, BUT this noise frequently excites resonances in the power transformer which is much greater than diode noise itself. The transformers literally ring for quite some time after each 120Hz waveform. My checking of a bunch of components shows this noise to be in the 100-300KHz range for most components. This is high enough to sail right through most power transformers but low enough to be affected only slightly by most AC line RF filters.

Components can even be excited by each other. I had one component that was not generating any of this on its own, but when plugged into a strip with another component that was ringing, its own resonances were excited, winding up with two different ringing frequencies in the box and on the power cords. With all my usual boxes plugged in I had four separate ringing frequencies throughout the system!

What does this have to do with power cords? They radiate all this stuff! It can get picked up by your ICs, speaker cables, components themselves etc. My tests show that different power cords can have radically different amounts of radiated fields from this source. The cords can also effect the resonance itself by changing the electrical parameters the transformer sees. Different cords can change the frequency of the resonance in some components.

I did some experiments with a spectrum analyzer and a short piece of wire connected to a probe and traced out the field around different parts of the system, plugs, cable, fuse, transformer etc. The results were kind of interesting. The transformer itself actually radiates little energy, the power cord itself ratiates more, but the amount varies significantly with cord. Cords with twisted wires radiate less, shielded twisted much less. The greatest radiation is at the fuse. This makes sense because this is where the wires separate. In the cord the fields from the two wires tend to cancel, there is no cancellation around the fuse where the wires separate. The plug is alao a place where the wires separate a little more than usual which also causes a significant increse in radiation.

So the upshot is that cords do make a big difference in the amount of radiated fields. Fuse holders and fuse wireing also make a huge difference. I didn't do any tests about different fuse types or materials, but I could easily see how this could make a difference. Anything which keeps the two AC wires close to each other will decrease the radiated filds.

BTW in my system I attacked the problem at the source, I dampped all my power transformers so they don't ring anymore. Jim Haggerman has a nice paper on this at his site, I followed his procedure to measure the resonant frequency and impedance of each transformer and compute a damping network for each. This made amazing improvements in the system sound. Unfortunately the measurements are not easy, I happened to have the right test equipment to do this.

As Al mentioned Higher frequency RF issues can also be an important part of power cord behavior, especially with all the digital stuff floating around in our environment, but I found that the transformer generated ringing is far more of a problem, at least in my system.

John S.


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Follow Ups Full Thread
Follow Ups
  • Radiating high frequency noise - John Swenson 12:29:44 05/03/07 (1)
    • Thanks! - Al Sekela 11:47:15 05/04/07 (0)

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