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In Reply to: RE: " better EMI/RFI rejection " is it really an issue for speaker cables ? posted by Dave_K on February 12, 2015 at 06:49:56
My experience with RFI problems in stereos and such is that the interfering signals usually enter the system through the speaker cables. After that typically the power cords then the interconnects.
It has little to do with gain, a lot to do with effective aperture: Speaker wires and power lines are great antennas! The junctions in your output bugs are rather directly connected to the speaker wires, especially if the design doesn't use a Zobel network, and your amplifier becomes an amplified crystal set.
The best cure is to just snap on a Ferrite Z-bead over the speaker cable (it has to go over both wires) near the amplifier. The audio, being low frequency and differential won't see it but the RF gets reflected and absorbed.
Radio Rick
Follow Ups:
Hi Rick,
I'm having a little trouble wrapping my head around this:
It has little to do with gain, a lot to do with effective aperture: Speaker wires and power lines are great antennas! The junctions in your output bugs are rather directly connected to the speaker wires, especially if the design doesn't use a Zobel network, and your amplifier becomes an amplified crystal set.
For a typical solid state amplifier with low output impedance, and a similarly low impedance speaker cable, I presume the output stage should effectively become a sink for any induced noise currents at frequencies within the bandwidth of the output stage, just as it is a sink for back-EMF. In other words, it is close enough to an ideal voltage source within its bandwidth that the speaker only sees the noise induced voltage differential across the cable.
Assuming a typical push-pull output stage, I don't see how it could behave like a crystal set because one side or the other is always conducting so there would be no rectification.
I hadn't given much thought to noise frequencies outside of the bandwidth of the output stage, but wouldn't they just be shunted through stray capacitance?
"the output stage should effectively become a sink for any induced noise currents at frequencies within the bandwidth of the output stage"
Yes, spot on!
The rub is that the interfering signals, at least for "RFI" by definition, are outside of it's bandwidth and therin lies the rub. The loop can't control it because it's GBW is too low at the interfering frequency but yet the final device junctions are plenty fast to rectify and thus "detect" it. Things go rather downhill from there as it's modulations get cought up in various feedback loops.
In general all systems must be protected so that none of their ports are exposed to signals that they can't handle. Lots of the time it's just common sense. If you are having an audiophile lawn party and a fast front comes through you probably will expect some problems with your preamp after a lightening bolt hits the tonearm.
Well, RFI is just a tamer version of the same thing. The general term for this sort of thing is EMI (Electromagnetic interference) and the tendency to be susceptable to this ilk of problem is called it's "Susceptibility".
The bottom line is it's common to virtually everything and the cure is to prevent ports from getting hit by energy that they can't handle. That usually involves passive filtering between them and the cold hard world.
Rick
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