Bad news: radio waves are everywhere, and every wire is an antenna. Wide bandwidth is a red herring. Don't fall for it.
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I don't know that is the claim at all. Seems to me it is pure blather you heard someone repeat and thought maybe they knew what they were talking about. This by the way covers the vast majority of what you are likely to hear. Pure blather repeated endlessly back and forth without ever a thought as to what is really going on.
I'm not going into the weeds of what exactly is going on, other than to reiterate what I already said: radio waves are everywhere, every wire is an antenna. And add one thing: a tuner is a device that allows you to tune in to certain radio frequencies in order to hear them as music. Because otherwise it is just background noise. A tuner is deliberate. But you can accomplish the same thing quite by accident with almost any circuit. That is all that is going on here. Once in a while someone gets some really bad RFI. So bad they can hear a radio station. Then this guy, who has no clue why or what is going on, one of three neurons in his brain fires and he gets the bright idea, "bandwidth! It's the bandwidth!" Like Steve Martin, "Stay away from the cans!" https://www.youtube.com/watch?v=eXRM3lFRwRI
Wide bandwidth is good. You want wide bandwidth. What you don't want is a lot of misinformation from misinformed audiophiles who know just enough to be dangerous.
spacial-temporal resolution of the human ear is, as a pair, and to some extent individually: exceeds a requirement for a jitter free 200khz/20 bit reproduction in digitally based signals....
the amplifier tends to be the same. the seeming contradiction is a excellent sounding tube amplifier that may only go out flat to 17khz or the like.
the situation is complex, at best. clean non phase-shifted bandwidth and similarly important clean dynamics is helpful, regarding both being a specification involving an amplifier.
Yet... a tube amplifier can violate both to some degree and still sound good...
With the wrong interconnects in a bad place you can pick up a radio station even without a wide bandwidth amp! :)
So the theory is that a very wide amplifier is very fast, and can go very low, perhaps to 0Hz and 100kHz. The problems are that 0 Hz is a DC signal which can blow your speakers, and 100 kHz could fry your tweets if amplified and you'll never hear either.
So, sure, wide bandwidth amps are extremely capable in the audible zone, but does that alone make it better? Meh.
So you are saying don't fall for the claim wide bandwidth is bad?Wide bandwidth is used to maintain phase relationships within the audio passband. To do this they have to go from 2Hz to 200,000Hz; 1/10th the lowest frequency and 10X the highest frequency to be amplified.
The exception to this is if the amplifier has a very high amount of negative feedback, in excess of 35dB. In this case the bandwidth can be limited to just about 20KHz, and the feedback will allow the amp to correct the phase that would otherwise be present by a rolloff just outside the audio band.
Radio Frequency Interference is another matter altogether. Its caused when a strong RF signal gets rectified somehow in the audio circuit and gets amplified. As others have pointed out, wide bandwidth isn't needed for that to happen.
So don't fall for the claim that wide bandwidth is bad.