Guido & Noble100,
Why an FM radio? I would expect that an AM radio would provide a much more sensitive and potentially more meaningful test.
As you probably realize, FM is inherently vastly more immune to RF interference than AM, since the audio that ends up being reproduced corresponds to frequency variations in the airborne signal rather than to amplitude variations. Also, in contrast to the AM broadcast band the 88 to 108 MHz frequencies of the FM band (and even its 10.7 MHz IF frequency which follows down-conversion within the radio) are vastly higher than the bandwidths of any audio component, and are therefore much less likely to be relevant.
I suspect that if an FM radio audibly responds to airborne RFI generated by an audio component it is likely not what is picked up by its built-in antenna that results in the audible interference, but rather what is picked up by downstream circuitry within the radio that operates at much lower frequencies. That response probably being much less sensitive than an AM radio's response would be to interference picked up by its antenna.
FWIW, my guess is that you'll hear significant interference when an AM radio, preferably tuned near the low end of the band (e.g. 540 kHz), is placed close to the amp. But what will be of interest is how quickly the interference drops off as the distance increases.
Best regards,
-- Al
Why an FM radio? I would expect that an AM radio would provide a much more sensitive and potentially more meaningful test.
As you probably realize, FM is inherently vastly more immune to RF interference than AM, since the audio that ends up being reproduced corresponds to frequency variations in the airborne signal rather than to amplitude variations. Also, in contrast to the AM broadcast band the 88 to 108 MHz frequencies of the FM band (and even its 10.7 MHz IF frequency which follows down-conversion within the radio) are vastly higher than the bandwidths of any audio component, and are therefore much less likely to be relevant.
I suspect that if an FM radio audibly responds to airborne RFI generated by an audio component it is likely not what is picked up by its built-in antenna that results in the audible interference, but rather what is picked up by downstream circuitry within the radio that operates at much lower frequencies. That response probably being much less sensitive than an AM radio's response would be to interference picked up by its antenna.
FWIW, my guess is that you'll hear significant interference when an AM radio, preferably tuned near the low end of the band (e.g. 540 kHz), is placed close to the amp. But what will be of interest is how quickly the interference drops off as the distance increases.
Best regards,
-- Al