[sdiy] RC, LC we all c RFin HiZ

[mark s]

let's start with an ideal (wish):

is there an effective way to eliminate, at the input of the circuit, all 
frequencies above, say 50kHz, leaving the audio band untouched ? placed at 
the input of a circuit, it would prevent all the bad stuff from getting in 
through the input cable, at least.

ok now go ahead and school me :^)  but please share your techniques for 
reducing AM, FM and the rest of it.

i'll ante: a 10nF or even better 20nF cap across the input jack has worked 
wonders for me. but sometimes you need more....

mark