Some food for thought about the effect of high frequencies in audio power amplifier

[Scytales]

Of course RF can enter a unit as well through the output.
The mandatory EMI capacitor at the line output jack and the usual series resistor between the actual output stage and the output jack should usually suffice.

Fun (sad ?) fact : trough studying a lot of schematics and actual implementations in various apparatuses, mostly CD/SA-CD/DVD players, I found that EMI capacitors at the output jacks are provisioned in the schematics and PC boards, but not populated in actually produced units.

I do not know why. Perhaps, the manufacturers skimp on that components to save cost as long as a specific unit pass EMI regulation without them ?

 

[nanook]

The EMI caps have to be right at the jack to be most efficient (a wire inside the unit will serve as antenna when the cap is located further away).
Some designer may fear that the capacitor at the line output may create a resonant system with the inductance of the cable and possibly a capacitor right at the input of the receiving unit.
Cost should not really be a thing, but you never know.

 

[Cbdb2]

View attachment 379620

This is interesting and tells the story. Some quick calcs. For every 5db drop in RF input the output drops by 20db! When the input is down to -35dbv (still 18mv RMS) output on the worst Amp is already -100dbv. Typical antennae pickup is less than a 1mv (-60dbv) which would produce an output at -180dbv. Thats from an antennae, a shielded coax should drop the input another 20db.
Yea if your next to a 10Kwatt radio transmitter etc. you might have a problem otherwise nothing to worry about.

 

[nanook]

Well, you either have a new car (with the radio built according to todays EMI requirements) or you may hear that your mobile phone is about to ring a couple of seconds before it actually does. This at least holds true for my car radio built about 24 years ago ;-)