Amp picking up radio station

[agedhorse]

Sometimes the RF signal strength is so high that it can't be eliminated no matter how good the design.

 

[Wasnex]

This was something that used to happen often way back in the day.
I remember not only radio stations, but the occasional pilot or train communications and even cop radios suddenly blaring out at gigs.
I just figured it was something the amp manufacturers had overcome since I hadn't heard it in a loooong time.
Guess not.

I am sure there has been some improvement in design of amps, but plenty of people are still using vintage amps.

Lot's of things other have changed. The FCC has restructured the frequency band so it's possible some of those broadcast no longer occur in a frequency range, or with a modulation type that would readily be picked up by an amp. Also power levels of different types of radios may have been modified.

AM radio is pretty bad about sneaking into electronics, especially if the power level is really high. AM is super easy to detect...I believe something as simple as a rusty connection can do it.

I believe many modern radios use digital, which allows the same performance at a lower power level. The low power means the signal is less likely to bleed into audio circuits. Digital means if the signal does bleed into an audio circuit that it would not resemble the original transmission.

Another issue is manufacturing tolerances. If a transmitter is putting out too much power or over modulates the carrier, the signal it is more likely to be detected in places where it is not supposed to be heard. Back in the day there were tons of people running around with souped up (illegal) CB radios.

I believe CBs use AM modulation or something called Single Side Band (SSB), which is an advancement of AM modulation. IF I remember, AM has two side bands. SSB suppresses one of the side bands, so all of the modulation can be encoded in the other side bad. Using only one side band extends transmission range without increasing the power or bandwidth of the broadcast. I believe amps can detect SSB easier than a AM broadcast of the same signal strenght.