Speakers sound OK? If you didn't need shield speaker cables, who does?
Serious RFI, indeed! If you were to walk outside near that on a cold day, you wouldn't need tower a coat or jacket; It would be like being microwaved in a microwave oven. You's simply self heat.WTMJ puts out 50kW on AM during the day! Now that's what I call RFI
Yes, but there is no amplification stage after the speaker cables that might be upset by the RFI or might amplify the RFI signal to audible levels. Neither are your speakers going to be very good at reproducing RF interference frequencies so you are very unlikely to hear any benefit from putting a shield on said speaker cables.
It would be an interesting engineering challenge to come up with a situation where it did make a difference. But the aesthetics of a metal/conductive loudspeaker enclosure and metal flooring is a bit too industrial for me.
The issue with such reasoning (in general) is that EMI/RFI is very situational. Regardless of how susceptible or immune a system is; an aggressor (source) is also required to have any ill effects. For example, in most conditions a non-shielded untwisted pair RCA may be perfectly fine. However, if laid on a large power transformer one may pickup 60 Hz noise.
I would wager that it was more likely the 'noise' is being coupled back into the amplifier and finding a route to couple into the signal path prior to amplification.I believe I've mentioned this before elsewhere on ASR: When I was a lad, we lived a just few miles as the wave flies
a pair of LS-12 drivers, unbaffled.
Sensitivity of the LS-12, if memory serves, is ca. 96 dB/watt @ 1 meter (calculated from the driver's published EIA sensitivity specification)
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* not a "peanut whistle", but also not 50 kW (as, e.g., was, and still is, WBAL-AM @ 1090 kHz). I think WFBR may have been 5 kW (@ 1300 kHz in those days).
I once did some consulting for a large car audio company which made subwoofers. Once in a while a fork lift driver moved a pallet load along the wall next to many offices. The computer monitors would all have visual distortions moving from one side to the next as the forklift passed.I had shielded 18 ga speaker wire when I lived near a large radio transmitting tower. It was some kind of Belden with foil and an 18 or 20 drain wire. It worked fine without grounding the drain. Unshielded cable picked up the radio station. Magnetic shielding for the speakers - MuMetal is to keep the magnetic fields of the drivers in, so they don't interfere with the deflection coils of CRT displays, a problem we don't have now. YMMV.
The famous contemporary artist Nam Jun Paik used CRT displays because that was what was available at the time. He illustrates the impact of a permanent magnet on a display. The variable magnetic field of a speaker would have an effect depending on the speaker and the distance.
I would think the coupled HF noise would be magnitudes larger than the radiated and re-coupled EMI/RFI.
sounds like an installation issue to me. I can't think if a single home installation where that is something that can't be avoided rather easily.
sounds like an installation issue to me. I can't think if a single home installation where that is something that can't be avoided rather easily.
That seem like an argument of theory, which is not what I was referring to.
What happens is: the interference sneaks in thru the amplifier's feedback loop to the sensitive first stage.
This. It is what I have seen many times in the past. The output impedance of the audio amplifier is very low at audio frequencies, but high at RF frequencies, allowing RF on the speaker wires to be brought inside the amplifier. A good design will include RFI suppression, typically broadband capacitors, but in a poor design the RF signal can sneak through the feedback path to be detected by the input stage and thus converted to audible noise.
Old fart, what can I say? I worked with the Kokomo team a little many years ago, though never in Kokomo.
Most amps, especially in the days had inductors (often with a resistor bypassing it) and a Boucerot filter (cap + resistor) just for that (stability of the amp).
Understanding the effects of EMI/RFI certainly gets complicated, often the removal/movement of the source (aggressor) is the easiest method.
Yes, for stability with reactive loads, but that was usually not sufficient RFI protection. Too much leakage around the inductor, and the cap's self-resonant frequency usually too low. But IME, so may not match that of others'. Some had small ceramic caps, or an RC series snubber, for RFI on the speaker terminals. Back in the day, adding a snubber across the (external) speaker jacks was something I did fairly often, mainly in the days of the CB craze when illegal sweep-tube "linears" (RF amplifiers) were everywhere and most of them were horrible for spraying distortion.
Yah... When I was doing tech work and having to deal with RFI constantly, removing the source wasn't usually an option. It was often a nearby CB or Ham station (though I worked with a number of the owners on mitigation -- Hams were usually OK, CB owners a mixed bag) or drive-by "good buddy" with a CB, and shutting down the local AM radio station wasn't an option. Now and then the radio station was out of spec, and they'd fix it, but usually it was just poor RFI rejection by the audio components that was the culprit.
