high frequency intermittent noise

It’s 4:45 and rush hour is upon us, I can hear the trains below, and the noise through my system.

Coincidence? You might start keeping a log.

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You said in your OP you have a pair of pro-audio monitors. By chance do you have any RCA shorting plugs? Not the dust cover caps but a shorting plug?

How big is one of the monitors, and how heavy? Is it something you could carry around at different locations in your apartment without a lot of effort?

You already said when you plugged them into the wall outlet, that your audio system plugs into, you can hear the noise. Was that with the monitors hooked up to the preramp or just power on with nothing connected to the input?

Have you ever powered one of them up without anything connected to the input?

I would think if the RF noise signal is radiated through the air it would enter the open RCA input jack and you would hear it through the speaker pretty good. If that is true what would happen if you placed one of them in the farthest room, area, as far as possible from were the audio system is now? Plug it in a wall outlet in that room.
Turn the monitor speaker amp on and see if there is any difference in the sound, and strength, of the signal noise compared to the room your audio system is in.

The RCA shorting plug.
If you have an RCA input shorting plug install the plug on the monitor speaker amp’s input at the various locations you do the testing. Try it without the RCA plug in the input and then with it inserted in the RCA input jack. I AM SURE I DON"T NEED TO MENTION IT. BUT DON"T DO IT WITH THE AMP POWERED ON. You will get one heck of a loud buzz.

My thinking is if the RF noise is being radiated it will be louder with the RCA port open than it will be if the input is shorted out with the plug. If the RF noise is traveling on the mains wiring it won’t much matter even with the RCA input  shorted out using the shorting plug or the input jack is open. That is for the particular RF noise you have been hearing.

Would the same logic apply if the pro-audio monitors have a balanced input?  No RCA inputs, just XLR.

AM is WAY too low in frequency. Cell phones broadcast in the 700 to 2100 MHz range. Not every tower serves every cell phone user. If it’s not a microcell of some sort it may me an LTE based hotspot. Whatever the source, that IS a TDMA signal pattern. It’s not WiFi because WiFi uses a variation of frequency division for point access. Bluetooth uses a frequency hopping scheme and would only be audible briefly.

@kosst_amojam,

Why is james1969 audio system able to receive the RF signal?

AM is WAY too low in frequency. Cell phones broadcast in the 700 to 2100 MHz range.

Thanks for your informative inputs, Kosst. But of course the bandwidth of James’ audio components most likely extends only up to a frequency that is even lower than both the 540 kHz he had his radio tuned to and the 455 kHz IF frequency which I believe is used by AM radios in the USA.

So if cellphone traffic is indeed the source of the noise, his components are presumably responding to relatively low frequency modulations that are present on the near GHz carrier frequencies, which **might** also be picked up by an AM radio.

Regards,
-- Al